How to use AVAssetReader and AVAssetWriter for multiple tracks (audio and video) simultaneously?

Question

I know how to use AVAssetReader and AVAssetWriter, and have successfully used them to grab a video track from one movie and transcode it into another. However, I'd like to do this with audio as well. Do I have to create and AVAssetExportSession after I've done with the initial transcode, or is there some way to switch between tracks while in the midst of a writing session? I'd hate to have to deal with the overhead of an AVAssetExportSession.

I ask because, using the pull style method - while ([assetWriterInput isReadyForMoreMediaData]) {...} - assumes one track only. How could it be used for more than one track, i.e. both an audio and a video track?

Answer 1

AVAssetWriter will automatically interleave requests on its associated AVAssetWriterInputs in order to integrate different tracks into the output file. Just add an AVAssetWriterInput for each of the tracks that you have, and then call requestMediaDataWhenReadyOnQueue:usingBlock: on each of your AVAssetWriterInputs.

Here's a method I have that calls requestMediaDataWhenReadyOnQueue:usingBlock:. I call this method from a loop over the number of output/input pairs I have. (A separate method is good both for code readability and also because, unlike a loop, each call sets up a separate stack frame for the block.)

You only need one dispatch_queue_t and can reuse it for all of the tracks. Note that you definitely should not call dispatch_async from your block, because requestMediaDataWhenReadyOnQueue:usingBlock: expects the block to, well, block until it has filled in as much data as the AVAssetWriterInput will take. You don't want to return before then.

- (void)requestMediaDataForTrack:(int)i {
  AVAssetReaderOutput *output = [[_reader outputs] objectAtIndex:i];
  AVAssetWriterInput *input = [[_writer inputs] objectAtIndex:i];

  [input requestMediaDataWhenReadyOnQueue:_processingQueue usingBlock:
    ^{
      [self retain];
      while ([input isReadyForMoreMediaData]) {
        CMSampleBufferRef sampleBuffer;
        if ([_reader status] == AVAssetReaderStatusReading &&
            (sampleBuffer = [output copyNextSampleBuffer])) {

          BOOL result = [input appendSampleBuffer:sampleBuffer];
          CFRelease(sampleBuffer);

          if (!result) {
            [_reader cancelReading];
            break;
          }
        } else {
          [input markAsFinished];

          switch ([_reader status]) {
            case AVAssetReaderStatusReading:
              // the reader has more for other tracks, even if this one is done
              break;

            case AVAssetReaderStatusCompleted:
              // your method for when the conversion is done
              // should call finishWriting on the writer
              [self readingCompleted];
              break;

            case AVAssetReaderStatusCancelled:
              [_writer cancelWriting];
              [_delegate converterDidCancel:self];
              break;

            case AVAssetReaderStatusFailed:
              [_writer cancelWriting];
              break;
          }

          break;
        }
      }
    }
  ];
}