duplicates:

• SR-4397 #keyPath(...) incorrectly resolves paths for Operation state properties

Issue Description:

This UIViewController code worked back in Xcode 8.2, Swift 3:

    let q = OperationQueue()

    override func viewDidLoad() {
        super.viewDidLoad()
        let op = Operation()
        op.addObserver(self, forKeyPath: #keyPath(Operation.isFinished), options: [], context: nil)
        self.q.addOperation(op)
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print(keyPath)
        if let op = object as? Operation, let keyPath = keyPath {
            op.removeObserver(self, forKeyPath: keyPath)
        }
    }

In Xcode 8.3, Swift 3.1, that same code now crashes with "[object] deallocated while key value observers were still registered with it." Our `observeValue` is not being called!

I have not found any way to get `observeValue` to be called for the `finished` / `isFinished` property while using `#keyPath` syntax. The only workaround I have found is to revert to a raw string:

op.addObserver(self, forKeyPath: "isFinished", options: [], context: nil)

That solves the problem from a practical standpoint, but we are forced to give up the yummy goodness of `#keyPath`.

I believe the source of the problem is that there is something odd about NSOperation's `isFinished` KVO-observable property, but I don't know exactly what it is. It seems to have something to do with the `is` prefix. Swift 3.1 now insists on translating `#keyPath(isFinished)` into "finished", which is clever but wrong in the case of NSOperation — you must use a keypath of "isFinished" (even in Objective-C) or the KVO notification won't fire.