[Technotes] invalidate() method

In android system, the class View (android.view.View) is an important interface for rendering UI objects. Inside it, a method called invalidate() plays a key role.

The function of invalidate is simple, it marks a region on the view to be 'dirty', which tells the android system that region must be redrawn soon in the near future. The region can be the whole View or just a designated region passed to invalidate. And obviously, invalidating only a small portion of the view can have some performance advantages for the UI system. Therefore, make sure you use invalidate() wisely!

Also, an important point: never call any drawing functions except in the onDraw( ) method (a method of class View). Instead, you use the invalidate( ) method to mark rectangles as dirty. The window manager will combine all the dirty rectangles at some point in the future and call onDraw( ) again for you. The dirty rectangles become the clip region, so screen updates are optimized to only those areas that change.