/*
* 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)
*/
ViewGroup draw()
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绘制背景 drawBackground(canvas)
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绘制自己onDraw(canvas)
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绘制子View dispatchDraw(canvas)
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绘制前景,滚动条等装饰onDrawForeground(canvas)
View draw()
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绘制背景 drawBackground(canvas)
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绘制自己onDraw(canvas)
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绘制前景,滚动条等装饰onDrawForeground(canvas)