Hi, 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