title: Use AVFoundation to modify audio metadata
body: <p>I'm working on an app which needs to modify metadata of audio files. I have played with Apple's official demo AVReaderWriterOSX. I have tried to set the metadata of <code>AVAssetWriterInput</code> and <code>AVAssetWriter</code>, but I still can't make it work to write metadata to the output file. Does anyone have any examples for this?</p>
  
  <p>Thank you in advance.</p>