IOS adding transitions between videos - ios

Question

I'm trying to create a video By combining two videos
It works properly until I tried to add transition between videos
It gives me an unplayable video file.
The code :
AVMutableComposition* mixComposition = [AVMutableComposition composition];
NSString* a_inputFileName = #"v1.mp4";
NSString* a_inputFilePath = [NSString stringWithFormat:#"%#/%#",documentsDirectoryPath,a_inputFileName];
NSURL * a_inputFileUrl = [NSURL fileURLWithPath:a_inputFilePath];
NSString* b_inputFileName = #"v2.mp4";
NSString* b_inputFilePath = [NSString stringWithFormat:#"%#/%#",documentsDirectoryPath,b_inputFileName];
NSURL* b_inputFileUrl = [NSURL fileURLWithPath:b_inputFilePath];
NSString* outputFileName = #"outputFile.mp4";
NSString* outputFilePath = [NSString stringWithFormat:#"%#/%#",documentsDirectoryPath,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);
nextClipStartTime = CMTimeSubtract(nextClipStartTime, CMTimeMake(1, 1));
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];
AVMutableVideoCompositionInstruction * instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, mixComposition.duration);
AVMutableVideoCompositionLayerInstruction * firstlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:[[a_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]];
[firstlayerInstruction setOpacityRampFromStartOpacity:1.0f toEndOpacity:0.0f timeRange:CMTimeRangeMake(CMTimeMake(3,1), CMTimeMake(1, 1))];
AVMutableVideoCompositionLayerInstruction * secondlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:[[b_videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]];
[secondlayerInstruction setOpacity:1.0f atTime:kCMTimeZero];
instruction.layerInstructions = [NSArray arrayWithObjects:firstlayerInstruction,secondlayerInstruction, nil];
AVMutableVideoComposition * videoComp = [AVMutableVideoComposition videoComposition];
videoComp.instructions = [NSArray arrayWithObject:instruction];
[videoComp setRenderSize:CGSizeMake(320, 480)];
[videoComp setFrameDuration:CMTimeMake(1, 30)];
AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
_assetExport.videoComposition = videoComp;
_assetExport.outputFileType = AVFileTypeMPEG4;
_assetExport.outputURL = outputFileUrl;
[_assetExport exportAsynchronouslyWithCompletionHandler:
^(void ) {
[self saveVideoToAlbum:outputFilePath];
}
];
If i remove this line : _assetExport.videoComposition = videoComp;
It works properly but no transition between videos.
Edit :
I found out that the exportsession's status is stuck on exporting nd it never finishes and i don't know why.
THX in advance.