Quando estou misturando dois vídeos com AVAssetExportSession no ios 9, está funcionando perfeitamente. mas quando eu misturo com AVAssetExportSession no iOS 10, ele não está funcionando. Por favor me ajude se alguém souber o motivo, obrigado.

atualmente, o código está funcionando para o iphone 6s e versões anteriores, mas não para o iPhone 7

por exemplo

-(void) blendVideoOverVideo:(NSURL*)mainVideoUrl andBlendVideoUrl:(NSURL*)liveEffectUrl
{
    AVURLAsset  *mainVideoUrlAsset =[AVURLAsset URLAssetWithURL:mainVideoUrl options:nil];
    //    AVPlayerItem* mainVideoPlayerItem =[[AVPlayerItem alloc]initWithAsset:mainVideoUrlAsset];
    AVAssetTrack* mainVideoTrack =[[mainVideoUrlAsset tracksWithMediaType:AVMediaTypeVideo]firstObject];
    CGSize mainVideoSize = [mainVideoTrack naturalSize];

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

    AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:mainVideoUrl options:nil];
    if(mainVideoUrl!=nil)
    {
        if([[audioAsset tracksWithMediaType:AVMediaTypeAudio] count])
        {
            AVMutableCompositionTrack *compositionCommentaryTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                                                preferredTrackID:kCMPersistentTrackID_Invalid];
            [compositionCommentaryTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, mainVideoUrlAsset.duration )
                                                ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
                                                 atTime:kCMTimeZero
                                                  error:nil];
        }
    }

    AVMutableCompositionTrack *mainVideoConpositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

    [mainVideoConpositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, mainVideoUrlAsset.duration) ofTrack:mainVideoTrack atTime:kCMTimeZero error:nil];

    AVMutableVideoCompositionLayerInstruction *mainVideoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:mainVideoConpositionTrack];

    //SEcond Track
    AVURLAsset  *blendVideoUrlAsset =[AVURLAsset URLAssetWithURL:liveEffectUrl options:nil];
    //    AVPlayerItem* blendVideoPlayerItem =[[AVPlayerItem alloc]initWithAsset:blendVideoUrlAsset];
    AVAssetTrack* blendVideoTrack =[[blendVideoUrlAsset tracksWithMediaType:AVMediaTypeVideo]firstObject];
    CGSize blendVideoSize = [blendVideoTrack naturalSize];

    AVMutableCompositionTrack *blendVideoConpositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

    CMTime oldTime=CMTimeMakeWithSeconds(CMTimeGetSeconds(blendVideoUrlAsset.duration), blendVideoUrlAsset.duration.timescale);

//    CMTime timeNew=CMTimeMakeWithSeconds(CMTimeGetSeconds(blendVideoUrlAsset.duration)/2, blendVideoUrlAsset.duration.timescale);


    [blendVideoConpositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, oldTime) ofTrack:blendVideoTrack atTime:kCMTimeZero error:nil];

    AVMutableVideoCompositionLayerInstruction *blendVideoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:blendVideoConpositionTrack];

    AVMutableVideoCompositionInstruction * MainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    MainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, mainVideoUrlAsset.duration);

    CGAffineTransform Scale = CGAffineTransformMakeScale(1.0f,1.0f);
    CGAffineTransform Move = CGAffineTransformMakeTranslation(0,0);
    [mainVideoLayerInstruction setTransform:CGAffineTransformConcat(Scale,Move) atTime:kCMTimeZero];

    [blendVideoLayerInstruction setOpacity:0.5 atTime:kCMTimeZero];
//    [blendVideoLayerInstruction setOpacity:0.0 atTime:timeNew];

    CGFloat cropOffX = 1.0;
    CGFloat cropOffY = 1.0;
    if(blendVideoSize.height>mainVideoSize.height)
    {
        cropOffY = mainVideoSize.height/blendVideoSize.height;
    }else{

        cropOffY = mainVideoSize.height/blendVideoSize.height;

    }
    if(blendVideoSize.width>mainVideoSize.width)
    {
        cropOffX = mainVideoSize.width/blendVideoSize.width;
    }
    Scale = CGAffineTransformMakeScale(cropOffX,cropOffY);
    Move = CGAffineTransformMakeTranslation(0.1,  0.1);
    [blendVideoLayerInstruction setTransform:CGAffineTransformConcat(Scale,Move) atTime:kCMTimeZero];

    MainInstruction.layerInstructions = [NSArray arrayWithObjects:blendVideoLayerInstruction,mainVideoLayerInstruction,nil];

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


    NSString *fullName= [NSString stringWithFormat:@"video%d.mov",arc4random() % 1000];



    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent:fullName];
    if([[NSFileManager defaultManager] fileExistsAtPath:myPathDocs])
    {
        [[NSFileManager defaultManager] removeItemAtPath:myPathDocs error:nil];
    }
    NSURL *url = [NSURL fileURLWithPath:myPathDocs];
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
    exporter.outputURL=url;

    CMTime start;
    CMTime duration;

    NSLog(@"Main Video dura %f blend dura - %f, ",CMTimeGetSeconds(mainVideoUrlAsset.duration),CMTimeGetSeconds(blendVideoUrlAsset.duration));


    if(CMTimeGetSeconds(blendVideoUrlAsset.duration)>CMTimeGetSeconds(mainVideoUrlAsset.duration))
    {
        start = CMTimeMakeWithSeconds(0.0, blendVideoUrlAsset.duration.timescale);
        duration = CMTimeMakeWithSeconds(CMTimeGetSeconds(mainVideoUrlAsset.duration), blendVideoUrlAsset.duration.timescale);
    }
    else if(CMTimeGetSeconds(mainVideoUrlAsset.duration)>CMTimeGetSeconds(blendVideoUrlAsset.duration))
    {
        start = CMTimeMakeWithSeconds(0.0, mainVideoUrlAsset.duration.timescale);
        duration = CMTimeMakeWithSeconds(CMTimeGetSeconds(mainVideoUrlAsset.duration), mainVideoUrlAsset.duration.timescale);
    }
    CMTimeRange range = CMTimeRangeMake(start, duration);

    exporter.timeRange = range;
    [exporter setVideoComposition:MainCompositionInst];
    exporter.outputFileType = AVFileTypeQuickTimeMovie;

    __weak typeof(self) weakSelf = self;

    [weakSelf createMBCircularProgress:exporter];


    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            [weakSelf exportDidFinish:exporter];
        });
    }];
}

esse código será executado no ios 9 e até no iOS 10 no iPhone 6s, 6,5 etc, mas esse código não será executado no simulador do iPhone 7.

A solução é que precisamos usar o XCode 8.1 beta mais recente para executar este