/**
   * 请求所有子 View 去 measure 自己，要考虑的部分有对子 View 的测绘要求 MeasureSpec 以及其自身的 padding
   * 这里跳过所有为 GONE 状态的子 View，最繁重的工作是在 getChildMeasureSpec 方法中处理的
   *
   * @param widthMeasureSpec 对该 View 的 width 测绘要求
   * @param heightMeasureSpec 对该 View 的 height 测绘要求
   */
  protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
    final int size = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < size; ++i) {
      final View child = children[i];
      if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
        measureChild(child, widthMeasureSpec, heightMeasureSpec);
      }
    }
  }

  protected void measureChild(View child, int parentWidthMeasureSpec,
      int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();//获取 Child 的 LayoutParams

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,// 获取 ChildView 的 widthMeasureSpec
        mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,// 获取 ChildView 的 heightMeasureSpec
        mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
  }

  /** 
   * 该方法是 measureChildren 中最繁重的部分，为每一个 ChildView 计算出自己的 MeasureSpec。
   * 目标是将 ChildView 的 MeasureSpec 和 LayoutParams 结合起来去得到一个最合适的结果。
   *
   * @param spec 对该 View 的测绘要求
   * @param padding 当前 View 在当前唯独上的 paddingand，也有可能含有 margins
   *
   * @param childDimension 在当前维度上（height 或 width）的具体指
   * @return 子视图的 MeasureSpec 
   */
  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {

      .........

    // 根据获取到的子视图的测量要求和大小创建子视图的 MeasureSpec
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode); 
  }

  /**
   *
   * 用于获取 View 最终的大小，父视图提供了宽、高的约束信息
   * 一个 View 的真正的测量工作是在 onMeasure(int, int) 中，由该方法调用。
   * 因此，只有 onMeasure(int, int) 可以而且必须被子类复写
   *
   * @param widthMeasureSpec 在水平方向上，父视图指定的的 Measure 要求
   * @param heightMeasureSpec 在竖直方向上，控件上父视图指定的 Measure 要求
   *
   */
  public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
   ...

   onMeasure(widthMeasureSpec, heightMeasureSpec);

   ...
  }

  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
        getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
  }

@Override
  protected void onLayout(boolean changed, int l, int t, int r, int b) {
    if (mOrientation == VERTICAL) {
      layoutVertical(l, t, r, b);
    } else {
      layoutHorizontal(l, t, r, b);
    }
  }

  /**
   * 遍历所有的子 View，为其设置相对父视图的坐标
   */
  void layoutVertical(int left, int top, int right, int bottom) {
  for (int i = 0; i < count; i++) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
          childTop += measureNullChild(i);
        } else if (child.getVisibility() != GONE) {//不需要立即展示的 View 设置为 GONE 可加快绘制
          final int childWidth = child.getMeasuredWidth();//measure 过程确定的 Width
          final int childHeight = child.getMeasuredHeight();//measure 过程确定的 height

          ...确定 childLeft、childTop 的值

          setChildFrame(child, childLeft, childTop + getLocationOffset(child),
              childWidth, childHeight);
        }
      }
  }

  private void setChildFrame(View child, int left, int top, int width, int height) {    
    child.layout(left, top, left + width, top + height);
  }  

  View.java
  public void layout(int l, int t, int r, int b) {
    ...
    setFrame(l, t, r, b)
  }

  /**
   * 为该子 View 设置相对其父视图上的坐标
   */
   protected boolean setFrame(int left, int top, int right, int bottom) {
    ...
   }

 /**
   * Manually render this view (and all of its children) to the given Canvas.
   * The view must have already done a full layout before this function is
   * called. When implementing a view, implement
   * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
   * If you do need to override this method, call the superclass version.
   *
   * @param canvas The Canvas to which the View is rendered. 
   *
   * 根据给定的 Canvas 自动渲染 View（包括其所有子 View）。在调用该方法之前必须要完成 layout。当你自定义 view 的时候，
   * 应该去是实现 onDraw(Canvas) 方法，而不是 draw(canvas) 方法。如果你确实需要复写该方法，请记得先调用父类的方法。
   */
  public void draw(Canvas canvas) {

    / * Draw traversal performs several drawing steps which must be executed
     * in the appropriate order:
     *
     *   1. Draw the background if need
     *   2. If necessary, save the canvas' layers to prepare for fading
     *   3. Draw view's content
     *   4. Draw children (dispatchDraw)
     *   5. If necessary, draw the fading edges and restore layers
     *   6. Draw decorations (scrollbars for instance)
     */

  // Step 1, draw the background, if needed
    if (!dirtyOpaque) {
      drawBackground(canvas);
    }

     // skip step 2 & 5 if possible (common case)
    final int viewFlags = mViewFlags;
    if (!verticalEdges && !horizontalEdges) {
      // Step 3, draw the content
      if (!dirtyOpaque) onDraw(canvas);

      // Step 4, draw the children
      dispatchDraw(canvas);

      // Step 6, draw decorations (scrollbars)
      onDrawScrollBars(canvas);

      if (mOverlay != null && !mOverlay.isEmpty()) {
        mOverlay.getOverlayView().dispatchDraw(canvas);
      }

      // we're done...
      return;
    }

    // Step 2, save the canvas' layers
    ...

    // Step 3, draw the content
    if (!dirtyOpaque) 
      onDraw(canvas);

    // Step 4, draw the children
    dispatchDraw(canvas);

    // Step 5, draw the fade effect and restore layers

    // Step 6, draw decorations (scrollbars)
    onDrawScrollBars(canvas);
  }

dispatchDraw(Canvas canvas){

...

 if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {//处理 ChildView 的动画
  final boolean buildCache = !isHardwareAccelerated();
      for (int i = 0; i < childrenCount; i++) {
        final View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {//只绘制 Visible 状态的布局，因此可以通过延时加载来提高效率
          final LayoutParams params = child.getLayoutParams();
          attachLayoutAnimationParameters(child, params, i, childrenCount);// 添加布局变化的动画
          bindLayoutAnimation(child);//为 Child 绑定动画
          if (cache) {
            child.setDrawingCacheEnabled(true);
            if (buildCache) {
              child.buildDrawingCache(true);
            }
          }
        }
      }

  final LayoutAnimationController controller = mLayoutAnimationController;
      if (controller.willOverlap()) {
        mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
      }

  controller.start();// 启动 View 的动画
}

 // 绘制 ChildView
 for (int i = 0; i < childrenCount; i++) {
      int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
      final View child = (preorderedList == null)
          ? children[childIndex] : preorderedList.get(childIndex);
      if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
        more |= drawChild(canvas, child, drawingTime);
      }
    }

...

}

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    return child.draw(canvas, this, drawingTime);
}

/**
   * This method is called by ViewGroup.drawChild() to have each child view draw itself.
   * This draw() method is an implementation detail and is not intended to be overridden or
   * to be called from anywhere else other than ViewGroup.drawChild().
   */
  boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
    ...
  }