title: AVVideoCompositionCoreAnimationTool and custom CALayer: setNeedsDisplay each frame?
body: <p>I'm using the <code>AVVideoCompositionCoreAnimationTool</code> with a <code>AVAssetExportSession</code> to draw overlays into a video on iPhone.</p>
  
  <p>particularly i'm looking to draw something really similar to Timecode into the video.  I can do this witha custom <code>CALayer</code> (or layer delegate, either way). I have this working with one issue,</p>
  
  <p>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 <code>drawInContext</code> call.</p>
  
  <p>Is there someway to ensure that <code>drawInContext</code> is called before rendering each frame of the video during export?</p>
  
  <p>Thanks!</p>