IPhone avcomposition issue - iphone

Question

Im trying to create a video that shows two videos one after the other using avcomposition on the iphone. This code works, however i can only see one of the videos for the entire duration of the newly created video
- (void) startEdit{
AVMutableComposition* mixComposition = [AVMutableComposition composition];
NSString* a_inputFileName = #"export.mov";
NSString* a_inputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:a_inputFileName];
NSURL* a_inputFileUrl = [NSURL fileURLWithPath:a_inputFilePath];
NSString* b_inputFileName = #"output.mov";
NSString* b_inputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:b_inputFileName];
NSURL* b_inputFileUrl = [NSURL fileURLWithPath:b_inputFilePath];
NSString* outputFileName = #"outputFile.mov";
NSString* outputFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:outputFileName];
NSURL* outputFileUrl = [NSURL fileURLWithPath:outputFilePath];
if ([[NSFileManager defaultManager] fileExistsAtPath:outputFilePath])
[[NSFileManager defaultManager] removeItemAtPath:outputFilePath error:nil];
CMTime nextClipStartTime = kCMTimeZero;
AVURLAsset* a_videoAsset = [[AVURLAsset alloc]initWithURL:a_inputFileUrl options:nil];
CMTimeRange a_timeRange = CMTimeRangeMake(kCMTimeZero,a_videoAsset.duration);
AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[a_compositionVideoTrack insertTimeRange:a_timeRange ofTrack:[[a_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];
nextClipStartTime = CMTimeAdd(nextClipStartTime, a_timeRange.duration);
AVURLAsset* b_videoAsset = [[AVURLAsset alloc]initWithURL:b_inputFileUrl options:nil];
CMTimeRange b_timeRange = CMTimeRangeMake(kCMTimeZero, b_videoAsset.duration);
AVMutableCompositionTrack *b_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[b_compositionVideoTrack insertTimeRange:b_timeRange ofTrack:[[b_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];
AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetLowQuality];
_assetExport.outputFileType = #"com.apple.quicktime-movie";
_assetExport.outputURL = outputFileUrl;
[_assetExport exportAsynchronouslyWithCompletionHandler:
^(void ) {
[self saveVideoToAlbum:outputFilePath];
}
];
}
- (void) saveVideoToAlbum:(NSString*)path{
if(UIVideoAtPathIsCompatibleWithSavedPhotosAlbum(path)){
UISaveVideoAtPathToSavedPhotosAlbum (path, self, #selector(video:didFinishSavingWithError: contextInfo:), nil);
}
}
- (void) video: (NSString *) videoPath didFinishSavingWithError: (NSError *) error contextInfo: (void *) contextInfo {
NSLog(#"Finished saving video with error: %#", error);
}
I've posted the whole code as it may help someone else.
Shouldn't
nextClipStartTime = CMTimeAdd(nextClipStartTime, a_timeRange.duration);
[b_compositionVideoTrack insertTimeRange:b_timeRange ofTrack:[[b_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];
add the second video to the end of the first
Cheers

Answer 1

Figured it out. Should have only had one AVMutableCompositionTrack.
Like so:
CMTime nextClipStartTime = kCMTimeZero;
AVURLAsset* a_videoAsset = [[AVURLAsset alloc]initWithURL:a_inputFileUrl options:nil];
CMTimeRange a_timeRange = CMTimeRangeMake(kCMTimeZero,a_videoAsset.duration);
AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[a_compositionVideoTrack insertTimeRange:a_timeRange ofTrack:[[a_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];
nextClipStartTime = CMTimeAdd(nextClipStartTime, a_timeRange.duration);
AVURLAsset* b_videoAsset = [[AVURLAsset alloc]initWithURL:b_inputFileUrl options:nil];
CMTimeRange b_timeRange = CMTimeRangeMake(kCMTimeZero, b_videoAsset.duration);
[a_compositionVideoTrack insertTimeRange:b_timeRange ofTrack:[[b_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];

Answer 2

I haven't spotted the flaw yet, but I do have a couple of suggestions:
First, capture the error in insertTimeRange and every other opportunity and inspect it.
Second, for the simple case of appending videos, you can use AVMutableComposition without so much track mucking. Use "insertTimeRange:ofAsset:atTime:error:" with AVAssets initialized from the files and you will simplify your code greatly. If you need to do something more complicated such as crossfades, you'll need to use a video composition and audio mix as well, and at that point you can deal with the complexity of tracks.