我像这样从AVPlayerItem观察轨道属性的键值:overridefuncobserveValue(forKeyPathkeyPath:String?,ofobject:Any?,change:[NSKeyValueChangeKey:Any]?,context:UnsafeMutableRawPointer?){ifcontext==&PlayerItemObserverContext{[...morecode...]}elseifkeyPath==#keyPath(AVPlayerItem.tracks){ifletplayerItem=self.playerItem{fort