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.

Answer 1

You can use dispatch groups!

Check out the AVReaderWriter example for MacOSX...

I am quoting directly from the sample RWDocument.m:

- (BOOL)startReadingAndWritingReturningError:(NSError **)outError
{
    BOOL success = YES;
    NSError *localError = nil;

    // Instruct the asset reader and asset writer to get ready to do work
    success = [assetReader startReading];
    if (!success)
        localError = [assetReader error];
    if (success)
    {
        success = [assetWriter startWriting];
        if (!success)
            localError = [assetWriter error];
    }

    if (success)
    {
        dispatch_group_t dispatchGroup = dispatch_group_create();

        // Start a sample-writing session
        [assetWriter startSessionAtSourceTime:[self timeRange].start];

        // Start reading and writing samples
        if (audioSampleBufferChannel)
        {
            // Only set audio delegate for audio-only assets, else let the video channel drive progress
            id  delegate = nil;
            if (!videoSampleBufferChannel)
                delegate = self;

            dispatch_group_enter(dispatchGroup);
            [audioSampleBufferChannel startWithDelegate:delegate completionHandler:^{
                dispatch_group_leave(dispatchGroup);
            }];
        }
        if (videoSampleBufferChannel)
        {
            dispatch_group_enter(dispatchGroup);
            [videoSampleBufferChannel startWithDelegate:self completionHandler:^{
                dispatch_group_leave(dispatchGroup);
            }];
        }

        // Set up a callback for when the sample writing is finished
        dispatch_group_notify(dispatchGroup, serializationQueue, ^{
            BOOL finalSuccess = YES;
            NSError *finalError = nil;

            if (cancelled)
            {
                [assetReader cancelReading];
                [assetWriter cancelWriting];
            }
            else
            {
                if ([assetReader status] == AVAssetReaderStatusFailed)
                {
                    finalSuccess = NO;
                    finalError = [assetReader error];
                }

                if (finalSuccess)
                {
                    finalSuccess = [assetWriter finishWriting];
                    if (!finalSuccess)
                        finalError = [assetWriter error];
                }
            }

            [self readingAndWritingDidFinishSuccessfully:finalSuccess withError:finalError];
        });

        dispatch_release(dispatchGroup);
    }

    if (outError)
        *outError = localError;

    return success;
}