Weird behaviour of AVMutableComposition freezing while using AVMutableVideoComposition

问题

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.

回答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()
}       

来源：https://stackoverflow.com/questions/31402674/weird-behaviour-of-avmutablecomposition-freezing-while-using-avmutablevideocompo