title: iOS AVPlayer replaceCurrentItemWithPlayerItem:nil block UI Thread
body: <p>It seems that the api replaceCurrentItemWithPlayerItem: will stuck the main thread for some seconds, I understand that replacing the item need the information of the new item which might take some time to preload. But questions come up that why replaceCurrentItemWithPlayerItem: with a <strong>nil item object</strong> would also stuck the main thread?? It happens to me that sometimes it take more than 5 seconds to replace a nil playerItem.
  I wonder what can I do to avoid the issue. Thanks for any advices!</p>