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<article id="post-Android嵌套滚动" class="article article-type-post" itemscope itemprop="blogPost">
<div class="article-meta">
<a href="/2020/11/14/Android%E5%B5%8C%E5%A5%97%E6%BB%9A%E5%8A%A8/" class="article-date">
<time datetime="2020-11-14T01:26:07.000Z" itemprop="datePublished">2020-11-14</time>
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<h1 itemprop="name">
<a class="article-title" href="/2020/11/14/Android%E5%B5%8C%E5%A5%97%E6%BB%9A%E5%8A%A8/">Android嵌套滚动</a>
</h1>
</header>
<div class="article-entry" itemprop="articleBody">
<p>Android Design包通过提供NestedScrollingChild和NestedScrollingParent来帮助开发实现嵌套滑动效果</p>
<h2 id="NestedScrollingChild与NestedScrollingParent关系简述"><a href="#NestedScrollingChild与NestedScrollingParent关系简述" class="headerlink" title="NestedScrollingChild与NestedScrollingParent关系简述"></a>NestedScrollingChild与NestedScrollingParent关系简述</h2><table>
<thead>
<tr>
<th>NestedScrollingChild</th>
<th>NestedScrollingParent</th>
<th>备注</th>
</tr>
</thead>
<tbody>
<tr>
<td>dispatchNestedScroll</td>
<td>onNestedScroll</td>
<td>分发嵌套滑动事件,在子View滑动处理完之后调用， unconsumed表示未被子View滚动消费的距离, consumed表示被子View消费的滚动距离</td>
</tr>
<tr>
<td>startNestedScroll</td>
<td>onStartNestedScroll</td>
<td>前者的调用会触发后者的调用,然后后者的返回值将决定后续的嵌套滑动事件是否能传递给父View,如果返回false,父View将不处理嵌套滑动事件,一般前者的返回值即后者的返回值</td>
</tr>
<tr>
<td></td>
<td>onNestedScrollAccepted</td>
<td>如果onStartNestedScroll返回true,则回调此方法</td>
</tr>
<tr>
<td>stopNestedScroll</td>
<td>onStopNestedScroll</td>
</tr>
<tr>
<td>dispatchNestedPreScroll</td>
<td>onNestedPreScroll</td>
<td>分发预嵌套滑动事件,在子View滑动处理之前调用, 通过consumed数组得到NestedScrollingParent消耗掉的滚动距离</td>
</tr>
<tr>
<td>dispatchNestedFling</td>
<td>onNestedFling</td>
<td></td>
</tr>
<tr>
<td>dispatchNestedPreFling</td>
<td>onNestedPreFling</td>
<td></td>
</tr>
<tr>
<td></td>
<td>getNestedScrollAxes</td>
<td>获得滑动方向,没有回调,为主动调用的方法</td>
</tr>
</tbody>
</table>
<h2 id="举例说明-SwipeRefreshLayout-amp-amp-RecyclerView嵌套"><a href="#举例说明-SwipeRefreshLayout-amp-amp-RecyclerView嵌套" class="headerlink" title="举例说明: SwipeRefreshLayout && RecyclerView嵌套"></a>举例说明: SwipeRefreshLayout && RecyclerView嵌套</h2><p>本例子中，SwipeRefreshLayout为NestedScrollingParent，而RecyclerView为NestedScrollingChild</p>
<p>根据触摸事件分发机制，ACTION_DOWN首先会来到<code>SwipeRefreshLayout#onInterceptTouchEvent</code>, 而SwipeRefreshLayout未做拦截，因此来到RecyclerView#onTouchEvent, 而RecyclerView#onTouchEvent针对ACTION_DOWN返回true；之后的所有事件都会传递到RecyclerView中，之后我们看下RecyclerView中对ACTION_MOVE事件的处理</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">case</span> MotionEvent.ACTION_MOVE: {</span><br><span class="line"> <span class="comment">// ... </span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (dispatchNestedPreScroll(dx, dy, mScrollConsumed, mScrollOffset, TYPE_TOUCH)) {</span><br><span class="line"> dx -= mScrollConsumed[<span class="number">0</span>]; <span class="comment">// 得到parent消费滚动后的剩余y距离</span></span><br><span class="line"> dy -= mScrollConsumed[<span class="number">1</span>]; <span class="comment">// 得到parent消费滚动后的剩余x距离</span></span><br><span class="line"> vtev.offsetLocation(mScrollOffset[<span class="number">0</span>], mScrollOffset[<span class="number">1</span>]);</span><br><span class="line"> <span class="comment">// Updated the nested offsets</span></span><br><span class="line"> mNestedOffsets[<span class="number">0</span>] += mScrollOffset[<span class="number">0</span>];</span><br><span class="line"> mNestedOffsets[<span class="number">1</span>] += mScrollOffset[<span class="number">1</span>];</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (mScrollState == SCROLL_STATE_DRAGGING) {</span><br><span class="line"> mLastTouchX = x - mScrollOffset[<span class="number">0</span>];</span><br><span class="line"> mLastTouchY = y - mScrollOffset[<span class="number">1</span>];</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (scrollByInternal(</span><br><span class="line"> canScrollHorizontally ? dx : <span class="number">0</span>,</span><br><span class="line"> canScrollVertically ? dy : <span class="number">0</span>,</span><br><span class="line"> vtev)) {</span><br><span class="line"> getParent().requestDisallowInterceptTouchEvent(<span class="keyword">true</span>);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (mGapWorker != <span class="keyword">null</span> && (dx != <span class="number">0</span> || dy != <span class="number">0</span>)) {</span><br><span class="line"> mGapWorker.postFromTraversal(<span class="keyword">this</span>, dx, dy);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">} <span class="keyword">break</span>;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>关注这里的<code>dispatchNestedPreScroll</code>和<code>scrollByInternal</code>方法， <code>dispatchNestedPreScroll</code>最终会触发<code>SwipeRefreshLayout#onNestedPreScroll</code>方法，<code>scrollByInternal</code>会调用<code>dispatchNestedScroll</code>方法，最终也来到<code>SwipeRefreshLayout#onNestedScroll</code></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="meta">@Override</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onNestedScroll</span><span class="params">(<span class="keyword">final</span> View target, <span class="keyword">final</span> <span class="keyword">int</span> dxConsumed, <span class="keyword">final</span> <span class="keyword">int</span> dyConsumed,</span></span></span><br><span class="line"><span class="function"><span class="params"> <span class="keyword">final</span> <span class="keyword">int</span> dxUnconsumed, <span class="keyword">final</span> <span class="keyword">int</span> dyUnconsumed)</span> </span>{</span><br><span class="line"> <span class="comment">//将nestedScroll传递给Parent</span></span><br><span class="line"> dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed,</span><br><span class="line"> mParentOffsetInWindow);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">final</span> <span class="keyword">int</span> dy = dyUnconsumed + mParentOffsetInWindow[<span class="number">1</span>];</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (dy < <span class="number">0</span> && !canChildScrollUp()) { <span class="comment">//注意canChildScrollUp方法</span></span><br><span class="line"> mTotalUnconsumed += Math.abs(dy);</span><br><span class="line"></span><br><span class="line"> moveSpinner(mTotalUnconsumed); <span class="comment">//处理refreshView的滑动</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="meta">@Override</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onNestedPreScroll</span><span class="params">(View target, <span class="keyword">int</span> dx, <span class="keyword">int</span> dy, <span class="keyword">int</span>[] consumed)</span> </span>{</span><br><span class="line"> <span class="comment">//只有当refreshView已经出现在屏幕中，并且手指往上移动才会调用下面的代码</span></span><br><span class="line"> <span class="keyword">if</span> (dy > <span class="number">0</span> && mTotalUnconsumed > <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">if</span> (dy > mTotalUnconsumed) {</span><br><span class="line"> consumed[<span class="number">1</span>] = dy - (<span class="keyword">int</span>) mTotalUnconsumed;</span><br><span class="line"> mTotalUnconsumed = <span class="number">0</span>;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> mTotalUnconsumed -= dy;</span><br><span class="line"> consumed[<span class="number">1</span>] = dy; <span class="comment">//消耗掉的距离, 这里回返回给RecyclerView</span></span><br><span class="line"> }</span><br><span class="line"> moveSpinner(mTotalUnconsumed);<span class="comment">//处理refreshView的滑动</span></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// refreshView移除屏幕</span></span><br><span class="line"> <span class="keyword">if</span> (mUsingCustomStart && dy > <span class="number">0</span> && mTotalUnconsumed == <span class="number">0</span></span><br><span class="line"> && Math.abs(dy - consumed[<span class="number">1</span>]) > <span class="number">0</span>) {</span><br><span class="line"> mCircleView.setVisibility(View.GONE);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 将nestedPreScroll传递到Parent去（本文可以忽略）</span></span><br><span class="line"> <span class="keyword">final</span> <span class="keyword">int</span>[] parentConsumed = mParentScrollConsumed;</span><br><span class="line"> <span class="keyword">if</span> (dispatchNestedPreScroll(dx - consumed[<span class="number">0</span>], dy - consumed[<span class="number">1</span>], parentConsumed, <span class="keyword">null</span>)) {</span><br><span class="line"> consumed[<span class="number">0</span>] += parentConsumed[<span class="number">0</span>];</span><br><span class="line"> consumed[<span class="number">1</span>] += parentConsumed[<span class="number">1</span>];</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>嵌套滚动时一个核心问题是当手指滑动时，这个滑动的距离由谁消费？为了解决这个问题，NestedScrollingParent引入了consumed这个概念，通过一个consumed数组的引用，可以告知上层View消费掉了多少距离，而子View则可以根据这个消费掉的距离，以及滑动的总距离，来处理自己的滑动距离</p>
<p>在本例子中，<code>手指移动的距离 = refreshView滑动的距离 + RecyclerView滑动的距离</code></p>
<h2 id="如何在代码中实现嵌套滚动效果"><a href="#如何在代码中实现嵌套滚动效果" class="headerlink" title="如何在代码中实现嵌套滚动效果"></a>如何在代码中实现嵌套滚动效果</h2><p>对一个NestedScrollingChild首先调用startNestedScroll，之后调用dispatchNestedScroll即可</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">recyclerView.startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL)</span><br><span class="line"><span class="keyword">val</span> arr = IntArray(<span class="number">2</span>)</span><br><span class="line">recyclerView.dispatchNestedPreScroll(<span class="number">0</span>, offsetY, arr, <span class="literal">null</span>)</span><br><span class="line"><span class="keyword">val</span> dyUnconsumed = offsetY - arr[<span class="number">1</span>]</span><br><span class="line"><span class="keyword">if</span> (dyUnconsumed != <span class="number">0</span>) {</span><br><span class="line"> recyclerView.scrollBy(<span class="number">0</span>, dyUnconsumed)</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
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<article id="post-FFMpeg-Android开发-简单音视频同步" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/09/13/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E9%9F%B3%E8%A7%86%E9%A2%91%E5%90%8C%E6%AD%A5/" class="article-date">
<time datetime="2020-09-13T02:39:03.000Z" itemprop="datePublished">2020-09-13</time>
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<a class="article-title" href="/2020/09/13/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E9%9F%B3%E8%A7%86%E9%A2%91%E5%90%8C%E6%AD%A5/">FFMpeg-Android开发-简单音视频同步</a>
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<p>本文代码可参考代码可以参考<a href="https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v2.1" target="_blank" rel="noopener">https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v2.1</a></p>
<p>我们之前实现了视频和音频的播放，但其中最大的问题是我们的音频和视频之间的播放速度没有同步，视频按照解码的速度，以最快速度进行了上屏，那么很有可能会出现视频播放完后音频还在播放的情况。这次我们就来尝试解决这个问题，正式解决问题前，我们先对一些基本概念做出介绍</p>
<h2 id="FFMpeg-中的-dts-和-pts"><a href="#FFMpeg-中的-dts-和-pts" class="headerlink" title="FFMpeg 中的 dts 和 pts"></a>FFMpeg 中的 dts 和 pts</h2><p>FFmpeg 里有两种时间戳：DTS（Decoding Time Stamp）和 PTS（Presentation Time Stamp）。前者是解码的时间，后者是显示的时间。要仔细理解这两个概念，需要先了解 FFmpeg 中的 packet 和 frame 的概念。</p>
<p>FFmpeg 中用 AVPacket 结构体来描述解码前或编码后的压缩包，用 AVFrame 结构体来描述解码后或编码前的信号帧。 对于视频来说，AVFrame 就是视频的一帧图像。这帧图像什么时候显示给用户，就取决于它的 PTS。DTS 是 AVPacket 里的一个成员，表示这个压缩包应该什么时候被解码。 如果视频里各帧的编码是按输入顺序（也就是显示顺序）依次进行的，那么解码和显示时间应该是一致的。可事实上，在大多数编解码标准（如 H.264 或 HEVC）中，编码顺序和输入顺序并不一致。 于是才会需要 PTS 和 DTS 这两种不同的时间戳。</p>
<p>具体到代码中为</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line">AVPacket *packet = av_packet_alloc();</span><br><span class="line">av_init_packet(packet);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// packet->dts == AV_NOPTS_VALUE</span></span><br><span class="line"><span class="keyword">while</span> (av_read_frame(format_context, packet) >= <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// do decoding ...</span></span><br><span class="line"> <span class="comment">// packet->dts != AV_NOPTS_VALUE here</span></span><br><span class="line"></span><br><span class="line"> result = avcodec_send_packet(video_codec_context, packet);</span><br><span class="line"> <span class="keyword">if</span> (result < <span class="number">0</span> && result != AVERROR(EAGAIN) && result != AVERROR_EOF) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : codec step 1 fail"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> result = avcodec_receive_frame(video_codec_context, frame);</span><br><span class="line"> <span class="keyword">if</span> (result < <span class="number">0</span> && result != AVERROR_EOF) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : codec step 2 fail"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> LOGI(<span class="string">"Frame %c (%d, size=%d) pts %d dts %d key_frame %d [codec_picture_number %d, display_picture_number %d]"</span>,</span><br><span class="line"> av_get_picture_type_char(frame->pict_type), video_codec_context->frame_number,</span><br><span class="line"> frame->pkt_size,</span><br><span class="line"> frame->pts,</span><br><span class="line"> frame->pkt_dts,</span><br><span class="line"> frame->key_frame, frame->coded_picture_number, frame->display_picture_number);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>那么为什么要区分 DTS 和 PTS 呢？这里就涉及到编解码标准中编码顺序和输入顺序不一致的问题，这里以 H.264 规范为例举一个例子</p>
