Cropping a video with AVAsset/AVCaptureSession - ios

Question

I'm trying to crop a video into a square. I already have an AVCaptureSession setup that produces a rectangle resolution .mov file. So I'm just trying to crop it with some code I found here. Here is my code:
-(void)cropVideo:(NSURL*)videoURL{
// input file
AVAsset* asset = [AVAsset assetWithURL:videoURL];
AVMutableComposition *composition = [AVMutableComposition composition];
[composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
// input clip
AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
// make it square
AVMutableVideoComposition* videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.renderSize = CGSizeMake(clipVideoTrack.naturalSize.height, clipVideoTrack.naturalSize.height);
videoComposition.frameDuration = CMTimeMake(1, 30);
AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(60, 30) );
// rotate to portrait
AVMutableVideoCompositionLayerInstruction* transformer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];
CGAffineTransform t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height, -(clipVideoTrack.naturalSize.width - clipVideoTrack.naturalSize.height) /2 );
CGAffineTransform t2 = CGAffineTransformRotate(t1, M_PI_2);
CGAffineTransform finalTransform = t2;
[transformer setTransform:finalTransform atTime:kCMTimeZero];
instruction.layerInstructions = [NSArray arrayWithObject:transformer];
videoComposition.instructions = [NSArray arrayWithObject: instruction];
// export
exporter = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality] ;
exporter.videoComposition = videoComposition;
NSURL *tmpDirURL = [NSURL fileURLWithPath:NSTemporaryDirectory() isDirectory:YES];
NSURL *fileURL = [[tmpDirURL URLByAppendingPathComponent:#"final"] URLByAppendingPathExtension:#"mov"];
exporter.outputURL=fileURL;
exporter.outputFileType=AVFileTypeQuickTimeMovie;
[exporter exportAsynchronouslyWithCompletionHandler:^(void){
NSLog(#"Exporting done!");
NSLog(#"exporter's outputURL is %#", exporter.outputURL);
self.moviePlayer = [[MPMoviePlayerController alloc] initWithContentURL:fileURL];
[self.view addSubview:self.moviePlayer.view];
self.moviePlayer.fullscreen = YES;
[self.moviePlayer play];
}];}
However, after running the exporter, the movie doesn't play in self.moviePlayer. It just goes "Loading..". Note that I tested the moviePlayer with the original videoURL parameter and that plays fine. Any ideas on what I'm doing wrong?
Thanks

Answer 1

Need:
dispatch_async(dispatch_get_main_queue(), ^{
//play movie here
});
in the completion handler.