Exporting Multiple Tracks: AVMutableComposition and AVAssetExportSession

Question

I have 4 videos files which I have edited and added to a mutable composition. I am trying to use the export session to export the files, however, when I am exporting, only the first track in the list of tracks below gets exported

<AVAssetExportSession: 0x60800001da50, asset = <AVMutableComposition: 0x6080002240a0 tracks = (
    "<AVMutableCompositionTrack: 0x600000224ca0 trackID = 1, mediaType = vide, editCount = 8>",
    "<AVMutableCompositionTrack: 0x600000226da0 trackID = 2, mediaType = vide, editCount = 10>",
    "<AVMutableCompositionTrack: 0x60000023e180 trackID = 3, mediaType = vide, editCount = 3>",
    "<AVMutableCompositionTrack: 0x60000023e500 trackID = 4, mediaType = vide, editCount = 7>"
)>, presetName = AVAssetExportPreset1280x720, outputFileType = (null)

Only the first track with trackID = 1 gets exported. Here is the export session source:

   // Create path to output file
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                             [NSString stringWithFormat:@"ProcessedVideo-%d.mov", arc4random() % 1000]];
    NSURL *url = [NSURL fileURLWithPath:myPathDocs];

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:batchComposition presetName:AVAssetExportPreset1280x720];

    NSLog(@"%@", exporter);

    exporter.outputURL = url;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;

    [exporter exportAsynchronouslyWithCompletionHandler:^(void) {
        switch (exporter.status) {
            case AVAssetExportSessionStatusCompleted:
                NSLog(@"Completed");
                break;
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Failed:%@",exporter.error);
                break;
            case AVAssetExportSessionStatusCancelled:
                NSLog(@"Canceled:%@",exporter.error);
                break;
            default:
                break;
        }
    }];

Does anyone have any ideas how I can get all 4 tracks to export to a single .mov file using export session?

Answer 1

Thanks to jlw for his help. What I found the problem was that I was adding multiple video tracks to mutable composition. What instead I should have done is made a single video track and applied all edits from the other asset tracks onto the single video tracks. Seems AVAssetExportSession will only export a single track as noted by jlw.

Summary:

1. Create mutable composition
2. Create mutable composition track
3. Apply asset tracks to the composition (insertTimeRange)
4. Export the mutable composition