Swift AVPlayer addObserver从未被调用

我正在尝试获取AVPlayer timedMetadata，但从未调用观察者

self.metaItem!.addObserver(self, forKeyPath: "metaData", options: NSKeyValueObservingOptions(), context: nil)
        metPlayer=AVPlayer(playerItem: metaItem)
        metPlayer.play()
...

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change:
        [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath != "metaData" { return }
        print("...")
    }

但MPMoviePlayerController工作正常

谢谢。

metaItem AVPlayerItem应遵守密钥路径，但不遵守元数据：

self.metaItem!.addObserver(self, forKeyPath: "timedMetadata", options: NSKeyValueObservingOptions(), context: nil)