Weird behaviour of AVMutableComposition freezing while using AVMutableVideoComposition - ios

Question

I am trying to merge multiple videos using AVMutableComposition. The problem I face is that whenever I try to add any AVMutableVideoComposition for applying any instructions, my playback freezes in AVPlayer at exact 6 seconds duration.
Another interesting thing is that it plays fine if I play it in Photos app of iPad after exporting using same videoComposition. So why does it freezes in AVPlayer at 6 seconds?
Code:
AVMutableComposition *mutableComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *videoCompositionTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *audioCompositionTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
for (AVURLAsset *asset in assets)
{
AVAssetTrack *assetTrack;
assetTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
AVAssetTrack *audioAssetTrack = [asset tracksWithMediaType:AVMediaTypeAudio].firstObject;
NSError *error;
[videoCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, assetTrack.timeRange.duration )
ofTrack:assetTrack
atTime:time
error:&error];
if (error) {
NSLog(#"asset url :: %#",assetTrack.asset);
NSLog(#"Error1 - %#", error.debugDescription);
}
[audioCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAssetTrack.timeRange.duration)
ofTrack:audioAssetTrack
atTime:time
error:&error];
if (error) {
NSLog(#"Error2 - %#", error.debugDescription);
}
time = CMTimeAdd(time, assetTrack.timeRange.duration);
if (CGSizeEqualToSize(size, CGSizeZero)) {
size = assetTrack.naturalSize;;
}
}
AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
AVMutableVideoCompositionLayerInstruction *videoTrackLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompositionTrack];
mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, time);
mainInstruction.layerInstructions = [NSArray arrayWithObjects:videoTrackLayerInstruction, nil];
AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];
mainCompositionInst.frameDuration = CMTimeMake(1, 30);
mainCompositionInst.renderSize = size;
pi = [AVPlayerItem playerItemWithAsset:mutableComposition];
pi.videoComposition = mainCompositionInst;
Also, I know problem is mostly videoComposition because if I remove the videoComposition, then it plays fine on AVPlayer.
UPDATE 1: I just found out that when it freezes after 6 seconds, if I drag the slider back or forward(i.e. use seekToTime), it starts playing normally again without any further freeze.
Also audio keeps on playing fine even when video is frozen.
UPDATE 2: If I just go ahead and export it using AVAssetExportSession with same AVMutableComposition, and load asset from of exported video, it works fine. So its just when I play AVMutableComposition directly, problem arises.

Answer 1

Finally, I got the solution to fix this.
You should play after playerItem's status is changed to .ReadyToPlay.
Please see as below.
func startVideoPlayer() {
let playerItem = AVPlayerItem(asset: self.composition!)
playerItem.videoComposition = self.videoComposition!
let player = AVPlayer(playerItem: playerItem)
player.actionAtItemEnd = .None
videoPlayerLayer = AVPlayerLayer(player: player)
videoPlayerLayer!.frame = self.bounds
/* add playerItem's observer */
player.addObserver(self, forKeyPath: "player.currentItem.status", options: .New, context: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: "playerItemDidReachEnd:", name: AVPlayerItemDidPlayToEndTimeNotification, object: playerItem);
self.layer.addSublayer(videoPlayerLayer!)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
if keyPath != nil && keyPath! == "player.currentItem.status" {
if let newValue = change?[NSKeyValueChangeNewKey] {
if AVPlayerStatus(rawValue: newValue as! Int) == .ReadyToPlay {
playVideo() /* play after status is changed to .ReadyToPlay */
}
}
} else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
}
}
func playerItemDidReachEnd(notification: NSNotification) {
let playerItem = notification.object as! AVPlayerItem
playerItem.seekToTime(kCMTimeZero)
playVideo()
}
func playVideo() {
videoPlayerLayer?.player!.play()
}