Detecting iOS Dark Mode Change

Question

I read through the documentation regarding: https://developer.apple.com/documentation/appkit/supporting_dark_mode_in_your_interface

When the user changes the system appearance, the system automatically asks each window and view to redraw itself. During this process, the system calls several well-known methods for both macOS and iOS, listed in the following table, to update your content.

In our legacy app we create our views as lazy variables in the init of each class. This means the views won't get drawn out with the correct color if the user goes into settings and switches to dark mode.

If you make appearance-sensitive changes outside of these methods, your app may not draw its content correctly for the current environment. The solution is to move your code into these methods.

Our application is quite big and a refactor will be done to support this in a better way in the future but I'm wondering if there is a way to detect this changes with the notification center like what can be done for Mac OS:

How to detect switch between macOS default & dark mode using Swift 3

Answer 1

Swift 5:

traitCollectionDidChange also gets called a few times. This is how I detect DarkMode runtime change and setColors() only once.

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)

        guard UIApplication.shared.applicationState == .inactive else {
            return
        }

        setColors()
    }

In setColors() func I update the colors. Detecting current colorScheme:

extension UIViewController {
    var isDarkMode: Bool {
        if #available(iOS 13.0, *) {
            return self.traitCollection.userInterfaceStyle == .dark
        }
        else {
            return false
        }
    }

}

I have colors defined like this (for iOS < 13):

enum ColorCompatibility {
    static var myOlderiOSCompatibleColorName: UIColor {
        if UIViewController().isDarkMode {
            return UIColor(red: 33, green: 35, blue: 37, alpha: 0.85)
        }
        else {
            return UIColor(hexString: "#F3F3F3", alpha: 0.85)
        }
    }
}

Example:

private func setColors() {
  myView.backgroundColor = ColorCompatibility.myOlderiOSCompatibleColorName
}

Also you might need to call setColors in ViewDidLoad/Will/DidAppear depending on your case like this:

viewDidLoad() {
...
setColors()
...
}

For iOS11+ you could use "named Colors", defined in Assets and much easier to use in IB.

Cheers