Build AVMutableComposition from AVURLAssets in loop - ios

Question

I'm working on an app that will need to concat a group of videos recorded from camera. Ultimately I'll have an array of URL's to work with but I can't figure out how to get two movie assets to concat properly. Here's some standalone code:
- (void)buildComposition {
NSString *path1 = [[NSBundle mainBundle] pathForResource:#"IMG_1049" ofType:#"MOV"];
NSString *path2 = [[NSBundle mainBundle] pathForResource:#"IMG_1431" ofType:#"MOV"];
NSURL *url1 = [NSURL fileURLWithPath:path1];
NSURL *url2 = [NSURL fileURLWithPath:path2];
AVMutableComposition *composition = [AVMutableComposition composition];
AVMutableVideoCompositionInstruction *compositionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
NSMutableArray *layerInstructions = [NSMutableArray array];
CGSize renderSize = CGSizeZero;
NSUInteger count = 0;
for (NSURL *url in #[url1, url2]) {
NSDictionary *options = #{ AVURLAssetPreferPreciseDurationAndTimingKey: #(YES) };
AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:url options:options];
CMTimeRange editRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(1.0, 600));
NSError *error = nil;
CMTime insertionTime = composition.duration;
NSArray *videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
AVAssetTrack *videoTrack = videoTracks.firstObject;
AVMutableCompositionTrack *videoCompositionTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[videoCompositionTrack insertTimeRange:editRange ofTrack:videoTrack atTime:insertionTime error:&error];
if (count == 0) {
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
CGAffineTransform scale = CGAffineTransformMakeScale(0.6, 0.6);
[layerInstruction setTransform:CGAffineTransformConcat(videoTrack.preferredTransform, scale) atTime:kCMTimeZero];
[layerInstructions addObject:layerInstruction];
}
else {
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
CGAffineTransform scale = CGAffineTransformMakeScale(0.9, 0.9);
[layerInstruction setTransform:CGAffineTransformConcat(videoTrack.preferredTransform, scale) atTime:kCMTimeZero];
[layerInstructions addObject:layerInstruction];
}
// set the render size
CGRect transformed = CGRectApplyAffineTransform(CGRectMakeWithCGSize(videoTrack.naturalSize), videoTrack.preferredTransform);
renderSize = CGSizeUnion(renderSize, transformed.size);
NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];
AVAssetTrack *audioTrack = audioTracks.firstObject;
AVMutableCompositionTrack *audioCompositionTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[audioCompositionTrack insertTimeRange:editRange ofTrack:audioTrack atTime:insertionTime error:&error];
++count;
}
// set the composition instructions
compositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, composition.duration);
compositionInstruction.layerInstructions = layerInstructions;
AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoCompositionWithPropertiesOfAsset:composition];
videoComposition.frameDuration = CMTimeMake(1, 30);
videoComposition.instructions = #[compositionInstruction];
videoComposition.renderSize = renderSize;
// export the composition
NSTimeInterval time = [NSDate timeIntervalSinceReferenceDate];
NSString *filename = [[NSString stringWithFormat:#"video-export-%f", time] stringByAppendingPathExtension:#"mov"];
NSString *pathTo = [NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:#"Documents/%#", filename]];
NSURL *fileUrl = [NSURL fileURLWithPath:pathTo];
AVAssetExportSession *assetExport = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetHighestQuality];
assetExport.videoComposition = videoComposition;
assetExport.outputFileType = AVFileTypeQuickTimeMovie;
assetExport.shouldOptimizeForNetworkUse = YES;
assetExport.outputURL = fileUrl;
[assetExport exportAsynchronouslyWithCompletionHandler:^{
switch (assetExport.status) {
case AVAssetExportSessionStatusFailed:
NSLog(#"\n\nFailed: %#\n\n", assetExport.error);
break;
case AVAssetExportSessionStatusCancelled:
NSLog(#"\n\nCancelled: %#\n\n", assetExport.error);
break;
default:
NSLog(#"\n\nExported: %#\n\n", fileUrl);
break;
}
}];
}
What I expect to happen is the first video plays for 1 second at 60% scale, and then the second video plays for 1 second at 90% scale.
What actually happens is the first video plays at both 60% and 90% at the start of the video. After 1 second, the video goes black but the audio plays correctly.
Any ideas? Thanks!

Answer 1

Figured it out for anyone who is curious. In my layer instructions, I was mistakenly building them using the AVURLAsset's videoTrack, not the AVMutableComposition's compositionTrack!
This line:
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
Should be:
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompositionTrack];