-[view.layer setNeedsDisplay]
-[layer.delegate drawLayer:incontext:]
CALayer uploads backing store to GPU
CALayer creats backing store(CGContextRef)
layer has a delegate
-[CALayer display]
layer.delegate responseTo -displayLayer:
-[CALayer drawInContext:]
-[UIView drawRect:]
-[layer.delegate displayLayer:]
-[UIview setNeedsDisplay]
finish