<p>假设有一个解码帧序列为<code>IBP</code>，当解码器对其解码时，解码第一帧肯定是序列中的第一帧 I, 但我们思考一下第二帧，已知 B 帧的解码需要参考前一个 I 帧或 P 帧以及后面一个 P 帧，那么解码的第二帧显然不能是序列中的第二帧 B 帧，而是第三帧 P 帧，因此我们得到了下面的一个 PTS 和 DTS 关系</p>
<pre><code>PTS: 1 2 3
DTS: 1 3 2
</code></pre><p>这里我们就会发现，DTS 和 PTS 是不同的</p>
<p>DTS 主要用于视频的解码,在解码阶段使用.PTS 主要用于视频的同步和输出.在 display 的时候使用.在没有 B frame 的情况下.DTS 和 PTS 的输出顺序是一样的.</p>
<h2 id="Timebase"><a href="#Timebase" class="headerlink" title="Timebase"></a>Timebase</h2><p>在 FFMpeg 中，同时还引入了 timebase 这个概念。timebase 用来度量时间尺度，假设 timebase={1, 25}, 那么意味着时间尺度就是 1/25 秒，假设 pts=20，那么在 timebase={1, 25}的情况下。这一帧的时间为<code>(20 * 1/25)s</code></p>
<p>FFMpeg 中，不同的数据状态对应的 timebase 也不一致。例如，非压缩时的数据(即 YUV 或者其它)，在 ffmpeg 中对应的结构体为 AVFrame,它的 timebase 为 AVCodecContext 的 time_base ,AVRational{1,25}。<br>压缩后的数据(对应的结构体为 AVPacket)对应的 timebase 为 AVStream 的 time_base，AVRational{1,90000}。<br>因为数据状态不同，timebase 不一样，所以我们必须转换，在 1/25 时间刻度下佔 10 格，在 1/90000 下是佔多少格。这就是 pts 的转换。</p>
<h2 id="利用-pts-实现音视频同步"><a href="#利用-pts-实现音视频同步" class="headerlink" title="利用 pts 实现音视频同步"></a>利用 pts 实现音视频同步</h2><p>当我们得到 timebase 和 pts 数据后，我们就可以通过时间换算来同步视频和音频的播放，考虑到音频的播放速度固定，最简单的做法就是将视频的播放向音频同步。</p>
<p>我们需要定义一个<code>audio_clock</code>来记录音频播放的时钟</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (index == player->audio_stream_index) {</span><br><span class="line"> player->audio_clock = packet->pts * av_q2d(stream->time_base);</span><br><span class="line"> LOGD(<span class="string">"SyncPlayer: Playing audio loop"</span>);</span><br><span class="line"> audio_play(player, frame, env);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>然后在视频播放的时候利用这个<code>audio_clock</code>进行一定的 delay</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (index == player->video_stream_index) {</span><br><span class="line"> <span class="keyword">auto</span> audio_clock = player->audio_clock;</span><br><span class="line"> <span class="keyword">double</span> timestamp;</span><br><span class="line"> <span class="keyword">if</span> (packet->pts == AV_NOPTS_VALUE) {</span><br><span class="line"> timestamp = <span class="number">0</span>;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> timestamp = frame->best_effort_timestamp * av_q2d(stream->time_base);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">double</span> frame_rate = av_q2d(stream->avg_frame_rate); <span class="comment">// fps = 1 / stream->avg_frame_rate</span></span><br><span class="line"> frame_rate += frame->repeat_pict * (frame_rate * <span class="number">0.5</span>); <span class="comment">// repeat_dict代表当前frame必须delay的时间, extra_delay = repeat_pict / (2*fps)</span></span><br><span class="line"> <span class="keyword">if</span> (timestamp == <span class="number">0.0</span>) {</span><br><span class="line"> usleep(frame_rate * <span class="number">1000</span>);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">fabs</span>(timestamp - audio_clock) > AV_SYNC_THRESHOLD_MIN</span><br><span class="line"> && <span class="built_in">fabs</span>(timestamp - audio_clock) < AV_NOSYNC_THRESHOLD) {</span><br><span class="line"> <span class="keyword">if</span> (timestamp > audio_clock) {</span><br><span class="line"> usleep((<span class="keyword">unsigned</span> <span class="keyword">long</span>)((timestamp - audio_clock)*<span class="number">1000000</span>));</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> video_play(player, frame, env);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>至此，一个简单地音视频播放器就完成了.</p>
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<article id="post-Kotlin协程异常处理" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/09/12/Kotlin%E5%8D%8F%E7%A8%8B%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86/" class="article-date">
<time datetime="2020-09-12T01:09:48.000Z" itemprop="datePublished">2020-09-12</time>
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<a class="article-title" href="/2020/09/12/Kotlin%E5%8D%8F%E7%A8%8B%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86/">Kotlin协程异常处理</a>
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<p>Kotlin的协程满足结构化语义：</p>
<ol>
<li><p>A parent-Coroutine finishes only after all its child-Coroutines have finished.</p>
</li>
<li><p>When a parent-Coroutine or scope finishes abnormally, either through cancelation or through an exception, all its child-Coroutines, that are still active, are canceled and new child-Coroutines can no longer be launched.</p>
</li>
<li><p>When a child-Coroutine finishes abnormally, its parent-Coroutine or scope finishes abnormally.</p>
</li>
</ol>
<p>针对第三点，无论是<code>CoroutineScope.async</code>或<code>CoroutineScope.launch</code>，以异常结束，只要是在一个scope中，他的父协程也会以异常结束。<code>async</code>和<code>launch</code>唯一不同的地方在于<code>async</code>需要调用await才会执行协程内容。即使我们对子协程进行了try-catch处理异常，父协程仍旧会拿着这个异常作为结果结束</p>
<h2 id="async的异常处理"><a href="#async的异常处理" class="headerlink" title="async的异常处理"></a>async的异常处理</h2><pre><code>不管是哪个启动器(launch, async等)，在应用了作用域之后，都会按照**作用域的语义**进行异常扩散，进而触发相应的取消操作，对于 async 来说就算不调用 await 来获取这个异常，它也会在 coroutineScope 当中触发父协程的取消逻辑，这一点请大家注意。
</code></pre><p>以CoroutineScope(Dispatchers.Main)作为根作用域为例，下面展示几个case</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>上面的代码即使没有调用<code>foo.await()</code>, 也会扩散到<code>Thread.UncaughtExceptionHandler</code>中。这段代码的执行结果是</p>
<pre><code>D/Foo: [, , 0]:test
</code></pre><p>同时app crash</p>
<p>这里我们应该这么理解，虽然是对<code>await()</code>进行了try-catch，但这里是对执行结果的<code>try-catch</code>, 这不影响协程自己的异常传递规则，在async中的协程scope抛出异常后，此时异常是未捕获状态；因此会向父协程scope<code>coroutineScope</code>转播, <code>coroutineScope</code>继续向<code>viewModelScope</code>传播，最终来到UncaughtExceptionHandler处处理</p>
<p>那么考虑一下直接加入<code>try-catch</code>:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> foo.await()</span><br><span class="line"> } <span class="keyword">catch</span> (e : IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"CoroutineScope caught exception: <span class="variable">$e</span>"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>输出结果为:</p>
<pre><code>E/Foo: [, , 0]:CoroutineScope caught exception: java.lang.IllegalStateException: test
D/Foo: [, , 0]:test
</code></pre><p>同时app crash</p>
<p>这里虽然我们对await进行了try-catch，打印了异常信息，但是根据作用域规则，这里我们的异常行为发生了扩散，从子协程扩散到根协程，最终扩散到<code>Thread.UncaughtExceptionHandler</code>中，对于安卓系统而言，就是引发了crash</p>
<p>那么如果我们在扩散的scope外层进行try-catch能否解决问题呢？尝试如下的代码:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> coroutineScope {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (e: IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"catch expection from outerScope: <span class="variable">$e</span>"</span>)</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>输出结果为:</p>
<pre><code>E/Foo: [, , 0]:CoroutineScope caught exception: java.lang.IllegalStateException: test
D/Foo: [, , 0]:test
</code></pre><p>此时app 不再crash</p>
<p>如果我们在添加await()同时进行try-catch:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> coroutineScope {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> foo.await()</span><br><span class="line"> } <span class="keyword">catch</span> (e: IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"catch expection from innerScope: <span class="variable">$e</span>"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (e: IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"catch expection from outerScope: <span class="variable">$e</span>"</span>)</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>输出结果为:</p>
<pre><code>E/Foo: [, , 0]:catch expection from innerScope: java.lang.IllegalStateException: test
D/Foo: [, , 0]:test
E/Foo: [, , 0]:catch expection from outerScope: java.lang.IllegalStateException: test
</code></pre><p>此时app 也不再crash，但我们会发现我们的两处<code>try-catch</code>均被触发</p>
<p>这也说明<strong>在协程中，异常的扩散并不遵循try-catch语法构成的作用域</strong></p>
<p>另一种方式就是换用一个不进行扩散语义的协程作用域，即使用supervisorScope</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> supervisorScope {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>这段代码的输出结果为</p>
<pre><code>D/Foo: [, , 0]:test
</code></pre><p>app 不发生crash, 同时没有IllegalStateException(“test”)的日志打印</p>
<p>这里的核心在于，我们的async是在<code>supervisorScope</code>作用域下，根据文档描述，这个作用域下的协程出现取消情况（异常抛出时）不会向外扩散</p>
<p>假如加上try-catch</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> supervisorScope {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> foo.await()</span><br><span class="line"> } <span class="keyword">catch</span> (e: IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"CoroutineScope caught exception: <span class="variable">$e</span>"</span>)</span><br><span class="line"> }</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"test"</span>)</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>我们得到结果</p>
<pre><code>E/Foo: [, , 0]:CoroutineScope caught exception: java.lang.IllegalStateException: test
D/Foo: [, , 0]:test
</code></pre><p>app 不发生crash</p>
<p>同理，我们的async是在<code>supervisorScope</code>作用域下，根据文档描述，这个作用域下的协程出现取消情况（异常抛出时）不会向外扩散，因此不会向外扩散到<code>Thread.UncaughtExceptionHandler</code>。同时，try-catch对await()发生作用，我们打印出了异常的信息</p>
<h2 id="withContext的异常处理"><a href="#withContext的异常处理" class="headerlink" title="withContext的异常处理"></a>withContext的异常处理</h2><p>那么对比一下withContext的处理我们就会发现不同</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">viewModelScope.launch {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> withContext(Dispatchers.Default) {</span><br><span class="line"> <span class="keyword">throw</span> IllegalArgumentException(<span class="string">"Test"</span>)</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (e: Exception) {</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>这里<code>withContext</code>直接就是一个协程scope，我们的try-catch直接作用于整个withContext构造的scope，因此异常被捕获的同时，不再向<code>viewModelScope</code>传播</p>
<h2 id="supervisorScope"><a href="#supervisorScope" class="headerlink" title="supervisorScope"></a>supervisorScope</h2><p>如何能够让子协程抛出异常的情况下，父协程不会终止所以其子协程和自己呢？这就引入了supervisorScope</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">val</span> result = supervisorScope {</span><br><span class="line"> ...</span><br><span class="line"> <span class="keyword">val</span> supervisedChild1 = <span class="keyword">this</span>.launch { ... }</span><br><span class="line"> ...</span><br><span class="line"> <span class="keyword">val</span> supervisedChild2 = <span class="keyword">this</span>.async { ... }</span><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>此时，若child1或child2任意一个抛出异常，也不会使另一个child和<code>supervising parent</code>停止</p>
<h2 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h2><p>最终我们得到了协程的Structured Concurrency含义:</p>
<ol>
<li><p>A parent-Coroutine finishes only after all its child-Coroutines have finished.</p>
</li>
<li><p>When a parent-Coroutine or scope finishes abnormally, either through cancelation or through an exception, all its child-Coroutines, that are still active, are canceled and new child-Coroutines can no longer be launched.</p>
</li>
<li><p>When a child-Coroutine finishes abnormally, its parent-Coroutine or scope (a) finishes abnormally if the parent is not a supervisor or (b) keeps running if the parent is a supervisor.</p>
</li>
</ol>
<h2 id="P-S-协程中的异常最佳实践"><a href="#P-S-协程中的异常最佳实践" class="headerlink" title="P.S 协程中的异常最佳实践"></a>P.S 协程中的异常最佳实践</h2><p>通过上文中对await，withContext的异常处理的方式，我们会发现在协程中处理异常其实是一件非常麻烦的事情，其异常的扩散规则并不合<code>try-catch</code>的作用域对应。因此这里简单提一下如何在实际项目中处理这种异常</p>
<p>Kotlin中对异常处理有两种推荐方式:</p>
<ol>
<li>default value</li>
<li>Wrapper Seal class</li>
</ol>
