I recently needed to determine when the user has manually switched between dark mode and light mode on macOS. In my menu bar app, Lucifer, the icon reflects the current appearance setting when you change it from the app — an inverted pentagram for dark mode and an upright pentagram for light mode. But there’s a bug. If the user manually changes the appearance setting from System Preferences, or if they are using the new “auto” setting in macOS Catalina, the icon gets stuck in its previous state.

I needed to get notified when the system appearance changed. On iOS, this is very straight-forward and a first-class API. On iOS 13, the interface style is part of UITraitCollection. In your view controllers, you can observe and respond to trait collection changes:

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {

    switch self.traitCollection.userInterfaceStyle {
    case .dark:
    case .light:
    case .unspecified:
    @unknown default:

I was hoping to find an equivalent NSTraitCollection in AppKit, but unfortunately that does not exist. Solving this will require some creativity.

After some searching online, I found that the UserDefaults key for the appearance setting on macOS is "AppleInterfaceStyle". Of course, this is not officially documented. If you did not know, you can respond to changes in UserDefaults via key-value observing and since iOS 9.3 and macOS Sierra KVO will notify of changes made by other programs.

// in a view controller or similar

private static var observerContext = 0

                                  forKeyPath: "AppleInterfaceStyle",
                                  options: [.new, .old, .initial, .prior],
                                  context: &Self.observerContext)

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

This works but it is a hack. And in my testing it can take a few seconds before observeValue(forKeyPath: of: change: context:) gets called.

It turns out there is a better option. Instead, you can KVO on NSApp.effectiveappearance. This is a much better and more reliable solution, and the observation closure is called immediately.

let observer = NSApp.observe(\.effectiveAppearance, options: [.new, .old, .initial, .prior]) { app, change in
    // respond to change...

That is exactly what I needed. Using KVO still does not feel great, but this is the best solution I could find without having an explicit API like iOS. The bug is fixed! Now my menu bar icon will stay in sync with the current appearance setting. I would still rather have an API similar to iOS, but this will do. Also note, if you are in the context of an NSView then you can implement viewDidChangeEffectiveAppearance().