WatchValue function is called UserDefaults twice

My main view controller is added as an observer for some key in UserDefault

. But every time the value changed, it observeValue

is called twice.

The stacks are different for these two calls.

The first of frame #6: forEachObserver + 332 frame #7: -[CFPrefsSource didChangeValues:forKeys:count:] + 68 frame #8: -[CFPrefsSource setValues:forKeys:count:removeValuesForKeys:count:] + 340 frame #9: -[CFPrefsSource setValue:forKey:] + 56

Second of frame #6: forEachObserver + 332 frame #7: -[CFPrefsSource _notifyObserversOfChangeFromValuesForKeys:toValuesForKeys:] + 68 frame #8: __84-[CFPrefsSearchListSource asynchronouslyNotifyOfChangesFromDictionary:toDictionary:]_block_invoke_2 + 36

The only related post I found is cocoa -dev mail list https://lists.apple.com/archives/cocoa-dev/2016/Nov/msg00084.html . This problem seems to originate from macOS 10.12

Can anyone provide more details on why this is happening or how to suppress duplicate messages?

Minimal example of reproduction:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        UserDefaults.standard.addObserver(
            self,
            forKeyPath: "test",
            options: .new,
            context: nil
        )

        UserDefaults.standard.set(true, forKey: "test")
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print("Observed: \(object ?? "None")")
    }
}