How to draw a moving rectangle in anroid ?

Question

I want to start android game development and I want to use some sort of canvas where you can draw shapes and possibly images.I have read about the onDraw method so I created a costume view and I have overridden the onDraw method to make that "canvas". But the problem is that I can't call the onDraw method to refresh my "canvas". I have also read that apparently validate() or postValidate should do the trick but they do nothing.

So I am asking for help by either an explanation or an example code (which would be better)

Thanks in advance