<p>第一种方式就是返回一个跟函数签名同类型的默认值，例如</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">toIntSafely</span><span class="params">(defaultValue: <span class="type">Int</span>)</span></span> : <span class="built_in">Int</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">try</span> {</span><br><span class="line"> parseInt(<span class="keyword">this</span>)</span><br><span class="line"> } <span class="keyword">catch</span> (e: NumberFormatException) {</span><br><span class="line"> <span class="keyword">return</span> defaultValue</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>第二种方式则是在API设计上使用一个Wrapper Class来包装出现的异常，使用这个Wrapper Class来保证Type-Check，举例来说:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">sealed</span> <span class="class"><span class="keyword">class</span> <span class="title">ParsedDate</span> </span>{</span><br><span class="line"> <span class="keyword">data</span> <span class="class"><span class="keyword">class</span> <span class="title">Success</span></span>(<span class="keyword">val</span> date: Date) : ParsedDate()</span><br><span class="line"> <span class="keyword">data</span> <span class="class"><span class="keyword">class</span> <span class="title">Failure</span></span>(<span class="keyword">val</span> errorOffset: <span class="built_in">Int</span>) : ParsedDate()</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">fun</span> DateFormat.<span class="title">tryParse</span><span class="params">(text: <span class="type">String</span>)</span></span>: ParsedDate =</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> ParsedDate.Success(parse(text))</span><br><span class="line"> } <span class="keyword">catch</span> (e: ParseException) {</span><br><span class="line"> ParsedDate.Failure(e.errorOffset)</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>那么对调用者而言,<code>tryParse</code>的结果一定是一个<code>ParsedDate</code>对象，而对异常无感知</p>
<p>结合协程使用来说，我们应该直接在一个作用域内，就返回一个default value或Wrapper Seal class，例如</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">sealed</span> <span class="class"><span class="keyword">class</span> <span class="title">ParsedDate</span> </span>{</span><br><span class="line"> <span class="keyword">data</span> <span class="class"><span class="keyword">class</span> <span class="title">Success</span></span>(<span class="keyword">val</span> date: Date) : ParsedDate()</span><br><span class="line"> <span class="keyword">data</span> <span class="class"><span class="keyword">class</span> <span class="title">Failure</span></span>(<span class="keyword">val</span> errorOffset: <span class="built_in">Int</span>) : ParsedDate()</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">CoroutineScope(Dispatchers.Main).launch {</span><br><span class="line"> <span class="keyword">val</span> foo = async {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">throw</span> IllegalStateException(<span class="string">"test"</span>)</span><br><span class="line"> } <span class="keyword">catch</span> (e: IllegalStateException) {</span><br><span class="line"> Log.e(<span class="string">"Foo"</span>, <span class="string">"catch exception inside: <span class="variable">$e</span>"</span>)</span><br><span class="line"> ParsedDate.Failure(-<span class="number">1</span>)</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">val</span> rst = foo.await()</span><br><span class="line"> Log.d(<span class="string">"Foo"</span>, <span class="string">"return rst: <span class="subst">${rst.errorOffset}</span>"</span>)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>输出结果:</p>
<pre><code>E/Foo: [, , 0]:catch exception inside: java.lang.IllegalStateException: test
D/Foo: [, , 0]:return rst: -1
</code></pre><p>同时app无crash，无异常日志打印</p>
<p>ref: </p>
<p><a target="_blank" rel="noopener" href="https://www.bennyhuo.com/2019/04/23/coroutine-exceptions/#4-%E5%BC%82%E5%B8%B8%E4%BC%A0%E6%92%AD">https://www.bennyhuo.com/2019/04/23/coroutine-exceptions/#4-%E5%BC%82%E5%B8%B8%E4%BC%A0%E6%92%AD</a></p>
<p><a target="_blank" rel="noopener" href="https://johnnyshieh.me/posts/kotlin-coroutine-exception-handling/">https://johnnyshieh.me/posts/kotlin-coroutine-exception-handling/</a></p>
<p><a target="_blank" rel="noopener" href="https://medium.com/@elizarov/structured-concurrency-722d765aa952">https://medium.com/@elizarov/structured-concurrency-722d765aa952</a></p>
<p><a target="_blank" rel="noopener" href="https://medium.com/the-kotlin-chronicle/coroutine-exceptions-supervision-15056802998b">https://medium.com/the-kotlin-chronicle/coroutine-exceptions-supervision-15056802998b</a></p>
<p><a target="_blank" rel="noopener" href="https://medium.com/the-kotlin-chronicle/coroutine-exceptions-3378f51a7d33">https://medium.com/the-kotlin-chronicle/coroutine-exceptions-3378f51a7d33</a></p>
<p><a target="_blank" rel="noopener" href="https://github.com/Kotlin/kotlinx.coroutines/issues/753">https://github.com/Kotlin/kotlinx.coroutines/issues/753</a></p>
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<article id="post-FFMpeg-Android开发-简单播放音频" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/08/29/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E6%92%AD%E6%94%BE%E9%9F%B3%E9%A2%91/" class="article-date">
<time datetime="2020-08-29T01:04:55.000Z" itemprop="datePublished">2020-08-29</time>
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<a class="article-title" href="/2020/08/29/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E6%92%AD%E6%94%BE%E9%9F%B3%E9%A2%91/">FFMpeg-Android开发-简单播放音频</a>
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<p>本文部分对应的源代码可参考<a href="https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.2" target="_blank" rel="noopener">https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.2</a></p>
<h2 id="具体实现"><a href="#具体实现" class="headerlink" title="具体实现"></a>具体实现</h2><p>音频播放的整个流程和视频非常相似，都经历了下面几个步骤</p>
<ol>
<li>解析 container</li>
<li>根据 container，获得我们关心的媒体数据（例如播放视频，那我们只关心视频媒体）</li>
<li>根据媒体信息获得对应的解码器</li>
<li>将对应的数据送给解码器</li>
<li>解码器解码，输出帧</li>
<li>将帧渲染到目标区域(播放音频)</li>
</ol>
<p>1-5 的步骤几乎可以参考播放视频的流程，这里简单看下不同</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">auto</span> swr_context = swr_alloc();</span><br><span class="line"><span class="keyword">auto</span> out_buffer = (<span class="keyword">uint8_t</span> *) av_malloc(<span class="number">44100</span> * <span class="number">2</span>);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// expected sample output</span></span><br><span class="line"><span class="keyword">uint64_t</span> out_channel_layout = AV_CH_LAYOUT_STEREO;</span><br><span class="line"><span class="keyword">auto</span> out_format = AV_SAMPLE_FMT_S16;</span><br><span class="line"></span><br><span class="line"><span class="keyword">auto</span> out_sample_rate = audio_codec_context->sample_rate;</span><br><span class="line"></span><br><span class="line"><span class="comment">// expected sample out para end</span></span><br><span class="line"></span><br><span class="line">swr_alloc_set_opts(swr_context,</span><br><span class="line"> out_channel_layout, out_format, out_sample_rate,</span><br><span class="line"> audio_codec_context->channel_layout, audio_codec_context->sample_fmt, audio_codec_context->sample_rate,</span><br><span class="line"><span class="number">0</span>, <span class="literal">nullptr</span>);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">swr_init(swr_context);</span><br><span class="line"></span><br><span class="line"><span class="keyword">auto</span> out_channels = av_get_channel_layout_nb_channels(AV_CH_LAYOUT_STEREO);</span><br></pre></td></tr></table></figure>
<p>这一段的主要目的是配置音频的转码格式，跟视频不同，但基本逻辑一致</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">auto</span> player_class = env->GetObjectClass(instance);</span><br><span class="line"><span class="keyword">auto</span> create_audio_track_method_id = env->GetMethodID(player_class, <span class="string">"createAudioTrack"</span>, <span class="string">"(II)V"</span>);</span><br><span class="line">env->CallVoidMethod(instance, create_audio_track_method_id, <span class="number">44100</span>, out_channels);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">auto</span> play_audio_track_method_id = env->GetMethodID(player_class, <span class="string">"playAudioTrack"</span>, <span class="string">"([BI)V"</span>);</span><br></pre></td></tr></table></figure>
<p>这里我们通过 JNI 来调用 Java 层的 AudioTrack 相关 API 来播放音频</p>
<p>至此，我们已经完成了音频的播放；接下来就剩下如何同时播放视频和音频，以及音视频同步问题了。下篇文章，我们就会来着手实现一个时间同步的简单播放器</p>
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<article id="post-FFMpeg-Android开发-简单播放视频" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/08/23/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E6%92%AD%E6%94%BE%E8%A7%86%E9%A2%91/" class="article-date">
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<a class="article-title" href="/2020/08/23/FFMpeg-Android%E5%BC%80%E5%8F%91-%E7%AE%80%E5%8D%95%E6%92%AD%E6%94%BE%E8%A7%86%E9%A2%91/">FFMpeg-Android开发-简单播放视频</a>
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<p>本文部分对应的源代码可参考<a href="https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.1" target="_blank" rel="noopener">https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.1</a></p>
<h2 id="具体实现"><a href="#具体实现" class="headerlink" title="具体实现"></a>具体实现</h2><p>结合上一篇文章<code>FFMpeg-Android开发-解析视频格式</code>的内容，我们知道视频解码大致分为如下几个步骤</p>
<ol>
<li>解析 container</li>
<li>根据 container，获得我们关心的媒体数据（例如播放视频，那我们只关心视频媒体）</li>
<li>根据媒体信息获得对应的解码器</li>
<li>将对应的数据送给解码器</li>
<li>解码器解码，输出帧</li>
<li>将帧渲染到目标区域</li>
</ol>
<p>那么这次我们就来尝试将视频解码并渲染到设备屏幕上，首先看我们 1-3 步的代码</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="comment">// Record result</span></span><br><span class="line"><span class="keyword">int</span> result;</span><br><span class="line"><span class="comment">// R1 Java String -> C String</span></span><br><span class="line"><span class="keyword">const</span> <span class="keyword">char</span> *path = env->GetStringUTFChars(path_, <span class="number">0</span>);</span><br><span class="line"><span class="comment">// Register FFmpeg components</span></span><br><span class="line">av_register_all();</span><br><span class="line"><span class="comment">// R2 initializes the AVFormatContext context</span></span><br><span class="line">AVFormatContext *format_context = avformat_alloc_context();</span><br><span class="line"><span class="comment">// Open Video File</span></span><br><span class="line">result = avformat_open_input(&format_context, path, <span class="literal">NULL</span>, <span class="literal">NULL</span>);</span><br><span class="line"><span class="keyword">if</span> (result < <span class="number">0</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not open video file"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Finding Stream Information of Video Files</span></span><br><span class="line">result = avformat_find_stream_info(format_context, <span class="literal">NULL</span>);</span><br><span class="line"><span class="keyword">if</span> (result < <span class="number">0</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not find video file stream info"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Find Video Encoder</span></span><br><span class="line"><span class="keyword">int</span> video_stream_index = <span class="number">-1</span>;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < format_context->nb_streams; i++) {</span><br><span class="line"> <span class="comment">// Matching Video Stream</span></span><br><span class="line"> <span class="keyword">if</span> (format_context->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {</span><br><span class="line"> video_stream_index = i;</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// No video stream found</span></span><br><span class="line"><span class="keyword">if</span> (video_stream_index == <span class="number">-1</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not find video stream"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Initialization of Video Encoder Context</span></span><br><span class="line">AVCodecContext *video_codec_context = avcodec_alloc_context3(<span class="literal">NULL</span>);</span><br><span class="line">avcodec_parameters_to_context(video_codec_context, format_context->streams[video_stream_index]->codecpar);</span><br><span class="line"><span class="comment">// Initialization of Video Encoder</span></span><br><span class="line">AVCodec *video_codec = avcodec_find_decoder(video_codec_context->codec_id);</span><br><span class="line"><span class="keyword">if</span> (video_codec == <span class="literal">NULL</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not find video codec"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// R3 Opens Video Decoder</span></span><br><span class="line">result = avcodec_open2(video_codec_context, video_codec, <span class="literal">NULL</span>);</span><br><span class="line"><span class="keyword">if</span> (result < <span class="number">0</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not find video stream"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Getting the Width and Height of Video</span></span><br><span class="line"><span class="keyword">int</span> videoWidth = video_codec_context->width;</span><br><span class="line"><span class="keyword">int</span> videoHeight = video_codec_context->height;</span><br></pre></td></tr></table></figure>
<p>其中 video_codec 代表解码器对象，其他对象在上一篇文章和代码注释中有相关解释。</p>
<p>接下来的步骤就是解码和上屏，这里我们先看下我们的上屏是如何实现的；这里为了方便，我直接使用 SurfaceView 构建我们的上层 View</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="function"><span class="keyword">fun</span> <span class="title">onCreate</span><span class="params">(savedInstanceState: <span class="type">Bundle</span>?)</span></span> {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> surfaceHolder = surfaceView.holder</span><br><span class="line"> surfaceHolder!!.setFormat(PixelFormat.RGBA_8888)</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">private</span> <span class="function"><span class="keyword">fun</span> <span class="title">playVideo</span><span class="params">(path: <span class="type">String</span>)</span></span> {</span><br><span class="line"> Thread {</span><br><span class="line"> mPlayer.playVideo(path, surfaceHolder!!.surface)</span><br><span class="line"> }.start()</span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// R4 Initializes Native Window s for Playing Videos</span></span><br><span class="line">ANativeWindow *native_window = ANativeWindow_fromSurface(env, surface); <span class="comment">// surface对应java层的surface对象</span></span><br><span class="line"><span class="keyword">if</span> (native_window == <span class="literal">NULL</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not create native window"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Limit the number of pixels in the buffer by setting the width, not the physical display size of the screen.</span></span><br><span class="line"><span class="comment">// If the buffer does not match the display size of the physical screen, the actual display may be stretched or compressed images.</span></span><br><span class="line">result = ANativeWindow_setBuffersGeometry(native_window, videoWidth, videoHeight,WINDOW_FORMAT_RGBA_8888);</span><br><span class="line"><span class="keyword">if</span> (result < <span class="number">0</span>){</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not set native window buffer"</span>);</span><br><span class="line"> ANativeWindow_release(native_window);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// Define drawing buffer</span></span><br><span class="line">ANativeWindow_Buffer window_buffer;</span><br><span class="line"><span class="comment">// There are three declarative data containers</span></span><br><span class="line"><span class="comment">// Data container Packet encoding data before R5 decoding</span></span><br><span class="line">AVPacket *packet = av_packet_alloc();</span><br><span class="line">av_init_packet(packet);</span><br><span class="line"><span class="comment">// Frame Pixel Data of Data Container After R6 Decoding Can't Play Pixel Data Directly and Need Conversion</span></span><br><span class="line">AVFrame *frame = av_frame_alloc();</span><br><span class="line"><span class="comment">// R7 converted data container where the data can be used for playback</span></span><br><span class="line">AVFrame *rgba_frame = av_frame_alloc();</span><br><span class="line"><span class="comment">// Data format conversion preparation</span></span><br><span class="line"><span class="comment">// Output Buffer</span></span><br><span class="line"><span class="keyword">int</span> buffer_size = av_image_get_buffer_size(AV_PIX_FMT_RGBA, videoWidth, videoHeight, <span class="number">1</span>);</span><br><span class="line"><span class="comment">// R8 Application for Buffer Memory</span></span><br><span class="line"><span class="keyword">uint8_t</span> *out_buffer = (<span class="keyword">uint8_t</span> *) av_malloc(buffer_size * <span class="keyword">sizeof</span>(<span class="keyword">uint8_t</span>));</span><br><span class="line">LOGI(<span class="string">"outBuffer size: %d, videoWidth: %d, videoHeight: %d, pix_fmt: %d"</span>, buffer_size * <span class="keyword">sizeof</span>(<span class="keyword">uint8_t</span>), videoWidth, videoHeight, video_codec_context->pix_fmt);</span><br><span class="line">av_image_fill_arrays(rgba_frame->data, rgba_frame->linesize, out_buffer, AV_PIX_FMT_RGBA, videoWidth, videoHeight, <span class="number">1</span>);</span><br><span class="line"><span class="comment">// R9 Data Format Conversion Context</span></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">SwsContext</span> *<span class="title">data_convert_context</span> = <span class="title">sws_getContext</span>(</span></span><br><span class="line"><span class="class"> <span class="title">videoWidth</span>, <span class="title">videoHeight</span>, <span class="title">video_codec_context</span>-><span class="title">pix_fmt</span>,</span></span><br><span class="line"><span class="class"> <span class="title">videoWidth</span>, <span class="title">videoHeight</span>, <span class="title">AV_PIX_FMT_RGBA</span>,</span></span><br><span class="line"><span class="class"> <span class="title">SWS_BICUBIC</span>, <span class="title">NULL</span>, <span class="title">NULL</span>, <span class="title">NULL</span>);</span></span><br><span class="line"><span class="comment">// Start reading frames</span></span><br></pre></td></tr></table></figure>
<p>这里 ANative 相关代码都是 Android NDK 中的 api，可参考文档理解；这里主要看一下这里的 AVPacket 和 AVFrame 的使用</p>
<p>AVPacket 在上一篇文章中有过介绍，它在 FFMpeg 中用来表示未解码时的数据；而 AVFrame 则表示了解码后的帧数据。</p>
<p>但这里有一个小问题是我们解码后的图像格式可能是和我们的 surface 的渲染格式不同，这里我们的 surface 渲染格式是<code>RGBA8888</code>，而视频的图像格式不一定为这个格式。为了实现转换，我们就需要<code>SwsContext</code>对象来帮助我们实现图像格式的转换。</p>
<p>看完上面后，终于来到了我们的解码 runloop</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">while</span> (av_read_frame(format_context, packet) >= <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// Matching Video Stream</span></span><br><span class="line"> <span class="keyword">if</span> (packet->stream_index == video_stream_index) {</span><br><span class="line"> <span class="comment">// Decode video</span></span><br><span class="line"> result = avcodec_send_packet(video_codec_context, packet);</span><br><span class="line"> <span class="keyword">if</span> (result < <span class="number">0</span> && result != AVERROR(EAGAIN) && result != AVERROR_EOF) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : codec step 1 fail"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> result = avcodec_receive_frame(video_codec_context, frame);</span><br><span class="line"> <span class="keyword">if</span> (result < <span class="number">0</span> && result != AVERROR_EOF) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : codec step 2 fail"</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> LOGI(<span class="string">"Frame %c (%d, size=%d) pts %d dts %d key_frame %d [codec_picture_number %d, display_picture_number %d]"</span>,</span><br><span class="line"> av_get_picture_type_char(frame->pict_type), video_codec_context->frame_number,</span><br><span class="line"> frame->pkt_size,</span><br><span class="line"> frame->pts,</span><br><span class="line"> frame->pkt_dts,</span><br><span class="line"> frame->key_frame, frame->coded_picture_number, frame->display_picture_number);</span><br><span class="line"> <span class="comment">// Data Format Conversion</span></span><br><span class="line"> result = sws_scale(</span><br><span class="line"> data_convert_context,</span><br><span class="line"> frame->data, frame->linesize,</span><br><span class="line"> <span class="number">0</span>, videoHeight,</span><br><span class="line"> rgba_frame->data, rgba_frame->linesize);</span><br><span class="line"> <span class="comment">// play</span></span><br><span class="line"> result = ANativeWindow_lock(native_window, &window_buffer, <span class="literal">NULL</span>);</span><br><span class="line"> <span class="keyword">if</span> (result < <span class="number">0</span>) {</span><br><span class="line"> LOGE(<span class="string">"Player Error : Can not lock native window"</span>);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// Draw the image onto the interface</span></span><br><span class="line"> <span class="comment">// Note: The pixel lengths of rgba_frame row and window_buffer row may not be the same here.</span></span><br><span class="line"> <span class="comment">// Need to convert well or maybe screen</span></span><br><span class="line"> <span class="keyword">uint8_t</span> *bits = (<span class="keyword">uint8_t</span> *) window_buffer.bits;</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> h = <span class="number">0</span>; h < videoHeight; h++) {</span><br><span class="line"> <span class="built_in">memcpy</span>(bits + h * window_buffer.stride * <span class="number">4</span>,</span><br><span class="line"> out_buffer + h * rgba_frame->linesize[<span class="number">0</span>],</span><br><span class="line"> rgba_frame->linesize[<span class="number">0</span>]);</span><br><span class="line"> }</span><br><span class="line"> ANativeWindow_unlockAndPost(native_window);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// Release package references</span></span><br><span class="line"> av_packet_unref(packet);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>整个解码循环也是大致分为如下几个步骤</p>
<ol>
<li>通过 AVPacket 读取一块数据</li>
<li>将 AVPacket 送给解码器使用</li>
<li>通过 AVFrame 得到 2 中的解码后数据</li>
<li>对 AVFrame 的图像格式进行转换</li>
<li>将 4 中的结果上屏</li>
</ol>
<p>其他的注释在代码中有说明</p>
<p>最后在 run-loop 外完成资源回收</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// Release R9</span></span><br><span class="line">sws_freeContext(data_convert_context);</span><br><span class="line"><span class="comment">// Release R8</span></span><br><span class="line">av_free(out_buffer);</span><br><span class="line"><span class="comment">// Release R7</span></span><br><span class="line">av_frame_free(&rgba_frame);</span><br><span class="line"><span class="comment">// Release R6</span></span><br><span class="line">av_frame_free(&frame);</span><br><span class="line"><span class="comment">// Release R5</span></span><br><span class="line">av_packet_free(&packet);</span><br><span class="line"><span class="comment">// Release R4</span></span><br><span class="line">ANativeWindow_release(native_window);</span><br><span class="line"><span class="comment">// Close R3</span></span><br><span class="line">avcodec_close(video_codec_context);</span><br><span class="line"><span class="comment">// Release R2</span></span><br><span class="line">avformat_close_input(&format_context);</span><br><span class="line"><span class="comment">// Release R1</span></span><br><span class="line">env->ReleaseStringUTFChars(path_, path);</span><br></pre></td></tr></table></figure>
<p>此时，我们就完成了整个视频的播放，不过这个播放器显然是处在不可用的状态，主要有下面两个问题</p>
<ol>
<li>没有声音</li>
<li>视频的播放是按照解码器的解码速度播放的；只要解码器足够快，视频播放就有多快，这个显然是不符合播放器播放视频的预期的</li>
</ol>
<p>问题 2 本质就是我们常说的音视频同步问题；我们后面会简单介绍音频的解码播放后，通过引入 pts, dts 等概念后，尝试解决问题 2</p>
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<article id="post-FFMpeg-Android开发-解析视频格式" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/08/22/FFMpeg-Android%E5%BC%80%E5%8F%91-%E8%A7%A3%E6%9E%90%E8%A7%86%E9%A2%91%E6%A0%BC%E5%BC%8F/" class="article-date">
<time datetime="2020-08-22T01:07:51.000Z" itemprop="datePublished">2020-08-22</time>
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<a class="article-title" href="/2020/08/22/FFMpeg-Android%E5%BC%80%E5%8F%91-%E8%A7%A3%E6%9E%90%E8%A7%86%E9%A2%91%E6%A0%BC%E5%BC%8F/">FFMpeg-Android开发-解析视频格式</a>
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<p>在正式使用FFMpeg完成视频的播放之前，我想先写一篇文章，简单介绍一下如何使用FFMpeg去获取一个视频的基本信息。通过这篇文章，来简单介绍一下FFmpeg中的相关概念，以及视频的一些基本概念。</p>
<h2 id="视频基本概念"><a href="#视频基本概念" class="headerlink" title="视频基本概念"></a>视频基本概念</h2><p>当我们提及视频格式的时候，实际上对应的是视频的container(又或者称为format)这个概念，这个概念往往对应mpeg4，mkv, webm等。一个container可以是由多个编码器压缩后的媒体的集合，例如，一个mp4，可以包含视频，音频，字幕等不同媒体</p>
<p>编码器codec，则是按照某种编解码标准下的具体实现，这一层对应的概念往往是av1, h264, vp9等</p>
<p>因此，一个视频文件，从读取IO到播放大致可以分为以下几步：</p>
<ol>
<li>解析container</li>
<li>根据container，获得我们关心的媒体数据（例如播放视频，那我们只关心视频媒体）</li>
<li>根据媒体信息获得对应的解码器</li>
<li>将对应的数据送给解码器</li>
<li>解码器解码，输出帧</li>
<li>将帧渲染到目标区域</li>
</ol>
