title: A way to detect CATiledLayer?
body: <p>I am using CATiledLayer to display a pdf page..
  But the layer do have some time to draw context.</p>
  
  <p>Therefore, I try to add an background beside this layer and remove the background after CATiledLayer is successful display..</p>
  
  <p>My question is <strong>Is there any ways to detect the drawing status of CATiledLayer?</strong> </p>
  
  <p>Thanks for your help.</p>