AVAsset首选转换为自定义AVVideoComposition

Question

我创建了一个自定义的AVVideoComposition类，并使用它：

AVAsset *asset = ...
AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoCompositionWithPropertiesOfAsset:asset];
videoComposition.customVideoCompositorClass = [MyCustomCompositor class];
MyCustomInstruction *instruction = // custom instruction holding CIFilter that is applied to every video frame
videoComposition.instructions = @[instruction];

导出会话后使用如下：

AVAssetExportSession *session = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality];
session.outputURL = ...
session.outputFileType = AVFileTypeQuickTimeMovie;
session.videoComposition = videoComposition;

[session exportAsynchronouslyWithCompletionHandler:^{
    ...
}];

根据documentation，如果我使用AVVideoComposition，则曲目的preferredTransform将无效。另外，使用自定义AVVideoCompostion指令，我无法使用 setTransform：atTime：设置AVMutableVideoCompositionLayerInstruction。

如何以正确的方向获取视频？