invalidate() for call onDraw() dose not work in ViewFlipper

In my android app project,I designed a custom component(extends ImageView).At first It works well,I use invalidate() to call my Ondraw(canvas) method.
But then I decide to refact my code to bring the ImageView extendsed custom component in a ViewFlipper,it dose not show.
From debugging I see that my onDraw(canvas) method dose not excuted at all,even if I used invalidate() in my onlayout().
?I tried postInvalidate() method ,but it not work too?

Any suggestion would be appreciated.

I tried to change my custom component to extends View(originaly extend ImageView),it worked.But I don't know WHY?

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