// RecyclerView的measure child方法
public void measureChild(@NonNull View child, int widthUsed, int heightUsed) {
      final LayoutParams lp = (LayoutParams) child.getLayoutParams();
  //將getItemOffsets()獲取的值累加到測(cè)量值之中
      final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
      widthUsed += insets.left + insets.right;
      heightUsed += insets.top + insets.bottom;
      final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
          getPaddingLeft() + getPaddingRight() + widthUsed, lp.width,
          canScrollHorizontally());
      final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
          getPaddingTop() + getPaddingBottom() + heightUsed, lp.height,
          canScrollVertically());
      if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
        child.measure(widthSpec, heightSpec);
      }
    }

@Override
  public void draw(Canvas c) {
    super.draw(c);
 //在RecyclerView繪制完之后回調(diào)onDrawOver()方法
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
      mItemDecorations.get(i).onDrawOver(c, this, mState);
    }
 }

 public void onDraw(Canvas c) {
    super.onDraw(c);
 //先回調(diào)onDraw()方法，在繪制RecyclerView子view
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
      mItemDecorations.get(i).onDraw(c, this, mState);
    }
  }

private void setDecorInsetsBottom(RecyclerView.LayoutParams param, int bottom) {
    try {
      // 找到RecyclerView.LayoutParams中的mDecorInsets屬性值
      Field filed = RecyclerView.LayoutParams.class.getDeclaredField("mDecorInsets");
      filed.setAccessible(true);
      Rect decorRect = (Rect) filed.get(param);
      decorRect.bottom = bottom;
    } catch (Exception e) {
    }
  }