View的draw过程源码分析

/**
     *
     * 原创作者:
	 * 谷哥的小弟 http://blog.csdn.net/lfdfhl
	 *
	 * 文档描述:
	 * View的draw过程源码分析
	 *
	 * 绘制过程分为六步:
	 *
     * 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)
     *
     * 其中第2步和第5步可以忽略,正如源码中所说的:
     * skip step 2 & 5 if possible (common case)
     *
     * 所以关注的是第1,3,4,6步.
     *
     */
    public void draw(Canvas canvas) {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
        }

        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;

        /**
         * 第1步:
         * draw the background, if needed
         * 绘制View的背景
         */
        int saveCount;

        if (!dirtyOpaque) {
            final Drawable background = mBGDrawable;
            if (background != null) {
                final int scrollX = mScrollX;
                final int scrollY = mScrollY;

                if (mBackgroundSizeChanged) {
                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
                    mBackgroundSizeChanged = false;
                }

                if ((scrollX | scrollY) == 0) {
                    background.draw(canvas);
                } else {
                    canvas.translate(scrollX, scrollY);
                    background.draw(canvas);
                    canvas.translate(-scrollX, -scrollY);
                }
            }
        }

        /**
         * 第2,5步:
         * skip step 2 & 5 if possible (common case)
         * 忽略这两步.
         */
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
        	/**
        	 * 第3步:
        	 * draw the content
        	 * 调用onDraw()绘制View的内容
        	 * 在View和ViewGroup的源码中onDraw方法均为空.
        	 * 因为每个View的内容不同,所以需要由具体的子类去实现
        	 */
            if (!dirtyOpaque) onDraw(canvas);

            /**
             * 第4步:
             * draw the children.
             * 调用dispatchDraw()绘制View的子View.
             * 在View中dispatchDraw()方法是一个空方法,因为View没有孩子.
             * 但是ViewGroup重写了该方法,因为ViewGroup有子View
             */
            dispatchDraw(canvas);

            /**
             * 第6步:
             * draw decorations (scrollbars)
             * 绘制View的滚动条.
             * 其实View都是有滚动条的,但一般情况下将其隐藏了,没有显示
             */
            onDrawScrollBars(canvas);

            return;
        }

        //省略其余代码.........
        //省略其余代码.........
        //省略其余代码.........
        //省略其余代码.........

    }