<h2 id="FFmpeg中对上述概念的定义"><a href="#FFmpeg中对上述概念的定义" class="headerlink" title="FFmpeg中对上述概念的定义"></a>FFmpeg中对上述概念的定义</h2><p>那么在FFmpeg中，是如何定义上述概念的呢？</p>
<p>对于container，FFMpeg使用<code>AVFormatContext</code><br>对于具体的媒体数据，通过遍历<code>AVFormatContext->streams</code>，我们能通过一个<code>AVStream</code>对象表示</p>
<p><code>AVStream</code>中的压缩分片用<code>AVPacket</code>表示，通过解码器解码后我们就能得到帧数据，用<code>AVFrame</code>表示</p>
<p>这篇文章就先简单介绍下1，2，3步的实现</p>
<h2 id="使用FFMpeg解析container，获取基本信息"><a href="#使用FFMpeg解析container，获取基本信息" class="headerlink" title="使用FFMpeg解析container，获取基本信息"></a>使用FFMpeg解析container，获取基本信息</h2><p>代码如下</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">AVFormatContext *avFormatContext = <span class="literal">nullptr</span>;</span><br><span class="line">LOGI(<span class="string">"video_config_create, open file uri: %s"</span>, uri);</span><br><span class="line"><span class="keyword">if</span> (avformat_open_input(&avFormatContext, uri, <span class="literal">nullptr</span>, <span class="literal">nullptr</span>)) { <span class="comment">// open IO, 如果成功, avFormatContext将有值</span></span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nullptr</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> (avformat_find_stream_info(avFormatContext, <span class="literal">nullptr</span>) < <span class="number">0</span>) {</span><br><span class="line"> avformat_free_context(avFormatContext);</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nullptr</span>;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">int</span> pos = <span class="number">0</span>; pos < avFormatContext->nb_streams; pos++) {</span><br><span class="line"> <span class="comment">// Getting the name of a codec of the very first video stream</span></span><br><span class="line"> AVCodecParameters *parameters = avFormatContext->streams[pos]->codecpar;</span><br><span class="line"> <span class="keyword">if</span> (parameters->codec_type == AVMEDIA_TYPE_VIDEO) {</span><br><span class="line"> videoConfig->parameters = parameters;</span><br><span class="line"> videoConfig->avVideoCodec = avcodec_find_decoder(parameters->codec_id);</span><br><span class="line"> videoConfig->videoStreamIndex = pos;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>代码的基本逻辑，和上述介绍的步骤，是一致的；其中，视频的文件格式信息，显然就在<code>AVFormatContext</code>中，视频的宽高信息，就在<code>AVCodecParameters</code>中，这个对象顾名思义，是跟随codec的；而解码器的相关信息，就在<code>avcodec_find_decoder</code>的返回值中</p>
<p>本文对应的源代码请参考<a href="https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.0" target="_blank" rel="noopener">https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.0</a></p>
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<article id="post-FFMpeg-Android开发101-编译和引入" class="article article-type-post" itemscope itemprop="blogPost">
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<a href="/2020/08/15/FFMpeg-Android%E5%BC%80%E5%8F%91101-%E7%BC%96%E8%AF%91%E5%92%8C%E5%BC%95%E5%85%A5/" class="article-date">
<time datetime="2020-08-15T02:15:51.000Z" itemprop="datePublished">2020-08-15</time>
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<a class="article-title" href="/2020/08/15/FFMpeg-Android%E5%BC%80%E5%8F%91101-%E7%BC%96%E8%AF%91%E5%92%8C%E5%BC%95%E5%85%A5/">FFMpeg Android开发101-编译和引入</a>
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<p>本系列会尝试将FFMpeg引入到android项目中，并借助FFMpeg完成一些音视频的简单饮用；FFMpeg作为一个成熟的音视频编解码工具被大量项目使用，但将FFMpeg引入到Android开发的文档并不多，国内有一部分但大量已经过时，这个系列会重新尝试带领大家完成整个过程。</p>
<p>本文对应的源代码请参考<a href="https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.0" target="_blank" rel="noopener">https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.0</a></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">git clone https://github.com/TedaLIEz/MyFFMpegAndroid/tree/v1.0</span><br><span class="line"></span><br><span class="line">cd MyFFMpegAndroid</span><br><span class="line">git submodule update</span><br><span class="line"></span><br><span class="line">cd ffmpeg-android-maker</span><br><span class="line">export ANDROID_SDK_HOME=${YOUR_SDK_HOME}</span><br><span class="line">export ANDROID_NDK_HOME=${YOUR_NDK_HOME}</span><br><span class="line">./ffmpeg-android-maker.sh</span><br></pre></td></tr></table></figure>
<p>之后用Android Studio打开项目，即可运行</p>
<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>总的来说，将FFMpeg引入到Android项目开发分为下面几个步骤：</p>
<ol>
<li>使用Android NDK编译FFMpeg项目</li>
<li>在自己的项目部署FFMpeg</li>
<li>开始FFMpeg开发，验证效果</li>
</ol>
<h3 id="使用Android-NDK编译FFMpeg"><a href="#使用Android-NDK编译FFMpeg" class="headerlink" title="使用Android NDK编译FFMpeg"></a>使用Android NDK编译FFMpeg</h3><pre><code>TL;DR 参考https://github.com/Javernaut/ffmpeg-android-maker
</code></pre><p>首先需要简单了解些FFMpeg编译产物的一些职责:</p>
<p><code>libavformat</code>: 处理文件container, stream<br><code>libavcodec</code>: 编解码<br><code>libswscale</code>: 图像处理<br><code>libavutil</code>: util库</p>
<p>还有一些其他的库，可以自行参考FFmpeg文档</p>
<p>考虑到我们是编译一个Android的FFMpeg库，本质上这就是一个交叉编译，因此我简单介绍一下FFmpeg编译到Android上的一些配置</p>
<h4 id="编译配置"><a href="#编译配置" class="headerlink" title="编译配置"></a>编译配置</h4><p>由于Android有着ARM, x86的32，64位处理器架构，因此我们需要编译4种二进制文件；这时需要一个交叉编译器(cross-compiler)来帮我们处理问题。</p>
<p>第二，编译过程不只是compile，还包括链接(linker)等其他工具, 我们统称为binutils</p>
<p>第三，我们需要Android系统自己的一些库和头文件，这些文件的存储位置我们成为sysroot</p>
<p>因此，整个编译需要的工具链包括<code>cross-compiler</code>, <code>binutils</code>, <code>sysroot</code></p>
<p>那么我们怎么获得这些工具呢？答案显然就是在Android NDK中，如果之前你有查阅过Android-NDK的目录，那么你就会发现NDK的结构其实已经说明了这三个工具的划分</p>
<p>这里我们看一下configure的相关参数</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">./configure \</span><br><span class="line">--prefix=<span class="variable">${BUILD_DIR}</span>/<span class="variable">${ABI}</span> \</span><br><span class="line">--<span class="built_in">enable</span>-cross-compile \</span><br><span class="line">--target-os=android \</span><br><span class="line">--arch=<span class="variable">${TARGET_TRIPLE_MACHINE_BINUTILS}</span> \</span><br><span class="line">--sysroot=<span class="variable">${SYSROOT}</span> \</span><br><span class="line">--cross-prefix=<span class="variable">${CROSS_PREFIX}</span> \</span><br><span class="line">--cc=<span class="variable">${CC}</span> \</span><br><span class="line">--extra-cflags=<span class="string">"-O3 -fPIC"</span> \</span><br><span class="line">--<span class="built_in">enable</span>-shared \</span><br><span class="line">--<span class="built_in">disable</span>-static \</span><br><span class="line"><span class="variable">${EXTRA_BUILD_CONFIGURATION_FLAGS}</span> \</span><br></pre></td></tr></table></figure>
<p><code>prefix</code>指明了产物的路径</p>
<p><code>target-os=android</code>: 指明我们的编译操作系统是Android</p>
<p><code>--arch=${TARGET_TRIPLE_MACHINE_BINUTILS}</code>: 指明arm, aarch64, i686 and x86_64</p>
<p><code>--sysroot=${SYSROOT}</code>: 指明sysroot的路径，这个路径一定是在Android NDK的路径下的一个子目录</p>
<p><code>--cross-prefix=${CROSS_PREFIX}</code>: 指明binutils的工具名前缀，这个名字会追加到ld前当作linker使用，例如</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">armeabi-v7a: $TOOLCHAIN_PATH/bin/arm-linux-androideabi-</span><br><span class="line">arm64-v8a: $TOOLCHAIN_PATH/bin/aarch64-linux-android-</span><br><span class="line">x86: $TOOLCHAIN_PATH/bin/i686-linux-android-</span><br><span class="line">x86_64: $TOOLCHAIN_PATH/bin/x86_64-linux-android-</span><br></pre></td></tr></table></figure>
<p><code>--cc=${CC}</code>: 指明编译器, 例如</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">armeabi-v7a: $TOOLCHAIN_PATH/bin/armv7a-linux-androideabi16-clang</span><br><span class="line">arm64-v8a: $TOOLCHAIN_PATH/bin/aarch64-linux-android21-clang</span><br><span class="line">x86: $TOOLCHAIN_PATH/bin/i686-linux-android16-clang</span><br><span class="line">x86_64: $TOOLCHAIN_PATH/bin/x86_64-linux-android21-clang</span><br></pre></td></tr></table></figure>
<p><code>--enable-shard</code>和<code>--disable-static</code>表明我们是编译一个动态库</p>
<p><code>--extra-cflags=”-O3 -fPIC”</code>表明了其他的C flag, <code>-O3</code>表明编译器优化级别, <code>-fPIC</code>是编译Android上的动态库必须的参数</p>
<h4 id="FFMpeg自身的相关配置"><a href="#FFMpeg自身的相关配置" class="headerlink" title="FFMpeg自身的相关配置"></a>FFMpeg自身的相关配置</h4><p>除去上面编译工具链需要的参数外，我们还可以定制化我们的FFMpeg编译，例如:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">--disable-runtime-cpudetect \</span><br><span class="line">--disable-programs \</span><br><span class="line">--disable-muxers \</span><br><span class="line">--disable-encoders \</span><br><span class="line">--disable-avdevice \</span><br><span class="line">--disable-postproc \</span><br><span class="line">--disable-swresample \</span><br><span class="line">--disable-avfilter \</span><br><span class="line">--disable-doc \</span><br><span class="line">--disable-debug \</span><br><span class="line">--disable-pthreads \</span><br><span class="line">--disable-network \</span><br><span class="line">--disable-bsfs \</span><br><span class="line">--disable-decoders \</span><br><span class="line">${DECODERS_TO_ENABLE}</span><br></pre></td></tr></table></figure>
<p>这里<code>--disable</code>-xxx表示不需要FFMpeg的具体模块，这个可以根据自身app的开发来定制</p>
<p>上述工作全部完成后，就可以开始make && make install了</p>
<h3 id="在Android项目中引入FFmpeg"><a href="#在Android项目中引入FFmpeg" class="headerlink" title="在Android项目中引入FFmpeg"></a>在Android项目中引入FFmpeg</h3><p>编译完成后，你应该能获得如下的编译结果</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">.</span><br><span class="line">├── include</span><br><span class="line">│ ├── arm64-v8a</span><br><span class="line">│ ├── armeabi-v7a</span><br><span class="line">│ ├── x86</span><br><span class="line">│ └── x86_64</span><br><span class="line">└── lib</span><br><span class="line"> ├── arm64-v8a</span><br><span class="line"> ├── armeabi-v7a</span><br><span class="line"> ├── x86</span><br><span class="line"> └── x86_64</span><br></pre></td></tr></table></figure>
<p>下一步就是引入到Android项目中了，首先在你的app/build.gradle中作如下修改</p>
<figure class="highlight groovy"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line">android {</span><br><span class="line"> ...</span><br><span class="line"> defaultConfig {</span><br><span class="line"> ...</span><br><span class="line"> ndk {</span><br><span class="line"> abiFilters <span class="string">'x86'</span>, <span class="string">'x86_64'</span>, <span class="string">'armeabi-v7a'</span>, <span class="string">'arm64-v8a'</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> sourceSets {</span><br><span class="line"> main {</span><br><span class="line"> <span class="comment">// let gradle pack the shared library into the apk</span></span><br><span class="line"> jniLibs.srcDirs = [<span class="string">'../ffmpeg-android-maker/output/lib'</span>]</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> externalNativeBuild {</span><br><span class="line"> cmake {</span><br><span class="line"> path <span class="string">"CMakeLists.txt"</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>分别解释一下:</p>
<p><code>abiFilters</code>: 指定了app支持的架构</p>
<p><code>jniLibs.srcDirs</code>: 指定了app需要的动态库路径，android编译时会自动将这里指定的动态库打包进apk</p>
<p><code>externalNativeBuild</code>这里指定了我们的<code>CMakeLists.txt</code>路径</p>
<p>下一步就是配置CMakeLists</p>
