Combining multiple videos in iOS using AVMutableVideoComposition

• Object AVMutableComposition . It will have our paths.
  AVMutableComposition

  AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];

• An array of instructions for tracks and an AVMutableVideoCompositionInstruction object for them.
  AVMutableVideoCompositionInstruction

  NSMutableArray *arrayInstruction = [[NSMutableArray alloc] init];
  AVMutableVideoCompositionInstruction *MainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];

• Common audio track. The variable isSound indicates whether it is needed here (according to the task, the user could record video with or without sound).
  
  Common audio track

     AVMutableCompositionTrack *audioTrack;
     if(self.isSoundOn==YES)
  		audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
  											preferredTrackID:kCMPersistentTrackID_Invalid];

• A variable for storing duration (and also it is needed to determine the boundaries of gluing).

  	CMTime duration = kCMTimeZero;

• We go over all the files and create assets for each of them. All this can and should be done in a loop, the variable i-here is the file index.
  
  Asset Creation

  I recorded all the videos in a tempo directory, while increasing the value of i. Accordingly, assets are created from these files.

  AVAsset *currentAsset = [AVAsset assetWithURL:[NSURL fileURLWithPath:[NSString stringWithFormat:@"%@%@%d%@", NSTemporaryDirectory(), @"Movie",i,@".mov"]]];

• In the loop, AVMutableCompositionTrack is created , in each of which a CMTimeRange time interval from the created asset is inserted.
  Track creation

  //VIDEO TRACK
  AVMutableCompositionTrack *currentTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
  [currentTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, currentAsset.duration) ofTrack:[[currentAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:duration error:nil];

  Particular attention should be paid to where exactly you want to insert the segment, if you want to avoid black bars during playback, or even worse, overlapping one video on another during playback. And do not even ask why I focus on this.

• AVMutableVideoCompositionLayerInstruction is used to create instructions for a specific layer of the track. It is in him that a possible transformation is recorded.
  AVMutableVideoCompositionLayerInstruction

  AVMutableVideoCompositionLayerInstruction *currentAssetLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:currentTrack];

  With the correct definition of the position of the video (portrait or landscapescaped) at the output, you can get a gorgeous picture in which the horizontal or vertical video will be centered depending on the track in which mode you go first.

• An AVAssetTrack object is needed to create a track based on an existing asset of type AVMediaTypeVideo .
  
  AVAssetTrack

  AVAssetTrack *currentAssetTrack = [[currentAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

• Increase the duration value .
  
  duration UP

  CMTime even requires a separate post, of course.

  duration=CMTimeAdd(duration, currentAsset.duration);

• timeRange

  The total duration of the video track.

  MainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, duration);

• layerInstructions

  Here we need an array of instructions generated in the second stage.

  MainInstruction.layerInstructions = arrayInstruction;

• MainCompositionInst

  We create an instance of MainCompositionInst with the necessary parameters

  		AVMutableVideoComposition *MainCompositionInst = [AVMutableVideoComposition videoComposition];
  		MainCompositionInst.instructions = [NSArray arrayWithObject:MainInstruction];
  		MainCompositionInst.frameDuration = CMTimeMake(1, 30);
  		MainCompositionInst.renderSize = CGSizeMake(320.0, 480.0);

• Where to save something?

  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];

• We set quality and create a session

  		NSString *quality = AVAssetExportPresetHighestQuality;
  		if(self.isHighQuality==NO)
  		quality = AVAssetExportPresetMediumQuality;
  		AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:quality]; 

• Save!

  exporter.outputURL=url;
  		exporter.outputFileType = AVFileTypeQuickTimeMovie;
  		exporter.videoComposition = MainCompositionInst;
  		exporter.shouldOptimizeForNetworkUse = YES;
  		[exporter exportAsynchronouslyWithCompletionHandler:^
  		 { //здесь можно удалить темповые файлы.
  }];

		 switch (exporter.status)
			 {
				 case AVAssetExportSessionStatusCompleted:
				 NSLog(@"Completed exporting!");
				 break;
				 case AVAssetExportSessionStatusFailed:
				 NSLog(@"Failed:%@", exporter.error.description);
				 break;
				 case AVAssetExportSessionStatusCancelled:
				 NSLog(@"Canceled:%@", exporter.error);
				 break;
				 case AVAssetExportSessionStatusExporting:
				 NSLog(@"Exporting!");
				 break;
				 case AVAssetExportSessionStatusWaiting:
				 NSLog(@"Waiting");
				 break;
				 default:
				 break;
			 }

Conclusion