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<!DOCTYPE html>
<html class="theme-next gemini use-motion" lang="zh-Hans">
<head>
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<title>姚陆浩的博客</title><!-- hexo-inject:begin --><!-- hexo-inject:end -->
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page-home">
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<p class="site-subtitle">用文字构建一个理想的世界</p>
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<h1 class="post-title" itemprop="name headline">
<a class="post-title-link" href="/2018/07/21/thread-local/" itemprop="url">ThreadLocal 的作用和实现原理</a></h1>
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2018-07-21
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<h1 id="ThreadLocal-简介"><a href="#ThreadLocal-简介" class="headerlink" title="ThreadLocal 简介"></a>ThreadLocal 简介</h1><p>这个类提供一个线程内部变量。通常情况下，我们创建的变量是可以被任何一个线程访问并修改的。而使用 ThreadLocal 创建的变量只能被当前线程访问，其他线程则无法访问和修改。</p>
<p>在日常开发中用到 ThreadLocal 的地方比较少，但是在某些特殊的场景下，通过 ThreadLocal 可以轻松地实现一些看起来很复杂的功能，这一点在 Android 的源码中也有所体现，比如 Looper、ActivityThread 以及 AMS 中都用到了 ThreadLocal。一般来说，当某些数据是以线程为作用域并且不同线程具有不同的数据副本的时候，就可以考虑采用 ThreadLocal。</p>
<h1 id="ThreadLocal-使用"><a href="#ThreadLocal-使用" class="headerlink" title="ThreadLocal 使用"></a>ThreadLocal 使用</h1><h2 id="创建，支持泛型"><a href="#创建，支持泛型" class="headerlink" title="创建，支持泛型"></a>创建，支持泛型</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">ThreadLocal<String> mStringThreadLocal = <span class="keyword">new</span> ThreadLocal<String>();</div></pre></td></tr></table></figure>
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<a class="post-title-link" href="/2018/07/15/android-runtime-permissions/" itemprop="url">动态权限</a></h1>
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<h1 id="动态权限"><a href="#动态权限" class="headerlink" title="动态权限"></a>动态权限</h1><p>动态权限又称<strong>运行时权限</strong>（高于 6.0 版本）。如果设备是运行在 Android 6.0 版本以上 —— app 的目标 sdk 版本是 23 以上时，在安装时，任何 app 权限用户都没有通知。在运行时，你的 app 必须请求用户授予危险权限，用户会看到一个系统弹窗（见下图左边）告诉用户，你的 app 正试图访问那个权限组。弹窗包括<strong>拒绝</strong>和<strong>允许</strong>按钮。</p>
<p><img src="http://otqmfc4ah.bkt.clouddn.com/permission_dialog.jpg" alt="运行时权限弹窗"></p>
<p>如果用户拒绝权限请求，下一次你的 app 请求权限，弹窗会包含一个复选框，当这个复选框被选中，表示了用户不想在提示这个权限（见上图右边）。</p>
<p>如果用户选中 <strong>Never ask again</strong> 并且点击 <strong>DENY</strong>，那么如果你之后试图请求相同的权限，系统将不再展示给用户。</p>
<p>及时用户授予即使用户授予您的应用程序所请求的权限，您也不能总是依赖它，用户还可以选择在系统设置中逐个启用和禁用权限，您应始终在运行时检查并请求权限以防止运行时错误。</p>
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<h1 id="如何理解设计模式的六大基本原则"><a href="#如何理解设计模式的六大基本原则" class="headerlink" title="如何理解设计模式的六大基本原则"></a>如何理解设计模式的六大基本原则</h1><p>设计模式是一套理论，由软件界的先辈们总结出的一套可以反复使用的经验，它可以提高代码的可重用性，增强系统的可维护性，以及解决一系列的复杂问题。在做软件的时候，需求是最难把握的，我们可以分析现有的需求，预测可能发生的变更，但是我们不能控制需求的变更。既然需求的变更是不可控的，那如何拥抱变化呢？幸运的是，设计模式给了我们指导，出现了 6 大设计原则。</p>
<p>这六大设计原则分别是：</p>
<ul>
<li>单一职责原则 (<strong>Single Responsibility Principle，简称SRP </strong>)</li>
<li>里氏替换原则 (<strong>Liskov Substitution Principle,简称LSP</strong>)</li>
<li>依赖导致原则 (<strong>Dependence Inversion Principle,简称DIP</strong>)</li>
<li>接口隔离原则 (<strong>Interface Segregation Principle,简称ISP</strong>)</li>
<li>迪米特原则 (<strong>Law of Demeter,简称LoD</strong>)</li>
<li>开闭原则 (<strong>Open Close Principle,简称OCP</strong>)</li>
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<p>通常情况下，图片的原始尺寸要比 ImageView 尺寸大，这时候把整张图片加载进 ImageView 会造成内存内存浪费，甚至造成 OOM (Out Of Memory Killer)。</p>
<p>在 Android App 中，为什么加载位图（Bitmap）是很棘手，是有很多原因的：</p>
<ul>
<li>Bitmap 可以很轻松的消耗 App 的内存预算（Android 分配给每个 APP 的最大内存）。例如，Pixel 手机上的相机可拍摄高达 4048x3036 像素（1200万像素）如果 bitmap 配置使用 ARGB_8888（每个像素占 4 字节）—— Android 2.3（API 9 版本）及以上版本的默认值，将这张照片加载到内存中需要大约48MB 的内存(4048 * 3036 * 4 字节)。如此大的内存能够立即耗尽 App 可用的所有的内存。</li>
<li>在 UI 线程中加载 bitmap 会降低你的 App 的性能，引发响应缓慢甚至发生 ANR (Application Not Responding) 的信息。因此，在使用 bitmap 时适当的管理线程是非常重要的。</li>
<li>如果你的 App 正在加载多张 bitmap 到内存中，你需要熟练的管理内存和磁盘缓存。否则，你的 App 可能要忍受响应性和滑动性差的体验</li>
</ul>
<p>多数情况下，我们推荐你在 App 中，使用 <a href="https://github.com/bumptech/glide" target="_blank" rel="external">Glide</a> 框架获得，解析展示 bitmap。在 Android中，Glide 抽象出来很多复杂的 bitmap 和其他图像和其他任务的相关工作。</p>
<p>然而，在使用开源框架的同时，我们仍然要明白一个像 Glide 这样优秀的框架要做哪些事情。</p>
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