<figure class="highlight cmake"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">cmake_minimum_required</span>(VERSION <span class="number">3.4</span>.<span class="number">1</span>) <span class="comment"># 指定Cmake最低版本</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">set</span>(ffmpeg_dir <span class="variable">${CMAKE_SOURCE_DIR}</span>/../ffmpeg-android-maker/output) <span class="comment"># 设置ffmpeg_dir变量</span></span><br><span class="line"><span class="keyword">include_directories</span>(<span class="variable">${ffmpeg_dir}</span>/<span class="keyword">include</span>/<span class="variable">${ANDROID_ABI}</span>) <span class="comment"># 设置需要include的头文件路径，注意这里的ANDROID_ABI代表了在gradle中指定的abiFilters的每一个变量</span></span><br><span class="line"><span class="keyword">set</span>(ffmpeg_libs <span class="variable">${ffmpeg_dir}</span>/lib/<span class="variable">${ANDROID_ABI}</span>) <span class="comment"># 设置ffmpeg_libs变量，指明shared library路径</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(avutil SHARED IMPORTED) <span class="comment"># 声明avutil库</span></span><br><span class="line"><span class="keyword">set_target_properties</span>(avutil PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libavutil.so) <span class="comment"># 指定avutil库shared library路径</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(avformat SHARED IMPORTED) <span class="comment"># 类似上面的声明</span></span><br><span class="line"><span class="keyword">set_target_properties</span>(avformat PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libavformat.so)</span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(avfilter SHARED IMPORTED)</span><br><span class="line"><span class="keyword">set_target_properties</span>(avfilter PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libavfilter.so)</span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(avcodec SHARED IMPORTED)</span><br><span class="line"><span class="keyword">set_target_properties</span>(avcodec PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libavcodec.so)</span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(swscale SHARED IMPORTED)</span><br><span class="line"><span class="keyword">set_target_properties</span>(swscale PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libswscale.so)</span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(swresample SHARED IMPORTED)</span><br><span class="line"><span class="keyword">set_target_properties</span>(swresample PROPERTIES IMPORTED_LOCATION <span class="variable">${ffmpeg_libs}</span>/libswresample.so)</span><br><span class="line"></span><br><span class="line"><span class="keyword">find_library</span>(log-lib log) <span class="comment"># 使用Android 的native log库，命名为log-lib</span></span><br><span class="line"><span class="keyword">find_library</span>(jnigraphics-lib jnigraphics) <span class="comment"># 同上，使用jnigraphics</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">add_library</span>(test_ffmpeg</span><br><span class="line"> SHARED</span><br><span class="line"> src/main/cpp/video_config.cpp</span><br><span class="line"> src/main/cpp/video_config_jni.cpp</span><br><span class="line"> src/main/cpp/utils.cpp</span><br><span class="line"> src/main/cpp/main.cpp</span><br><span class="line"> src/main/cpp/player.cpp) <span class="comment"># 添加我们自己项目中的代码，并命名为test_ffmpeg库</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">target_link_libraries</span>(</span><br><span class="line"> test_ffmpeg</span><br><span class="line"> <span class="variable">${log-lib}</span></span><br><span class="line"> <span class="variable">${jnigraphics-lib}</span></span><br><span class="line"> android</span><br><span class="line"> avformat</span><br><span class="line"> avcodec</span><br><span class="line"> swscale</span><br><span class="line"> avutil</span><br><span class="line"> swresample</span><br><span class="line"> avfilter) <span class="comment"># 通知linker将上述所有library链接</span></span><br></pre></td></tr></table></figure>
<p>具体含义已经在注释中</p>
<p>全部配置完成后，可以尝试进行编译；剩下的问题就是JNI相关的知识和Android自身的相关开发知识了，本文就不再赘述。需要注意的一个小点就是在Java层System.loadLibrary时需要注意加载顺序</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">init</span> {</span><br><span class="line"> listOf(<span class="string">"avutil"</span>, <span class="string">"avcodec"</span>, <span class="string">"avformat"</span>, <span class="string">"swscale"</span>, <span class="string">"test_ffmpeg"</span>).forEach {</span><br><span class="line"> System.loadLibrary(it)</span><br><span class="line"> }</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>之后运行app，如果没有出现crash，就基本证明我们的引入是ok的了。Have Fun！</p>
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<article id="post-Jetpack-Compose简介与思考" class="article article-type-post" itemscope itemprop="blogPost">
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<time datetime="2020-07-25T01:22:01.000Z" itemprop="datePublished">2020-07-25</time>
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<a class="article-title" href="/2020/07/25/Jetpack-Compose%E7%AE%80%E4%BB%8B%E4%B8%8E%E6%80%9D%E8%80%83/">Jetpack Compose简介与思考</a>
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<p>Jetpack Compose可以认为是Android对UI代码架构的演进，核心目的是为了让安卓自身的UI代码能够跟随现代UI开发的步伐</p>
<h2 id="现代UI开发的特性"><a href="#现代UI开发的特性" class="headerlink" title="现代UI开发的特性"></a>现代UI开发的特性</h2><p>现代UI开发的一个重要特点在于其代码为声明式的代码结构，从数学关系上来看，UI的代码构成可以描述为对某一时刻状态的一个函数，即<code>UI=F(n)</code>，其中n表示了当前的交互状态。参考MVVM的设计来说，这个状态可以由ViewModel来管理</p>
<h2 id="安卓的UI开发现状"><a href="#安卓的UI开发现状" class="headerlink" title="安卓的UI开发现状"></a>安卓的UI开发现状</h2><p>安卓的UI开发思路和代码结构已经明显老化，主要体现在下面一些方面:</p>
<ol>
<li>仍旧有xml这样的配置文件，即使出现了ViewBinding也无济于事</li>
<li>构建UI时有大量的命令式代码，这种代码散落在Activity/Fragment中，不利于维护</li>
</ol>
<h2 id="命令式编程与声明式编程的争论"><a href="#命令式编程与声明式编程的争论" class="headerlink" title="命令式编程与声明式编程的争论"></a>命令式编程与声明式编程的争论</h2><p>命令式编程风格的问题在于程序非常依赖过程，换言之，如果代码编写的顺序出错，那么程序也会出错；而声明式编程则是只关心结果，编写代码时，只需要把开发者最终想要的结果直接写入代码，如果需要变化，则直接修改你的声明代码。并且声明式风格的UI代码在状态改变的任意时刻都是<strong>同一套代码</strong>构建UI, 这有助于提升代码质量</p>
<p>声明式风格编码特点在于:</p>
<pre><code>Describe your UI right now. For any value of now.
</code></pre><p>这种风格是一种状态无关代码(status independent)</p>
<h2 id="Jetpack-Compose"><a href="#Jetpack-Compose" class="headerlink" title="Jetpack Compose"></a>Jetpack Compose</h2><p>Jetpack Compose的目的就在于改变现有的UI开发思路，完全向声明式函数演进，同时提供了一些便捷功能，例如一个常规的UI声明如下:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MainActivity</span> : <span class="type">AppCompatActivity</span></span>() {</span><br><span class="line"> <span class="keyword">override</span> <span class="function"><span class="keyword">fun</span> <span class="title">onCreate</span><span class="params">(savedInstanceState: <span class="type">Bundle</span>?)</span></span> {</span><br><span class="line"> <span class="keyword">super</span>.onCreate(savedInstanceState)</span><br><span class="line"> setContentView(R.layout.activity_main)</span><br><span class="line"> findViewById<TextView>(R.id.tv).text = <span class="string">"Android"</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于复杂的UI界面，这种UI代码的编码方式是可以极其零散的, 可能一部分写在xml里面，一部分又写在代码里，同时代码里的编码可以任意地乱序（命令式代码的弊端）。而Compose出现后，我们的编码方式就可以变化为</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MainActivity</span> : <span class="type">AppCompatActivity</span></span>() {</span><br><span class="line"> <span class="keyword">override</span> <span class="function"><span class="keyword">fun</span> <span class="title">onCreate</span><span class="params">(savedInstanceState: <span class="type">Bundle</span>?)</span></span> {</span><br><span class="line"> <span class="keyword">super</span>.onCreate(savedInstanceState)</span><br><span class="line"> setContent {</span><br><span class="line"> MyApp {</span><br><span class="line"> Greeting(name = <span class="string">"Android"</span>)</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="meta">@Composable</span></span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">MyApp</span><span class="params">(content: @<span class="type">Composable</span>()</span></span> () -> <span class="built_in">Unit</span>) {</span><br><span class="line"> MaterialTheme {</span><br><span class="line"> content()</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="meta">@Composable</span></span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">Greeting</span><span class="params">(name: <span class="type">String</span>)</span></span> {</span><br><span class="line"> Surface(color = Color.Yellow) {</span><br><span class="line"> Text(text = <span class="string">"Hello <span class="variable">$name</span>!"</span>, modifier = Modifier.padding(<span class="number">24</span>.dp))</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>乍一看这种编码风格非常像Flutter，React，Vue的UI代码风格，本质上这些前端框架的UI设计都遵循了声明式函数风格，通过将编码方式转化为声明式函数风格后，我们间接地获得了以下收益</p>
<ol>
<li>各个函数独立，复用性提升</li>
<li>UI代码紧凑</li>
</ol>
<p>复用性提升的直接效果之一就是我们可以在IDE里面直接preview UI效果，这个也是Android Studio 4.2后内置的能力，只需要给你的<code>@Composable</code>函数加上一个<code>@Preview</code>标签并进行编译，这个函数自己的UI效果就能直接在IDE上展示</p>
<p>UI代码紧凑则明确了任何对UI的修改都有唯一的入口，例如在上面的例子中，<code>Activity</code>中一定是<code>setContent</code></p>
<h3 id="Jetpack-Compose的状态管理"><a href="#Jetpack-Compose的状态管理" class="headerlink" title="Jetpack Compose的状态管理"></a>Jetpack Compose的状态管理</h3><p>声明式风格的UI都面临一个问题就是如何处理状态改变，Jetpack Compose和其他常见的声明式UI框架一致，采用了state这个概念，通过声明一个state对象，我们根据这个state对象构造自己的UI和其子View，当state改变时，动态的改变这个UI的声明，例如:</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Composable</span></span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">MyScreenContent</span><span class="params">()</span></span> {</span><br><span class="line"> <span class="keyword">val</span> counterState = state { <span class="number">0</span> }</span><br><span class="line"> Counter(</span><br><span class="line"> count = counterState.value,</span><br><span class="line"> updateCount = { newCount -></span><br><span class="line"> counterState.value = newCount</span><br><span class="line"> }</span><br><span class="line"> )</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="meta">@Composable</span></span><br><span class="line"><span class="function"><span class="keyword">fun</span> <span class="title">Counter</span><span class="params">(count: <span class="type">Int</span>, updateCount: (<span class="type">Int</span>)</span></span> -> <span class="built_in">Unit</span>) {</span><br><span class="line"> Button(onClick = { updateCount(count+<span class="number">1</span>) }) {</span><br><span class="line"> Text(<span class="string">"I've been clicked <span class="variable">$count</span> times"</span>)</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这个例子中，我们就能发现<code>MyScreenContent</code>中内含了一个counterState对象，它通过counterState构建了一个Counter，而Counter也是counterState的一个函数，当counterState变化时，Counter也会重新构建，UI因此也发生变化</p>
<h3 id="Jetpack-Compose中Lifecycle"><a href="#Jetpack-Compose中Lifecycle" class="headerlink" title="Jetpack Compose中Lifecycle"></a>Jetpack Compose中Lifecycle</h3><p>Android开发非常强调Lifecycle-aware，compose也是如此。为此Jetpack Compose提出了Efforts这个概念，在这个概念中提供了onCommit, onPreCommit, onActive, onDispose一系列函数，用来监听生命周期变化</p>
<h2 id="感想和思考"><a href="#感想和思考" class="headerlink" title="感想和思考"></a>感想和思考</h2><p>似乎在UI开发上，声明式编程已经成为了主流的方向。随着SwiftUI，Flutter，Jetpack Compose的出现，越来越多的新兴UI开发框架抛弃了现有的MVC，MVP模式，而走向声明式UI方向，随着声明式UI被引入移动端开发，新兴的MVI模式也进入了大众视野，并逐渐被人们接受</p>
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<article id="post-MMKV源码简析-跨进程" class="article article-type-post" itemscope itemprop="blogPost">
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<a class="article-title" href="/2020/05/04/MMKV%E6%BA%90%E7%A0%81%E7%AE%80%E6%9E%90-%E8%B7%A8%E8%BF%9B%E7%A8%8B/">MMKV源码简析---跨进程</a>
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<p>MMKV 一个重要特性就是增加了 android 侧对跨进程读写的支持，我们单独用一篇文章来分析一下 MMKV 对跨进程存储的实现方式<br>典型的初始化调用如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">MMKV mmkv = MMKV.mmkvWithID(MMKV_ID, MMKV.MULTI_PROCESS_MODE, CryptKey);</span><br><span class="line"></span><br><span class="line">mmkv.encode(...)</span><br><span class="line">mmkv.decode(...)</span><br></pre></td></tr></table></figure>
<p>MMKV 会把每个文件都通过 mmap 到进程的访问空间，因此每个进程本身就可以直接访问存储，但这里没有解决跨进程的并发访问问题，解决并发需要别的手段，我们看看 MMKV.MULTI_PROCESS_MODE 造成了什么影响</p>
<h2 id="MMKV-MULTI-PROCESS-MODE"><a href="#MMKV-MULTI-PROCESS-MODE" class="headerlink" title="MMKV.MULTI_PROCESS_MODE"></a>MMKV.MULTI_PROCESS_MODE</h2><p>参考初始化流程的代码和<code>MMKV.MULTI_PROCESS_MODE=2</code>, 来到 MMKV 构造函数, 可以发现此时 MMKV::m_isInterProcess=true, 这个对代码逻辑有什么影响呢?</p>
<ol>
<li>checkLoadData 的后半段代码会执行</li>
<li>m_sharedProcessLock, m_exclusiveProcessLock 全部 enable</li>
</ol>
<p>先来看看 checkLoadData 的后半段代码是啥</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (!m_isInterProcess) {</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> (!m_metaFile->isFileValid()) {</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// <span class="doctag">TODO:</span> atomic lock m_metaFile?</span></span><br><span class="line">MMKVMetaInfo metaInfo;</span><br><span class="line">metaInfo.read(m_metaFile->getMemory()); <span class="comment">// 文件m_metaFile被mmap，每个进程都能读到, 但是m_metaInfo是内存对象，没有被进程共享，因此有可能存储的元数据和文件中的元数据不一致</span></span><br><span class="line"><span class="keyword">if</span> (m_metaInfo->m_sequence != metaInfo.m_sequence) {</span><br><span class="line"> MMKVInfo(<span class="string">"[%s] oldSeq %u, newSeq %u"</span>, m_mmapID.c_str(), m_metaInfo->m_sequence, metaInfo.m_sequence);</span><br><span class="line"> SCOPED_LOCK(m_sharedProcessLock);</span><br><span class="line"></span><br><span class="line"> clearMemoryCache();</span><br><span class="line"> loadFromFile();</span><br><span class="line"> notifyContentChanged();</span><br><span class="line">} <span class="keyword">else</span> <span class="keyword">if</span> (m_metaInfo->m_crcDigest != metaInfo.m_crcDigest) {</span><br><span class="line"> MMKVDebug(<span class="string">"[%s] oldCrc %u, newCrc %u, new actualSize %u"</span>, m_mmapID.c_str(), m_metaInfo->m_crcDigest,</span><br><span class="line"> metaInfo.m_crcDigest, metaInfo.m_actualSize);</span><br><span class="line"> SCOPED_LOCK(m_sharedProcessLock);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">size_t</span> fileSize = m_file->getActualFileSize();</span><br><span class="line"> <span class="keyword">if</span> (m_file->getFileSize() != fileSize) {</span><br><span class="line"> MMKVInfo(<span class="string">"file size has changed [%s] from %zu to %zu"</span>, m_mmapID.c_str(), m_file->getFileSize(), fileSize);</span><br><span class="line"> clearMemoryCache();</span><br><span class="line"> loadFromFile();</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> partialLoadFromFile();</span><br><span class="line"> }</span><br><span class="line"> notifyContentChanged();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>相比较于单进程，这里增加了对内存中的 meta 和文件的 meta 的 seq 和 crc 校验比较，如果不相等，则需要走文件重新加载逻辑，这一块，和<a href="https://github.com/Tencent/MMKV/wiki/android_ipc#%E7%8A%B6%E6%80%81%E5%90%8C%E6%AD%A5" target="_blank" rel="noopener">https://github.com/Tencent/MMKV/wiki/android_ipc#%E7%8A%B6%E6%80%81%E5%90%8C%E6%AD%A5</a>描述是一致的</p>
