title: How to build UIActivityIndicatorView
body: <ul>
  <li>When using a <code>UIActivityIndicatorView</code>, it is possible to add this view to your view hierarchy and start animating it. </li>
  <li>After that you are completely save to block the <code>main thread</code> (not that this is a good approach), there is basically nothing that I know of, that could stop the <code>activity indicator</code> from spinning.</li>
  </ul>
  
  <p>My question is: </p>
  
  <ol>
  <li>How is this done? It seems like <code>UIActivityIndicatorView</code> uses its own thread for rendering. </li>
  <li>Is this something that can be achieved with my own views? </li>
  <li>I have a <code>CoreAnimation</code> animation that I want to keep playing while the main thread might be blocking for a couple of milliseconds.</li>
  </ol>
  
  <p>How to do that? Thanks for any help or ideas!</p>
  
  <p>//EDIT: To clarify my question: I want to know what Apple does to get <code>UIActivityIndicatorView</code> animating even when you block the main thread. When I trigger my own CoreAnimation and I block the main thread, the animation itself stops. Furthermore to the question what Apple does to achieve that under the hood, I want to know, if I can achieve this myself, with public API.</p>