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Trouble using swift 4's KVO "observe" instead of addObserver

I'm having some trouble getting the new KVO syntax right. According to the Apple documentation:

Create an observer for the key path and call the observe(_:options:changeHandler) method. For more information on key paths, see Keys and Key Paths.

class MyObserver: NSObject {
    @objc var objectToObserve: MyObjectToObserve
    var observation: NSKeyValueObservation?

    init(object: MyObjectToObserve) {
        objectToObserve = object

        observation = observe(\.objectToObserve.myDate) { object, change in
            print("Observed a change to \(object.objectToObserve).myDate, updated to: \(object.objectToObserve.myDate)")

let observed = MyObjectToObserve()
let observer = MyObserver(object: observed)


I'm initializing my observation like so:

self.observation = self.webView!.observe(\.webView.isLoading, changeHandler: { (webView, observedChange) in

but am getting this error:

enter image description here

like image 341
Eden Avatar asked Mar 29 '18 18:03


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2 Answers

Turns out the syntax needs to be like this, using the object Type rather than the object instance name:

self.observation = self.webView!.observe(\WKWebView.isLoading, changeHandler: { (webView, observedChange) in

Misread the documentation ¯\_(ツ)_/¯

like image 128
Eden Avatar answered Sep 23 '22 11:09


If you use the \. syntax the root element is the observed object so it's simply

self.observation = self.webView!.observe(\.isLoading, ...

The compiler treats your syntax as webView.webView.isLoading which is obviously not intended.

like image 41
vadian Avatar answered Sep 22 '22 11:09