AVQueuePlayer.timeControlStatus上的KVO总是给出nil值

Question

我正在使用KVO监控AVQueuePlayer.timeControlStatus:

timeControlStatusObservation = avPlayer.observe(
    \AVQueuePlayer.timeControlStatus,
    options: [.old, .new],
    changeHandler: {
        [weak self] (player, change) in
        print("timeControlState: \(change.oldValue), \(change.newValue), \(self?.avPlayer.timeControlStatus)")
    })

change.oldValue并且change.newValue总是零,但avPlayer.timeControlStatus确实有正确的价值.我得到了以下日志:

timeControlState: nil, nil, Optional(playing) 
timeControlState: nil, nil, Optional(paused)

为什么KVO给出零值？这是图书馆的错误吗？