observer.swift

// Code (in app delegate)
 
    func setupObservers() {
        observations.append(
            UserDefaults.standard.observe(
                \UserDefaults.selectedPreset,
                options: [.initial, .new],
                changeHandler: selectedPresetChanged
            )
        )
 
        UserDefaults.standard.addObserver(self, forKeyPath: #keyPath(UserDefaults.selectedPreset), options: [.initial, .new], context: nil)
    }
 
    override func observeValue(
        forKeyPath keyPath: String?,
        of object: Any?,
        change: [NSKeyValueChangeKey : Any]?,
        context: UnsafeMutableRawPointer?
    ) {
        NSLog("""
           observeValue(
               forKeyPath: \(String(describing:keyPath)),
               of: \(String(describing:object)),
               change: \(String(describing:change)),
               context: \(String(describing:context))
           )
           """)
    }
 
    func selectedPresetChanged(
        _: UserDefaults,
        object: NSKeyValueObservedChange<ControlModeWithPresets>
    ) {
        NSLog(
            "selectedPresetChanged() " +
                "newValue = \(String(describing: object.newValue))"
        )
    }
 
 
// Output:
// 2019-02-05 22:20:17.883391-0200 BattChgCtrl[8152:282384] selectedPresetChanged() newValue = nil
// 2019-02-05 22:20:17.883846-0200 BattChgCtrl[8152:282384] observeValue(
//     forKeyPath: Optional("selectedPreset"),
//     of: Optional(<NSUserDefaults: 0x600000c409f0>),
//     change: Optional([__C.NSKeyValueChangeKey(_rawValue: kind): 1, __C.NSKeyValueChangeKey(_rawValue: new): {
//     mode = Automatic;
//     presetId = "F88B1590-2ABA-4AFF-81B5-BA1A128ACD48";
// }]),
//     context: nil
// )