optimize 'SelectableHelper'

add constructor:

- SelectableHelper(RecyclerView.Adapter, List<Integer>)

add method:

- getSelectedCount()

helper/src/main/java/recyclerview/helper/SelectableHelper.java

@@ -64,13 +64,22 @@ public class SelectableHelper {
     private OnSelectCountChangeListener mOnSelectCountChangeListener;
 
     public SelectableHelper(@NonNull RecyclerView.Adapter adapter) {
+        this(adapter, null);
+    }
+
+    public SelectableHelper(@NonNull RecyclerView.Adapter adapter, @Nullable List<Integer> selectedPositions) {
         NonNullUtil.requireNonNull(adapter);
 
         mAdapter = adapter;
         mRecyclerView = null;
 
-        mSelectMode = SelectMode.SINGLE;
-        mSelectedPositions = new ArrayList<>();
+        if (selectedPositions == null) {
+            mSelectMode = SelectMode.SINGLE;
+            mSelectedPositions = new ArrayList<>();
+        } else {
+            mSelectMode = SelectMode.MULTIPLE;
+            mSelectedPositions = new ArrayList<>(selectedPositions);
+        }
 
         initAdapterDataObserver();
     }
@@ -182,6 +191,15 @@ public boolean isSelected(int position) {
         return mSelectedPositions.contains(position);
     }
 
+    /**
+     * 获取被选中项的数量。
+     *
+     * @return 被选中项的数量。
+     */
+    public int getSelectedCount() {
+        return mSelectedPositions.size();
+    }
+
     /**
      * 获取所有已被选中的列表项的索引值。
      *