Bitmap change will make View auto refresh before onDraw() and regardless invalidate invoke

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http://stackoverflow.com – I had a strange issue about View.onDraw(), I had a Canvas that draw some text with a Bitmap, in onDraw method, I draw that Bitmap use the View hold Canvas. when user touch, I invoke startDragging then make some change of Bitmap, but I never perform View.invalidate(), I thought whatever change of Bitmap wouldn't affect the BoardView UI, because I didn't invalidate it to refresh, but it refresh with new change of Bitmap immediately without onDraw() invoke, I set a breakpoint at onDraw first line and Debugging, but onDraw not invoke. After that, I invoke invalidate() after startDragging(), I stop (HowTos)