<p>m_sharedProcessLock, m_exclusiveProcessLock 全部 enable 应该就是使锁生效，那么分析一下这个锁是如何实现跨进程锁的</p>
<h2 id="InterProcessLock"><a href="#InterProcessLock" class="headerlink" title="InterProcessLock"></a>InterProcessLock</h2><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">InterProcessLock</span> {</span></span><br><span class="line"> FileLock *m_fileLock;</span><br><span class="line"> LockType m_lockType;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> InterProcessLock(FileLock *fileLock, LockType lockType)</span><br><span class="line"> : m_fileLock(fileLock), m_lockType(lockType), m_enable(<span class="literal">true</span>) {</span><br><span class="line"> MMKV_ASSERT(m_fileLock);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">bool</span> m_enable;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">void</span> <span class="title">lock</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (m_enable) {</span><br><span class="line"> m_fileLock->lock(m_lockType);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"><span class="comment">// ...</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>可以看到，MMKV 的跨进程锁是基于文件锁实现的, <code>InterProcessLock#lock</code>基于<code>FileLock.lock</code>实现, 一路转发来到<code>FileLock::platformLock()</code></p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">bool</span> FileLock::doLock(LockType lockType, <span class="keyword">bool</span> wait) {</span><br><span class="line"> <span class="keyword">if</span> (!isFileLockValid()) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">bool</span> unLockFirstIfNeeded = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (lockType == SharedLockType) {</span><br><span class="line"> <span class="comment">// don't want shared-lock to break any existing locks</span></span><br><span class="line"> <span class="keyword">if</span> (m_sharedLockCount > <span class="number">0</span> || m_exclusiveLockCount > <span class="number">0</span>) {</span><br><span class="line"> m_sharedLockCount++;</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// don't want exclusive-lock to break existing exclusive-locks</span></span><br><span class="line"> <span class="keyword">if</span> (m_exclusiveLockCount > <span class="number">0</span>) {</span><br><span class="line"> m_exclusiveLockCount++;</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// prevent deadlock</span></span><br><span class="line"> <span class="keyword">if</span> (m_sharedLockCount > <span class="number">0</span>) {</span><br><span class="line"> unLockFirstIfNeeded = <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">auto</span> ret = platformLock(lockType, wait, unLockFirstIfNeeded);</span><br><span class="line"> <span class="keyword">if</span> (ret) {</span><br><span class="line"> <span class="keyword">if</span> (lockType == SharedLockType) {</span><br><span class="line"> m_sharedLockCount++;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> m_exclusiveLockCount++;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> ret;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">bool</span> FileLock::platformLock(LockType lockType, <span class="keyword">bool</span> wait, <span class="keyword">bool</span> unLockFirstIfNeeded) {</span><br><span class="line"><span class="meta"># <span class="meta-keyword">ifdef</span> MMKV_ANDROID</span></span><br><span class="line"> <span class="keyword">if</span> (m_isAshmem) {</span><br><span class="line"> <span class="keyword">return</span> ashmemLock(lockType, wait, unLockFirstIfNeeded);</span><br><span class="line"> }</span><br><span class="line"><span class="meta"># <span class="meta-keyword">endif</span></span></span><br><span class="line"> <span class="keyword">auto</span> realLockType = LockType2FlockType(lockType);</span><br><span class="line"> <span class="keyword">auto</span> cmd = wait ? realLockType : (realLockType | LOCK_NB);</span><br><span class="line"> <span class="keyword">if</span> (unLockFirstIfNeeded) {</span><br><span class="line"> <span class="comment">// try lock</span></span><br><span class="line"> <span class="keyword">auto</span> ret = flock(m_fd, realLockType | LOCK_NB);</span><br><span class="line"> <span class="keyword">if</span> (ret == <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// let's be gentleman: unlock my shared-lock to prevent deadlock</span></span><br><span class="line"> ret = flock(m_fd, LOCK_UN);</span><br><span class="line"> <span class="keyword">if</span> (ret != <span class="number">0</span>) {</span><br><span class="line"> MMKVError(<span class="string">"fail to try unlock first fd=%d, ret=%d, error:%s"</span>, m_fd, ret, strerror(errno));</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">auto</span> ret = flock(m_fd, cmd);</span><br><span class="line"> <span class="keyword">if</span> (ret != <span class="number">0</span>) {</span><br><span class="line"> MMKVError(<span class="string">"fail to lock fd=%d, ret=%d, error:%s"</span>, m_fd, ret, strerror(errno));</span><br><span class="line"> <span class="comment">// try recover my shared-lock</span></span><br><span class="line"> <span class="keyword">if</span> (unLockFirstIfNeeded) {</span><br><span class="line"> ret = flock(m_fd, LockType2FlockType(SharedLockType));</span><br><span class="line"> <span class="keyword">if</span> (ret != <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// let's hope this never happen</span></span><br><span class="line"> MMKVError(<span class="string">"fail to recover shared-lock fd=%d, ret=%d, error:%s"</span>, m_fd, ret, strerror(errno));</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>可以看到 MMKV 在文件锁 flock 的基础上，增加了一套计数器机制(代码中的 m_sharedLockCount 和 m_exclusiveLockCount)，来确保</p>
<ol>
<li>锁的重入特性</li>
<li>锁的共享和互斥性转换特性</li>
</ol>
<p>关于文件锁的细节，可以参考<a href="https://gavv.github.io/articles/file-locks" target="_blank" rel="noopener">https://gavv.github.io/articles/file-locks</a></p>
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<article id="post-MMKV源码简析-读写" class="article article-type-post" itemscope itemprop="blogPost">
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<time datetime="2020-05-04T07:22:29.000Z" itemprop="datePublished">2020-05-04</time>
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<a class="article-title" href="/2020/05/04/MMKV%E6%BA%90%E7%A0%81%E7%AE%80%E6%9E%90-%E8%AF%BB%E5%86%99/">MMKV源码简析---读写</a>
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<ul>
<li><a href="#mmkv-%e5%86%99">MMKV 写</a><ul>
<li><a href="#checkloaddata">checkLoadData()</a></li>
<li><a href="#appenddatawithkey">appendDataWithKey</a></li>
</ul>
</li>
<li><a href="#mmkv-%e8%af%bb">MMKV 读</a></li>
<li><a href="#%e5%b0%8f%e7%bb%93">小结</a></li>
<li><a href="#mmkv-%e7%a9%ba%e9%97%b4%e4%bc%98%e5%8c%96">MMKV 空间优化</a></li>
</ul>
<p>我们还是从 testcase 出发，一点点看 MMKV 如何实现读写的</p>
<h2 id="MMKV-写"><a href="#MMKV-写" class="headerlink" title="MMKV 写"></a>MMKV 写</h2><figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">val</span> mmkv = MMKV.mmkvWithID(<span class="string">"testKotlin"</span>)</span><br><span class="line">mmkv.encode(<span class="string">"bool"</span>, <span class="literal">true</span>)</span><br><span class="line">mmkv.encode(<span class="string">"int"</span>, Integer.MIN_VALUE)</span><br></pre></td></tr></table></figure>
<p>MMKV#encode 支持所有基本类型, 这里我们挑一个简单的 bool 类型分析一下，根据调用链<code>MMKV#encode</code>-><code>MMKV#encodeBool</code>来到 native-bridge.cpp 中的 JNI 接口，找到其实现<code>MMKV::set(bool value, MMKVKey_t key)</code></p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// MMKVKey_t = const string&</span></span><br><span class="line"><span class="keyword">bool</span> MMKV::<span class="built_in">set</span>(<span class="keyword">bool</span> value, MMKVKey_t key) {</span><br><span class="line"> <span class="keyword">if</span> (isKeyEmpty(key)) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">size_t</span> size = pbBoolSize();</span><br><span class="line"> <span class="function">MMBuffer <span class="title">data</span><span class="params">(size)</span></span>;</span><br><span class="line"> <span class="function">CodedOutputData <span class="title">output</span><span class="params">(data.getPtr(), size)</span></span>;</span><br><span class="line"> output.writeBool(value);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> setDataForKey(<span class="built_in">std</span>::move(data), key);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这里根据 pb 协议的规范，分配了一块和 pb 协议中商定的类型字段长度相同的 buffer MMBuffer, 将这个 buffer 利用 CodedOutputData 这个工具类写入一个 bool，之后调用<code>setDataForKey</code>来记录</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> MMKV::setDataForKey(MMBuffer &&data, MMKVKey_t key) {</span><br><span class="line"> <span class="keyword">if</span> (data.length() == <span class="number">0</span> || isKeyEmpty(key)) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> SCOPED_LOCK(m_lock);</span><br><span class="line"> SCOPED_LOCK(m_exclusiveProcessLock);</span><br><span class="line"> checkLoadData(); <span class="comment">// #1</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">auto</span> ret = appendDataWithKey(data, key); <span class="comment">// #2</span></span><br><span class="line"> <span class="keyword">if</span> (ret) {</span><br><span class="line"> m_dic[key] = <span class="built_in">std</span>::move(data);</span><br><span class="line"> m_hasFullWriteback = <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> ret;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="checkLoadData"><a href="#checkLoadData" class="headerlink" title="checkLoadData()"></a>checkLoadData()</h3><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">void</span> MMKV::checkLoadData() {</span><br><span class="line"> <span class="keyword">if</span> (m_needLoadFromFile) { <span class="comment">// m_needLoadFromFile在MMKV构造和clearMemoryCache调用后才会是true</span></span><br><span class="line"> SCOPED_LOCK(m_sharedProcessLock);</span><br><span class="line"></span><br><span class="line"> m_needLoadFromFile = <span class="literal">false</span>;</span><br><span class="line"> loadFromFile(); <span class="comment">// 初始化过程中也调用了这个函数，相当于加载存储中的内容</span></span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (!m_isInterProcess) {</span><br><span class="line"> <span class="comment">// 不是跨进程模式的MMKV，直接返回</span></span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (!m_metaFile->isFileValid()) {</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// <span class="doctag">TODO:</span> atomic lock m_metaFile?