ちなみに UserDefaults.didChangeNotification だと１回しかきてないとかいいよる

gistfile1.txt

import Cocoa

let testKey: String = "TESTKEY"
let userDefaults: UserDefaults = UserDefaults.standard
let defaultCenter: NotificationCenter = NotificationCenter.default

// Remove Perferences & register default
userDefaults.removePersistentDomain(forName: Bundle.main.bundleIdentifier!)
userDefaults.register(defaults: [testKey : false])


class testClass: NSObject {
    var callCount: Int = 0

    override init() {
        super.init()
        userDefaults.addObserver(self, forKeyPath: testKey, options: .new, context: nil)
        defaultCenter.addObserver(
            self,
            selector: #selector(userDefaultsDidChange),
            name: UserDefaults.didChangeNotification,
            object: nil
        )
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        self.callCount += 1
        if (keyPath == testKey) {
            print("\(testKey) changed: (\(self.callCount)times)")
        }
    }
    
    @objc func userDefaultsDidChange(_ notification: Notification) {
        print("Changed!")
    }
}

let testObj = testClass()
userDefaults.set(true, forKey: testKey)

Result:

TESTKEY changed: (1times)
TESTKEY changed: (2times)
Changed!