AVVideoCompositionCoreAnimationTool and custom CALayer: setNeedsDisplay each frame?

I'm using the AVVideoCompositionCoreAnimationTool with a AVAssetExportSession to draw overlays into a video on iPhone.

particularly i'm looking to draw something really similar to Timecode into the video. I can do this witha custom CALayer (or layer delegate, either way). I have this working with one issue,

The problem is I can't see a way of calling setNeedsDisplay each frame on my custom CALayer so that I can change the type every frame through the dra开发者_Python百科wInContext call.

Is there someway to ensure that drawInContext is called before rendering each frame of the video during export?

Thanks!