title: UIViewController: Is there a way to know when the modal transition animation is finished?
body: <p>In my app, I have a UIViewController presented modally with the default slide animation - let's call it 'A'.  During the time 'A' is busy popping up it's possible another piece of code will try to use A to present a further modal viewcontroller.<br>
  On these occasions, I get an exception, presumably as 'A' isn't fully ready yet.</p>
  
  <p>Does anyone know if there's a way of knowing if a UIViewController is still animating?  </p>
  
  <p>Thanks.</p>