Layout as the name，is to affirm the position of View，it include the position of View and it children view.
source code flow
layout method，call setFrame method to confirm own position，then call children View’s onLayout method to confirm their position.
onLayout method are different by different view，because different view have different child layout，for example, linearlayout have vertical and horizon two mode.
Draw will draw the view to screen
1.draw background bacground.draw(canvas)
2.draw itself onDraw
3.draw children dispatchDraw
4.draw decorate onDrawScrollBars
If call it to pass a true param, system will not draw this view，it’s false in most time，we can use it when the customer ViewGroup don’t need draw，can let system do some optimize.