UserDefaults.didChangeNotification not firing

Hmmm, this is tricky. The docs for this notification are pretty clear about its semantics:

This notification isn't posted when changes are made outside the current process, or when ubiquitous defaults change.

And Settings is definitely “outside the current process”.

Having said that, this is a pretty common case and it’s probably worth filing a bug about it. Please post your bug number, just for the record.

In the meantime, the solution is to use KVO to monitor for changes. Quoting the docs again:

You can use key-value observing to register observers for specific keys of interest in order to be notified of all updates, regardless of whether changes are made within or outside the current process.

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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Can you post an example how to do this? Thx ;)

Find a solution:

userDefaults.addObserver(self, forKeyPath: "youKey", options: .new, context: nil)

And:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {

    //do something 

}

If you’re using Swift then it has a much nicer interface to KVO, namely the observe(_:options:changeHandler:) method. For example, if I add code like this:

extension UserDefaults {
    @objc dynamic var myProperty: String? {
        get {
            self.string(forKey: "myProperty")
        }
        set {
            self.set(newValue, forKey: "myProperty")
        }
    }
}

to expose the defaults value as a property, I can then write code like this:

class MyClass {
    var myPropertyObservation: NSKeyValueObservation?

    func startObserving() {
        let defaults = UserDefaults.standard
        self.myPropertyObservation = defaults.observe(\.myProperty, options: [.new, .old]) { _, change in
            print(change)
        }
    }
}

I then modify the defaults value using defaults:

% defaults write "com.example.apple-samplecode.Test764675" "myProperty" "some value"

and the change handler runs and prints:

NSKeyValueObservedChange<Optional<String>>(kind: __C.NSKeyValueChange, newValue: Optional(Optional("some value")), oldValue: Optional(nil), indexes: nil, isPrior: false)

Nice!

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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"