/**     * 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) {        ...    }