UIScrollView and CATiledLayer question

Question

I am using a CATiledLayer as content of UIScrollView, but seems I can not get the correct (sometimes) [scrollView contentSize] and [scrollView contentOffset] from CATiledLayer's delegate method :

• (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx

I know CATiledLayer uses this method to render tile in a background thread. And I need to use contentSize and contentOffset to calculate some data (some coordinates). The app ran fine on simulator but on device, it sometimes got the wrong contentSize and contentOffset