title: generateCGImagesAsynchronouslyForTimes in ARC
body: <p>If I run the following in a project where ARC is enabled the completion handler never fires. But without ARC it works as expected. What am I missing here?</p>
  
  <pre><code>NSURL *url = [NSURL URLWithString:@"http://media.w3.org/2010/05/sintel/trailer.mp4"];
  AVURLAsset *asset=[[AVURLAsset alloc] initWithURL:url options:nil];
  AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
  generator.appliesPreferredTrackTransform = YES;
  CMTime thumbTime = CMTimeMakeWithSeconds(5,30);
  
  AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, 
                                                     CGImageRef im, 
                                                     CMTime actualTime, 
                                                     AVAssetImageGeneratorResult result, 
                                                     NSError *error){
      NSLog(@"completion handler");
  };
  
  generator.maximumSize = CGSizeMake(320, 180);
  [generator generateCGImagesAsynchronouslyForTimes:[NSArray arrayWithObject:[NSValue valueWithCMTime:thumbTime]] completionHandler:handler];
  </code></pre>