title: check video file size before playing on iphone
body: <p>I am working on an iphone app that downloads a video from a web server and then plays this video.
  my problem is that sometimes the server replies to me with an empty file not a valid video.
  how can i check that this is a valid video file before playing?</p>
  
  <pre><code>    player = [[MPMoviePlayerController alloc] initWithContentURL:url];
      [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(didFinishPlaying:) name:MPMoviePlayerPlaybackDidFinishNotification object:player];
      player.controlStyle = MPMovieControlStyleFullscreen;
      [player play];
  </code></pre>