AVFoundation

- (void)test {
    NSDictionary *options = @{ AVURLAssetPreferPreciseDurationAndTimingKey : @YES };
    NSURL *firstUrl = [[NSBundle mainBundle] URLForResource:@"test1" withExtension:@"MOV"];
    AVURLAsset *firstVideoAsset = [[AVURLAsset alloc] initWithURL:firstUrl options:options];
    NSURL *secondUrl = [[NSBundle mainBundle] URLForResource:@"test2" withExtension:@"MOV"];
    AVURLAsset *secondVideoAsset = [[AVURLAsset alloc] initWithURL:secondUrl options:options];
    
    AVMutableComposition *mutableComposition = [AVMutableComposition composition];
    AVMutableCompositionTrack *videoCompositionTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    AVMutableCompositionTrack *audioCompositionTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    
    AVAssetTrack *firstVideoAssetTrack = [[firstVideoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVAssetTrack *secondVideoAssetTrack = [[secondVideoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVAssetTrack *firstAudioAssetTrack = [[firstVideoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
    AVAssetTrack *secondAudioAssetTrack = [[secondVideoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
    
    [videoCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstVideoAssetTrack.timeRange.duration) ofTrack:firstVideoAssetTrack atTime:kCMTimeZero error:nil];
    [videoCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, secondVideoAssetTrack.timeRange.duration) ofTrack:secondVideoAssetTrack atTime:firstVideoAssetTrack.timeRange.duration error:nil];
    [audioCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAudioAssetTrack.timeRange.duration) ofTrack:firstAudioAssetTrack atTime:kCMTimeZero error:nil];
    [audioCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, secondAudioAssetTrack.timeRange.duration) ofTrack:secondAudioAssetTrack atTime:firstAudioAssetTrack.timeRange.duration error:nil];
    
    AVMutableAudioMix *mutableAudioMix = [AVMutableAudioMix audioMix];
    AVMutableAudioMixInputParameters *mixParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:audioCompositionTrack];
    // Set the volume ramp to slowly fade the audio out over the duration of the composition.
    [mixParameters setVolumeRampFromStartVolume:1.f toEndVolume:0.f timeRange:CMTimeRangeMake(kCMTimeZero, mutableComposition.duration)];
    mutableAudioMix.inputParameters = @[mixParameters];
    
    BOOL isFirstVideoPortrait = NO;
    CGAffineTransform firstTransform = firstVideoAssetTrack.preferredTransform;
    // Check the first video track's preferred transform to determine if it was recorded in portrait mode.
    if (firstTransform.a == 0 && firstTransform.d == 0 && (firstTransform.b == 1.0 || firstTransform.b == -1.0) && (firstTransform.c == 1.0 || firstTransform.c == -1.0)) {
        isFirstVideoPortrait = YES;
    }
    BOOL isSecondVideoPortrait = NO;
    CGAffineTransform secondTransform = secondVideoAssetTrack.preferredTransform;
    // Check the second video track's preferred transform to determine if it was recorded in portrait mode.
    if (secondTransform.a == 0 && secondTransform.d == 0 && (secondTransform.b == 1.0 || secondTransform.b == -1.0) && (secondTransform.c == 1.0 || secondTransform.c == -1.0)) {
        isSecondVideoPortrait = YES;
    }
    if ((isFirstVideoPortrait && !isSecondVideoPortrait) || (!isFirstVideoPortrait && isSecondVideoPortrait)) {
        UIAlertView *incompatibleVideoOrientationAlert = [[UIAlertView alloc] initWithTitle:@"Error!" message:@"Cannot combine a video shot in portrait mode with a video shot in landscape mode." delegate:self cancelButtonTitle:@"Dismiss" otherButtonTitles:nil];
        [incompatibleVideoOrientationAlert show];
        return;
    }

    AVMutableVideoCompositionInstruction *firstVideoCompositionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    // Set the time range of the first instruction to span the duration of the first video track.
    firstVideoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, firstVideoAssetTrack.timeRange.duration);
    AVMutableVideoCompositionInstruction * secondVideoCompositionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    // Set the time range of the second instruction to span the duration of the second video track.
    secondVideoCompositionInstruction.timeRange = CMTimeRangeMake(firstVideoAssetTrack.timeRange.duration, CMTimeAdd(firstVideoAssetTrack.timeRange.duration, secondVideoAssetTrack.timeRange.duration));
    
