I'm subclassing CALayer to provide my own drawing in method. For optimization I call -[MyLayer setNeedsDisplayInRect:] instead of -[MyLayer setNeedsDisplay]. In the drawing method I get the rect which should be redrawn via CGContextGetClipBoundingBox().
If I use this layer as the base layer of an UIView every thing works as expected. The problem arises, as soon as I use my custom layer as a sublayer of an other CALayer. Than CGContextGetClipBoundingBox() always returns the rect of the bounds of that layer.
Any ideas?
[EDIT]
It seems, that there is no guaranty, that the content of the CALayer is cached and only the dirty part gets redrawn. I did a small test and stored the rect that needs display as a separate property. The result was, that only this part was visible on the screen.
I'll now render to an image context and keep that image as a cache. In the draw method I'll only display the cached image.