Android Programming: The Big Nerd Ranch Guide, 4th Edition

Rendering Inside onDraw(Canvas)

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() function. 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, the top-level View is no longer invalid.

You can also manually specify that a view is invalid, even if it is currently on the screen. This will cause the system to redraw the view with any necessary updates. You will mark the BoxDrawingView as invalid any time the user creates a new box ...