    AVMutableVideoCompositionLayerInstruction *firstVideoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompositionTrack];
    // Set the transform of the first layer instruction to the preferred transform of the first video track.
    [firstVideoLayerInstruction setTransform:firstTransform atTime:kCMTimeZero];
    AVMutableVideoCompositionLayerInstruction *secondVideoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompositionTrack];
    // Set the transform of the second layer instruction to the preferred transform of the second video track.
    [secondVideoLayerInstruction setTransform:secondTransform atTime:firstVideoAssetTrack.timeRange.duration];
    firstVideoCompositionInstruction.layerInstructions = @[firstVideoLayerInstruction];
    secondVideoCompositionInstruction.layerInstructions = @[secondVideoLayerInstruction];
    
    AVMutableVideoComposition *mutableVideoComposition = [AVMutableVideoComposition videoComposition];
    mutableVideoComposition.instructions = @[firstVideoCompositionInstruction, secondVideoCompositionInstruction];
    
    CGSize naturalSizeFirst, naturalSizeSecond;
    // If the first video asset was shot in portrait mode, then so was the second one if we made it here.
    if (isFirstVideoPortrait) {
        // Invert the width and height for the video tracks to ensure that they display properly.
        naturalSizeFirst = CGSizeMake(firstVideoAssetTrack.naturalSize.height, firstVideoAssetTrack.naturalSize.width);
        naturalSizeSecond = CGSizeMake(secondVideoAssetTrack.naturalSize.height, secondVideoAssetTrack.naturalSize.width);
    }
    else {
        // If the videos weren't shot in portrait mode, we can just use their natural sizes.
        naturalSizeFirst = firstVideoAssetTrack.naturalSize;
        naturalSizeSecond = secondVideoAssetTrack.naturalSize;
    }
    float renderWidth, renderHeight;
    // Set the renderWidth and renderHeight to the max of the two videos widths and heights.
    if (naturalSizeFirst.width > naturalSizeSecond.width) {
        renderWidth = naturalSizeFirst.width;
    }
    else {
        renderWidth = naturalSizeSecond.width;
    }
    if (naturalSizeFirst.height > naturalSizeSecond.height) {
        renderHeight = naturalSizeFirst.height;
    }
    else {
        renderHeight = naturalSizeSecond.height;
    }
    mutableVideoComposition.renderSize = CGSizeMake(renderWidth, renderHeight);
    // Set the frame duration to an appropriate value (i.e. 30 frames per second for video).
    mutableVideoComposition.frameDuration = CMTimeMake(1,30);
    
    // Create a static date formatter so we only have to initialize it once.
    static NSDateFormatter *kDateFormatter;
    if (!kDateFormatter) {
        kDateFormatter = [[NSDateFormatter alloc] init];
        kDateFormatter.dateStyle = NSDateFormatterMediumStyle;
        kDateFormatter.timeStyle = NSDateFormatterShortStyle;
    }
    // Create the export session with the composition and set the preset to the highest quality.
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mutableComposition presetName:AVAssetExportPresetHighestQuality];
    // Set the desired output URL for the file created by the export process.
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"test3.mov"]];
    unlink([myPathDocs UTF8String]);
    NSURL* videoUrl = [NSURL fileURLWithPath:myPathDocs];
    NSFileManager *fm = [NSFileManager defaultManager];
    if ([fm fileExistsAtPath:videoUrl.path]) {
        [fm removeItemAtURL:videoUrl error:nil];
    }
    exporter.outputURL = videoUrl;
    // Set the output file type to be a QuickTime movie.
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = mutableVideoComposition;
    exporter.audioMix = mutableAudioMix;
    // Asynchronously export the composition to a video file and save this file to the camera roll once export completes.
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (exporter.status == AVAssetExportSessionStatusCompleted) {
              //do something
            }
        });
    }];


}