Android : Get A Custom View To Redraw Partially?

I have a custom view that fills my entire screen. (A piano keyboard) When a user touches the key, it causes invalidate() to be called and the whole keyboard gets redrawn to show the new state with a touched key.

Currently the view is very simple, but I plan to add a bit more nice graphics. Since the whole keyboard is dynamically rendered this would make redrawing the entire keyboard more expensive.

So I thought, let's look into partial redrawing. Now I call invalidate(Rect dirty) with the correct dirty region. I set my onDraw(Canvas canvas) method to only draw the keys in the dirty region if I do indeed want a partial redraw. This results in those keys being drawn, but the rest of the keyboard is totally black/not drawn at all.

Am I wrong in expecting that calling invalidate(Rect dirty) would "cache" the current canvas, and only "allows" drawing in the dirty region?

Is there any way I can achieve what I want? (A way to "cache" the canvas and only redraw the dirty area?"