title: How to get CABasicAnimation progress?
body: <p>For <code>NSAnimation</code> in developer documentation I found the <code>-addProgressMark</code> method. For <code>CABasicAnimation</code> I didn't find anything similar.</p>
  
  <pre><code>CABasicAnimation * rotateLayer = [CABasicAnimation animationWithKeyPath:@"transform.rotation.x"];
  </code></pre>
  
  <p>When animation progress <code>rotateLayer</code> = 50% I want do something...
  How do I do this?</p>