title: Does AVQueuePlayer support asynchronously adding AVPlayerItems?
body: <p>I have a video player that needs to play a sequence of videos from the network. The URLs for these videos are not known in advance, as they come from XML or JSON responses from other HTTP requests.</p>
  
  <p>As the video URLs become known, I create <code>AVPlayerItems</code> and add them to an <code>AVQueuePlayer</code>. I observe the player status and the item status to determine which item is playing etc.</p>
  
  <p>The problem is that the player seems to get into various weird states where it returns  <code>AVFoundationErrorDomain error -11800</code> and from then on the player refuses to play anything. Even after deallocating the <code>AVQueuePlayer</code> and starting a new one, videos that previously played refuse to play.</p>
  
  <p>Is adding new <code>AVPlayerItems</code> to a playing <code>AVQueuePlayer</code> a supported operation, or should I be doing this another way? If it is supported, what should I be looking out for/doing (Eg. thread issues? KVO issues?) to make this work?</p>