Android Programming: The Big Nerd Ranch Guide

Rendering Inside onDraw(…)

When your application is launched, all of its views are invalid. This means that they have not drawn anything to the screen. To fix this situation, Android calls the top-level View’s draw() method. This causes that view to draw itself, which causes its children to draw themselves. Those children’s children then draw themselves, and so on down the hierarchy. When all the views in the hierarchy have drawn themselves, then the top-level View is no longer invalid.

To hook into this drawing, you override the following View method:

    protected void onDraw(Canvas canvas)

The call to invalidate() that you make in response to ACTION_MOVE in onTouchEvent(…) makes the BoxDrawingView invalid again. This causes it to redraw itself ...