</span></span><br><span class="line"> MMKVMetaInfo metaInfo;</span><br><span class="line"> metaInfo.read(m_metaFile->getMemory());</span><br><span class="line"> <span class="keyword">if</span> (m_metaInfo->m_sequence != metaInfo.m_sequence) { <span class="comment">// 内存中的seq和文件中的seq不一致，文件有新的改动，清理缓存，重新读文件，通知内容更改</span></span><br><span class="line"> MMKVInfo(<span class="string">"[%s] oldSeq %u, newSeq %u"</span>, m_mmapID.c_str(), m_metaInfo->m_sequence, metaInfo.m_sequence);</span><br><span class="line"> SCOPED_LOCK(m_sharedProcessLock);</span><br><span class="line"></span><br><span class="line"> clearMemoryCache();</span><br><span class="line"> loadFromFile();</span><br><span class="line"> notifyContentChanged();</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (m_metaInfo->m_crcDigest != metaInfo.m_crcDigest) { <span class="comment">// 内存中的crc校验值和文件中crc校验值不一致</span></span><br><span class="line"> MMKVDebug(<span class="string">"[%s] oldCrc %u, newCrc %u, new actualSize %u"</span>, m_mmapID.c_str(), m_metaInfo->m_crcDigest,</span><br><span class="line"> metaInfo.m_crcDigest, metaInfo.m_actualSize);</span><br><span class="line"> SCOPED_LOCK(m_sharedProcessLock);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">size_t</span> fileSize = m_file->getActualFileSize();</span><br><span class="line"> <span class="keyword">if</span> (m_file->getFileSize() != fileSize) {</span><br><span class="line"> <span class="comment">// 有新内容写入，清理memory，重新读文件，通知内容更改</span></span><br><span class="line"> MMKVInfo(<span class="string">"file size has changed [%s] from %zu to %zu"</span>, m_mmapID.c_str(), m_file->getFileSize(), fileSize);</span><br><span class="line"> clearMemoryCache();</span><br><span class="line"> loadFromFile();</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// 现有内容有变化，存量加载</span></span><br><span class="line"> partialLoadFromFile();</span><br><span class="line"> }</span><br><span class="line"> notifyContentChanged(); <span class="comment">// 通知Java层通过setWantsContentChangeNotify注册的回调</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>逻辑比较清晰，主要看看 partialLoadFromFile 这个函数</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// read from last m_position</span></span><br><span class="line"><span class="keyword">void</span> MMKV::partialLoadFromFile() {</span><br><span class="line"> m_metaInfo->read(m_metaFile->getMemory());</span><br><span class="line"></span><br><span class="line"> <span class="keyword">size_t</span> oldActualSize = m_actualSize;</span><br><span class="line"> m_actualSize = readActualSize();</span><br><span class="line"> <span class="keyword">auto</span> fileSize = m_file->getFileSize();</span><br><span class="line"> MMKVDebug(<span class="string">"loading [%s] with file size %zu, oldActualSize %zu, newActualSize %zu"</span>, m_mmapID.c_str(), fileSize,</span><br><span class="line"> oldActualSize, m_actualSize);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (m_actualSize > <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">if</span> (m_actualSize < fileSize && m_actualSize + Fixed32Size <= fileSize) {</span><br><span class="line"> <span class="keyword">if</span> (m_actualSize > oldActualSize) {</span><br><span class="line"> <span class="comment">// 有新内容</span></span><br><span class="line"> <span class="keyword">size_t</span> bufferSize = m_actualSize - oldActualSize;</span><br><span class="line"> <span class="keyword">auto</span> ptr = (<span class="keyword">uint8_t</span> *) m_file->getMemory();</span><br><span class="line"> <span class="function">MMBuffer <span class="title">inputBuffer</span><span class="params">(ptr + Fixed32Size + oldActualSize, bufferSize, MMBufferNoCopy)</span></span>;</span><br><span class="line"> <span class="comment">// incremental update crc digest</span></span><br><span class="line"> m_crcDigest =</span><br><span class="line"> (<span class="keyword">uint32_t</span>) CRC32(m_crcDigest, (<span class="keyword">const</span> <span class="keyword">uint8_t</span> *) inputBuffer.getPtr(), inputBuffer.length());</span><br><span class="line"> <span class="keyword">if</span> (m_crcDigest == m_metaInfo->m_crcDigest) {</span><br><span class="line"> <span class="keyword">if</span> (m_crypter) {</span><br><span class="line"> decryptBuffer(*m_crypter, inputBuffer);</span><br><span class="line"> }</span><br><span class="line"> MiniPBCoder::greedyDecodeMap(m_dic, inputBuffer, bufferSize); <span class="comment">// 解析buffer内容到m_dic, buffer的结构是线性的string-MMBuffer KV结构</span></span><br><span class="line"> m_output->seek(bufferSize);</span><br><span class="line"> m_hasFullWriteback = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> MMKVDebug(<span class="string">"partial loaded [%s] with %zu values"</span>, m_mmapID.c_str(), m_dic.size());</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> MMKVError(<span class="string">"m_crcDigest[%u] != m_metaInfo->m_crcDigest[%u]"</span>, m_crcDigest, m_metaInfo->m_crcDigest);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// something is wrong, do a full load</span></span><br><span class="line"> clearMemoryCache();</span><br><span class="line"> loadFromFile();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="appendDataWithKey"><a href="#appendDataWithKey" class="headerlink" title="appendDataWithKey"></a>appendDataWithKey</h3><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> MMKV::appendDataWithKey(<span class="keyword">const</span> MMBuffer &data, MMKVKey_t key) {</span><br><span class="line"> <span class="keyword">size_t</span> keyLength = key.length();</span><br><span class="line"> <span class="comment">// size needed to encode the key</span></span><br><span class="line"> <span class="keyword">size_t</span> size = keyLength + pbRawVarint32Size((<span class="keyword">int32_t</span>) keyLength);</span><br><span class="line"> <span class="comment">// size needed to encode the value</span></span><br><span class="line"> size += data.length() + pbRawVarint32Size((<span class="keyword">int32_t</span>) data.length());</span><br><span class="line"></span><br><span class="line"> SCOPED_LOCK(m_exclusiveProcessLock);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">bool</span> hasEnoughSize = ensureMemorySize(size);</span><br><span class="line"> <span class="keyword">if</span> (!hasEnoughSize || !isFileValid()) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> m_output->writeString(key);</span><br><span class="line"> m_output->writeData(data); <span class="comment">// note: write size of data</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">auto</span> ptr = (<span class="keyword">uint8_t</span> *) m_file->getMemory() + Fixed32Size + m_actualSize;</span><br><span class="line"> <span class="keyword">if</span> (m_crypter) {</span><br><span class="line"> m_crypter->encrypt(ptr, ptr, size);</span><br><span class="line"> }</span><br><span class="line"> m_actualSize += size;</span><br><span class="line"> updateCRCDigest(ptr, size); <span class="comment">// 更新crc校验和seq</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="MMKV-读"><a href="#MMKV-读" class="headerlink" title="MMKV 读"></a>MMKV 读</h2><p>同样地，通过 JNI 找到<code>MMKV::getBool</code></p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> MMKV::getBool(MMKVKey_t key, <span class="keyword">bool</span> defaultValue) {</span><br><span class="line"> <span class="keyword">if</span> (isKeyEmpty(key)) {</span><br><span class="line"> <span class="keyword">return</span> defaultValue;</span><br><span class="line"> }</span><br><span class="line"> SCOPED_LOCK(m_lock);</span><br><span class="line"> <span class="keyword">auto</span> &data = getDataForKey(key); <span class="comment">// #1</span></span><br><span class="line"> <span class="keyword">if</span> (data.length() > <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="function">CodedInputData <span class="title">input</span><span class="params">(data.getPtr(), data.length())</span></span>;</span><br><span class="line"> <span class="keyword">return</span> input.readBool();</span><br><span class="line"> } <span class="keyword">catch</span> (<span class="built_in">std</span>::exception &exception) {</span><br><span class="line"> MMKVError(<span class="string">"%s"</span>, exception.what());</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> defaultValue;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>实现关键在 getDataForKey</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> MMBuffer &MMKV::getDataForKey(MMKVKey_t key) {</span><br><span class="line"> checkLoadData();</span><br><span class="line"> <span class="keyword">auto</span> itr = m_dic.find(key);</span><br><span class="line"> <span class="keyword">if</span> (itr != m_dic.end()) {</span><br><span class="line"> <span class="keyword">return</span> itr->second;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">static</span> MMBuffer nan;</span><br><span class="line"> <span class="keyword">return</span> nan;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>checkLoadData 函数通过调用 loadFromFile 会确保所有的文件数据被读进 m_dic 中，这里直接取就可以了</p>
<h2 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h2><p>MMKV 的读写逻辑比较简单，主要的实现还是依赖了 PB 方式的数据序列化和反序列化，根据代码逻辑，我们可以尝试还原出 MMKV 内部的存储结构如下:</p>
<figure class="highlight plain"><figcaption><span>buffer</span></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">message KV {</span><br><span class="line"> string key = 1;</span><br><span class="line"> buffer value = 2;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">message MMKV {</span><br><span class="line"> int32 size = 1; // 文件大小，用于校验新数据写入</span><br><span class="line"> repeated KV kv = 2;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>同时我们可以通过<code>MiniPBCoder::decodeOneMap</code>的代码实现可以发现，MMKV 对于新旧数据问题，采用的方式不是写覆盖，而是直接追加，同时读时以最后一次写为最新值。这种方式显然是会带来大量的 key 字段冗余，因此必然存在一套完整的空间优化</p>
<h2 id="MMKV-空间优化"><a href="#MMKV-空间优化" class="headerlink" title="MMKV 空间优化"></a>MMKV 空间优化</h2><p>核心实现在<code>MMKV::ensureMemorySize</code>中</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> MMKV::ensureMemorySize(<span class="keyword">size_t</span> newSize) {</span><br><span class="line"> <span class="keyword">if</span> (!isFileValid()) {</span><br><span class="line"> MMKVWarning(<span class="string">"[%s] file not valid"</span>, m_mmapID.c_str());</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// make some room for placeholder</span></span><br><span class="line"> <span class="keyword">constexpr</span> <span class="keyword">size_t</span> ItemSizeHolderSize = <span class="number">4</span>;</span><br><span class="line"> <span class="keyword">if</span> (m_dic.empty()) {</span><br><span class="line"> newSize += ItemSizeHolderSize;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (newSize >= m_output->spaceLeft() || m_dic.empty()) {</span><br><span class="line"> <span class="comment">// 空间不够，进行一次全量写入，这次全量写入会把过长的冗余字段全部覆盖写</span></span><br><span class="line"> <span class="comment">// try a full rewrite to make space</span></span><br><span class="line"> <span class="keyword">auto</span> fileSize = m_file->getFileSize();</span><br><span class="line"> MMBuffer data = MiniPBCoder::encodeDataWithObject(m_dic);</span><br><span class="line"> <span class="keyword">size_t</span> lenNeeded = data.length() + Fixed32Size + newSize;</span><br><span class="line"> <span class="keyword">size_t</span> avgItemSize = lenNeeded / <span class="built_in">std</span>::max<<span class="keyword">size_t</span>>(<span class="number">1</span>, m_dic.size());</span><br><span class="line"> <span class="keyword">size_t</span> futureUsage = avgItemSize * <span class="built_in">std</span>::max<<span class="keyword">size_t</span>>(<span class="number">8</span>, (m_dic.size() + <span class="number">1</span>) / <span class="number">2</span>);</span><br><span class="line"> <span class="comment">// 1. no space for a full rewrite, double it</span></span><br><span class="line"> <span class="comment">// 2. or space is not large enough for future usage, double it to avoid frequently full rewrite</span></span><br><span class="line"> <span class="keyword">if</span> (lenNeeded >= fileSize || (lenNeeded + futureUsage) >= fileSize) {</span><br><span class="line"> <span class="keyword">size_t</span> oldSize = fileSize;</span><br><span class="line"> <span class="keyword">do</span> {</span><br><span class="line"> fileSize *= <span class="number">2</span>; <span class="comment">// 2倍的扩充策略</span></span><br><span class="line"> } <span class="keyword">while</span> (lenNeeded + futureUsage >= fileSize);</span><br><span class="line"> MMKVInfo(<span class="string">"extending [%s] file size from %zu to %zu, incoming size:%zu, future usage:%zu"</span>, m_mmapID.c_str(),</span><br><span class="line"> oldSize, fileSize, newSize, futureUsage);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// if we can't extend size, rollback to old state</span></span><br><span class="line"> <span class="keyword">if</span> (!m_file->truncate(fileSize)) {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// check if we fail to make more space</span></span><br><span class="line"> <span class="keyword">if</span> (!isFileValid()) {</span><br><span class="line"> MMKVWarning(<span class="string">"[%s] file not valid"</span>, m_mmapID.c_str());</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> doFullWriteBack(<span class="built_in">std</span>::move(data)); <span class="comment">// 全量写入</span></span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>整个读写过程分析到这里，后面将分析一下 MMKV 对于跨进程存储的实现</p>
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