CATiledLayer drawInContext called after associated view is gone

Question

I ran into an interesting iOS problem today involving a CATiledLayer. This only happend on the device - not in the simulator.

My view draws in its CALayer via the drawLayer: inContext: delegate callback. This layer has a CATiledLayer-derived sublayer, which does its own drawing in an overridden drawInContext: method.

Both layers are rendering pdf content via CGContextDrawPDFPage(). (The CALayer draws a low res version, while the CATiledLayer sublayer draws hi-res content over the top.)

I ran into a scenario where I would be done with the view - would remove it from its superview and release it. dealloc() is called on the view. Sometime later, the CATiledLayer's drawInContext: method would be called (on a background thread), by the system. It would draw, but on return from the method Springboard would crash, and in doing so, bring down my app as well.

I fixed it by setting a flag in the CATiledLayer, telling it not to render anymore, from the view's dealloc method.

But I can only imagine there is a more elegant way. How come the CATiledLayer drawInContext: method was still called after the parent layer, and the parent-layer's view were deallocated? What is the correct way to shut down the view so this doesn't happen?

Answer 1

The slow, but best way to fix is to also set view.layer.contents = nil. This waits for the threads to finish.