Detecting changes in the preferences window

At a glance

Project structure

PreferencesWindowSample/
├── AppDelegate.swift
├── SceneDelegate.swift
├── ViewController.swift
├── Settings.bundle/
│   ├── Root.plist
│   └── additional preference panes
└── Base.lproj/Main.storyboard

Structure observations

• No settings view controller exists: Mac Catalyst renders the declarative Settings bundle.
• Registration/parsing is application startup logic in AppDelegate.
• Typed preference access, value mapping, and UI subscription are colocated in ViewController.swift.

Overall architecture

flowchart LR
    Schema["Settings.bundle plists"] --> Preferences["System Preferences window"]
    Schema --> App["AppDelegate"]
    App -->|"parse recursively + register defaults"| Defaults["UserDefaults.standard"]
    Preferences -->|"write preference"| Defaults
    Defaults -->|"KVO publisher"| Subscription["AnyCancellable pipeline"]
    Subscription -->|"map raw Int"| Colors["BackgroundColors"]
    Colors -->|"assign"| View["ViewController.view.backgroundColor"]
    Scene["SceneDelegate"] -->|"max window size"| Window["UIWindowScene"]

Reference code

PreferencesWindowSample/ViewController.swift:36 — the controller retains a publisher pipeline that converts preference values into colors and assigns them directly to the view.

var subscriber: AnyCancellable?

override func viewDidLoad() {
    super.viewDidLoad()

    subscriber = UserDefaults.standard
        .publisher(
            for: \.backgroundColorValue,
            options: [.initial, .new]
        )
        .map {
            BackgroundColors(rawValue: $0)?
                .currentColor()
        }
        .assign(
            to: \UIView.backgroundColor,
            on: view
        )
}

Interpretation

The system preferences window and app UI communicate through persistent state, not direct controller callbacks. UserDefaults is the boundary, and Combine turns its KVO-compliant property into a live UI binding.

Ownership and state

classDiagram
    SystemPreferences --> UserDefaults : writes values
    AppDelegate --> SettingsBundle : reads schemas
    AppDelegate --> UserDefaults : registers defaults
    ViewController *-- AnyCancellable : subscriber lifetime
    AnyCancellable --> UserDefaultsPublisher : subscription
    AnyCancellable --> UIView : assignment target
    ViewController *-- BackgroundColors : mapped value
    SceneDelegate o-- UIWindow : scene window

Ownership evidence

PreferencesWindowSample/AppDelegate.swift:36 — launch setup builds a defaults dictionary from the bundle and gives it to the shared defaults store.

func registerDefaultPreferenceValues() {
    let preferenceSpecifiers =
        retrieveSettingsBundlePreferenceSpecifiers(
            from: "Root.plist"
        )
    let defaultValuesToRegister =
        parse(preferenceSpecifiers)
    UserDefaults.standard.register(
        defaults: defaultValuesToRegister
    )
}

Class and protocol design

There is no app-defined protocol. The abstraction seams are declarative schema, a typed enum, and Combine’s publisher/subscriber contracts.

Access control

Reference code

PreferencesWindowSample/ViewController.swift:58 — the extension turns stringly stored keys into typed, observable properties without adding a separate preferences service.

extension UserDefaults {
    @objc dynamic var backgroundColorValue: Int {
        integer(forKey: "backgroundColorValue")
    }

    @objc dynamic var someRandomOption: Bool {
        bool(forKey: "someRandomOption")
    }
}

Logic ownership and placement

Design patterns

Naming conventions

• BackgroundColors names the value domain, while enum cases mirror settings plist values.
• backgroundColorValue ends in “Value” because it exposes persisted representation, not UIColor.
• registerDefaultPreferenceValues and retrieveSettingsBundlePreferenceSpecifiers use explicit lifecycle/data verbs.
• subscriber is generic; backgroundColorSubscription would describe ownership more precisely.

Architecture takeaways

• Use Settings.bundle as the contract when the system owns Preferences UI.
• Register defaults at launch, and keep registration separate from persisted user values.
• Map persisted primitives into typed app concepts before updating UI.
• Retain the cancellable at the screen owner so observation has an explicit lifetime.
• Make subscription and parsing helpers private in production when no cross-type collaboration needs them.

Source map

• Official Apple documentation