如何从Asset获取慢动作视频的文件URL？

Question

我正在获取普通视频的文件网址，但慢动作视频资产类型为AVComposition。我尝试使用AVAssetExportSession但是耗费了大量时间。

PHVideoRequestOptions *options=[[PHVideoRequestOptions alloc] init];
options.version = PHVideoRequestOptionsVersionCurrent;
options.networkAccessAllowed = YES;

[[PHImageManager defaultManager] requestAVAssetForVideo:asset options:options resultHandler:^(AVAsset *avAsset, AVAudioMix *audioMix, NSDictionary *info) {

     if(([avAsset isKindOfClass:[AVComposition class]] && ((AVComposition *)avAsset).tracks.count == 2)) {

         //Output URL of the slow motion file.
         NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
         NSString *documentsDirectory = paths.firstObject;
         NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"mergeSlowMoVideo-%d.mov",arc4random() % 1000]];
         NSURL *url = [NSURL fileURLWithPath:myPathDocs];

         //Begin slow mo video export
         AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:avAsset presetName:AVAssetExportPresetHighestQuality];
         exporter.outputURL = url;
         exporter.outputFileType = AVFileTypeQuickTimeMovie;
         exporter.shouldOptimizeForNetworkUse = YES;

         [exporter exportAsynchronouslyWithCompletionHandler:^{

             dispatch_async(dispatch_get_main_queue(), ^{
                 if (exporter.status == AVAssetExportSessionStatusCompleted) {
                 }
             });
         }];
     } else if ([avAsset isKindOfClass:[AVURLAsset class]]) {
     }
 }];