迅速观察（_：options：changeHandler :) newValue总是为零

Question

我具有此设置来观察AVCaptureDevice deviceWhiteBalanceGains的值更改：

@objc dynamic var videoDevice:AVCaptureDevice?
private var deviceWBGainsObservation: NSKeyValueObservation?

然后在代码中，

deviceWBGainsObservation = videoDevice?.observe(\.deviceWhiteBalanceGains,
                     options: [.old, .new]) {[unowned self] (object, change) in

    NSLog("\(change)")
}

问题是更改oldValue，而newValue始终为nil。如果我使用传统的键值观察方法observeValue(forKeypath:...)，则不是这种情况。我在做什么错了？