Ios exportAsynchronouslyWithCompletionHandler调用一次

我以前试过问这个问题，但没有得到回应。所以我会再试一次。我试图把图像放在视频的顶部（比如涂鸦或边框）。我调用了“addAnimation”方法三次，它会运行，但exportAsynchronouslyWithCompletionHandler只在第一次尝试时被调用。有什么想法吗

 - (void)addAnimation:(UIImage *)background for:(NSString *)socialSite
{

// 1 - Load Asset
AVURLAsset* videoAsset = [AVURLAsset URLAssetWithURL:self.videoUrl options:nil];

// 2 - Create AVMutableComposition Object
AVMutableComposition* mixComposition = [AVMutableComposition composition];

// 2.1 - VideoTrack
AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
// 2.2 - AudioTrack
AVMutableCompositionTrack *compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];


//If either of them are nil, then alloc
if( (self.clipVideoTrack == nil) || (self.clipAudioTrack == nil)){

    self.clipVideoTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    self.clipAudioTrack = [[videoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
}


[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration) ofTrack:self.clipVideoTrack atTime:kCMTimeZero error:nil];
[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration) ofTrack:self.clipAudioTrack atTime:kCMTimeZero error:nil];
CGSize videoSize = [self.clipVideoTrack naturalSize];

// 3 - Background layer
UIImage *myImage = background;
CALayer *aLayer = [CALayer layer];

if(socialSite == nil) {
    aLayer.contents = (id)myImage.CGImage;
    if(__IPHONE_4_INCH)
        aLayer.frame = CGRectMake(0, 0, 640, 640);
    else
        aLayer.frame = CGRectMake(0, 0, 320, 320);
}
else{
    aLayer.contents = (id)myImage.CGImage;
    aLayer.frame = CGRectMake(0, 0, myImage.size.width, myImage.size.height);
}

// 3.1 - Video layer
CALayer *parentLayer = [CALayer layer];
CALayer *videoLayer = [CALayer layer];
parentLayer.frame = CGRectMake(0, 0, myImage.size.width, myImage.size.height);
if(socialSite == nil){
    videoLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
}
else{
    videoLayer.frame = CGRectMake(0, 110, videoSize.width, videoSize.height);

}
[parentLayer addSublayer:videoLayer];
[parentLayer addSublayer:aLayer];


// 3.2 - Add instructions
AVMutableVideoComposition* videoComp = [AVMutableVideoComposition videoComposition];
videoComp.renderSize = videoSize;
videoComp.frameDuration = CMTimeMake(1, 30);
videoComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, [mixComposition duration]);
AVAssetTrack *videoTrack = [[mixComposition tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];


AVMutableVideoCompositionLayerInstruction* layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];
videoComp.instructions = [NSArray arrayWithObject: instruction];

AVAssetExportSession *assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];//AVAssetExportPresetPassthrough
assetExport.videoComposition = videoComp;


// 4 - Get path
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                         [NSString stringWithFormat:@"FinalVideo-%d.mov",arc4random() % 1000]];
NSURL *url = [NSURL fileURLWithPath:myPathDocs];

myPathDocs = [myPathDocs stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];


// 5 - Create exporter
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                  presetName:AVAssetExportPresetHighestQuality];

exporter.outputURL=url;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.shouldOptimizeForNetworkUse = YES;
exporter.videoComposition = videoComp;

[exporter exportAsynchronouslyWithCompletionHandler:^ {
        NSData *video;

        /*********************************** INSTAGRAM *********************************************/
        if([socialSite isEqualToString:@"instagram"]){
            video = [NSData dataWithContentsOfURL:exporter.outputURL];
            [MPOCreateMPManager sharedManager].createdMarcoPolo.instagramVideo = video;
            NSLog(@"IG SIZE (In CreateViewController): %lu",(unsigned long)[MPOCreateMPManager sharedManager].createdMarcoPolo.instagramVideo.length);
        }

        /*********************************** FB/TWITTER *********************************************/
        else if ([socialSite isEqualToString:@"facebook"]){
            video = [NSData dataWithContentsOfURL:exporter.outputURL];
            [MPOCreateMPManager sharedManager].createdMarcoPolo.fBTwitterVideo = video;
            NSLog(@"FB SIZE (In CreateViewController): %lu",(unsigned long)[MPOCreateMPManager sharedManager].createdMarcoPolo.fBTwitterVideo.length);
        }

        /***********************************   DOODLE   *********************************************/
        else if (socialSite == nil){
            video = [NSData dataWithContentsOfURL:exporter.outputURL];
            [MPOCreateMPManager sharedManager].createdMarcoPolo.video = video;
            [MPOCreateMPManager sharedManager].createdMarcoPolo.image = [self loadImage:exporter.outputURL];
            NSLog(@"*******************************DOODLE DONE*************************************");
        }
}];
}