iOS 5: Error merging 3 videos with AVAssetExportSession - ios

Question

I'm trying to merge (append) 3 videos using AVAssetExportSession, but I keep getting this error. Weirdly for 1 or 2 videos it worked.
Error Domain=AVFoundationErrorDomain Code=-11820 "Cannot Complete Export" UserInfo=0x458120 {NSLocalizedRecoverySuggestion=Try exporting again., NSLocalizedDescription=Cannot Complete Export}
I even tried to redo the function in case of error but what I got is only infinite error message. This is the snippet of my code.
AVMutableComposition *mixComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
NSError * error = nil;
NSMutableArray * timeRanges = [NSMutableArray arrayWithCapacity:arrayMovieUrl.count];
NSMutableArray * tracks = [NSMutableArray arrayWithCapacity:arrayMovieUrl.count];
for (int i=0; i<[arrayMovieUrl count]; i++) {
AVURLAsset *assetClip = [arrayMovieUrl objectAtIndex:i];
AVAssetTrack *clipVideoTrackB = [[assetClip tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[timeRanges addObject:[NSValue valueWithCMTimeRange:CMTimeRangeMake(kCMTimeZero, assetClip.duration)]];
[tracks addObject:clipVideoTrackB];
}
[compositionTrack insertTimeRanges:timeRanges ofTracks:tracks atTime:kCMTimeZero error:&error];
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPreset1280x720];
NSParameterAssert(exporter != nil);
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.outputURL = outputUrl;
[exporter exportAsynchronouslyWithCompletionHandler:^{
switch ([exporter status]) {
case AVAssetExportSessionStatusFailed:
NSLog(#"Export failed: %#", [exporter error]);
break;
case AVAssetExportSessionStatusCancelled:
NSLog(#"Export canceled");
break;
case AVAssetExportSessionStatusCompleted:
NSLog(#"Export successfully");
break;
default:
break;
}
if (exporter.status != AVAssetExportSessionStatusCompleted){
NSLog(#"Retry export");
[self renderMovie];
}
}];
Is there something wrong with my code or iOS 5 has some bug?

Answer 1

I've found the problem. So the problem was actually because I use AVPlayerLayer to display each video in preview mode simultaneously. Referring to this question AVPlayerItem fails with AVStatusFailed and error code "Cannot Decode" , there's undocumented limit of maximum 4 simultaneous AVPlayer to work. And this limit somehow hinders AVAssetExportSession from working when there's 4 AVPlayer instance at that moment.
The solution is to release the AVPlayer before exporting, or not using AVPlayer altogether.