title: Read multiple tracks with AVAssetReader
body: <p>I need to read raw samples of both audio and video from an AVAsset. What is the best way to do this? I can attache multiple AVAssetReaderTrackOutput objects to one AVAssetReader. But [AVAssetReaderOutput copyNextSampleBuffer] is synchronous. What is the best way to read the two tracks in chronological order Is it safe to assign a thread to each AVAssetReaderTrackOutput? </p>