"hidden" vs #keyPath(UIView.isHidden)

Question

KVO observer with #keyPath(UIView.isHidden) does not work, but "hidden" works.

Very strange. Is it bug or feature?

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("*");
    }
}

Answer 1

Is it bug or feature?

Let's say it's a known fact. "Renamification" means that Swift pretends that the name of an Objective-C Bool property starts with is... even when it doesn't. But the #keyPath mechanism didn't get the memo when it comes to KVO and property setter names, and property setter swizzling to implement KVO observing is purely an Objective-C feature, so you have to use the real name of the property / setter, i.e. the Objective-C name, so that communication with Objective-C works correctly for KVO observation purposes.

I've filed a bug report on it (https://bugs.swift.org/browse/SR-2415) on the grounds that Swift could behave a little smarter about this, but until the Swift gang respond, it's just something you have know and deal with.