Android View 绘制流程(Draw)步骤详情

View的draw过程
以DecorView为例
Draw的过程依旧是从ViewRootImpl的performTraversals开始，进过一系列的调用，最终调用了View的draw方法，由于ViewGroup没有重写draw方法，因此所有的View都是调用View#draw方法，因此，我们直接看它的源码：

    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

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

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

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

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

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

View的绘制流程有一下六步骤：
1、对View的背景进行绘制：View.drawBackground
2、保存当前图层信息(可跳过)
3、绘制View的内容：View.OnDraw或者重写的OnDraw
4、对View的子View进行绘制(如果有子View)：dispatchDraw 遍历调用所有子元素的draw方法。
5、绘制View的褪色的边缘，类似于阴影效果(可跳过)
6、绘制View的装饰：View.onDrawForeground