*与 *KVO的 观察者#keyPath(UIView.isHidden)不起作用，但是"hidden"起作用。

很奇怪。它是错误还是功能？

child.addObserver(self, forKeyPath: "hidden", options: [.initial,.new], context: nil);

override func observeValue(forKeyPath keyPath: String?, of object: Any?, .change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if let view = object as? UIView, view.superview === self && keyPath == "hidden" {
        print("*");
    }
}