merge video

gistfile1.m

 if (firstAsset !=nil && secondAsset!=nil) {
        [activityView startAnimating];
        // 1 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.
        AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
        // 2 - Video track
        AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo 
            preferredTrackID:kCMPersistentTrackID_Invalid];
        [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) 
            ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
        [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, secondAsset.duration) 
            ofTrack:[[secondAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:firstAsset.duration error:nil];
        // 3 - Audio track
        if (audioAsset!=nil){
            AVMutableCompositionTrack *AudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio 
                preferredTrackID:kCMPersistentTrackID_Invalid];
            [AudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeAdd(firstAsset.duration, secondAsset.duration)) 
                ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];
        } 
        // 4 - Get path
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectory = [paths objectAtIndex:0];
        NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
            [NSString stringWithFormat:@"mergeVideo-%d.mov",arc4random() % 1000]];
        NSURL *url = [NSURL fileURLWithPath:myPathDocs];
        // 5 - Create exporter
        AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition 
            presetName:AVAssetExportPresetHighestQuality];
        exporter.outputURL=url;
        exporter.outputFileType = AVFileTypeQuickTimeMovie;
        exporter.shouldOptimizeForNetworkUse = YES;
        [exporter exportAsynchronouslyWithCompletionHandler:^{
             dispatch_async(dispatch_get_main_queue(), ^{
                 [self exportDidFinish:exporter];
             });
         }];
    }