title: iphone- [AVAssetReaderAudioMixOutput copyNextSampleBuffer] Error
body: <p>I am working with AVAssetReader and AVAssetWriter to make movie with multi tracks.
  So I finished it using [AVAssetReaderAudioMixOutput copyNextSampleBuffer] and [AVAssetWriterInput appendSampleBuffer].</p>
  
  <p>But I kept getting the error: [AVAssetReaderTrackOutput copyNextSampleBuffer] cannot copy next sample buffer unless the asset reader is in the 'reading' state'.
  How solve this problem.</p>