swift - Adding dark mode to iOS app

Question

I am trying to add a theme to my app (a dark theme). So when the user clicks an activity switch it would then make the whole app go into the dark mode. I have hard coded the dark mode just to see what it would look like; however now I would like to be able to enable and disable it through and UISwitch, but I am not sure how to do this?

class DarkModeTableViewCell: UITableViewCell {

var DarkisOn = Bool()
let userDefaults = UserDefaults.standard


@IBOutlet var darkModeSwitchOutlet: UISwitch!

override func awakeFromNib() {
    super.awakeFromNib()


}

override func setSelected(_ selected: Bool, animated: Bool) {
    super.setSelected(selected, animated: animated)

    // Configure the view for the selected state
}


@IBAction func darkModeSwitched(_ sender: Any) {

    if darkModeSwitchOutlet.isOn == true {

        //enable dark mode

        DarkisOn = true

        userDefaults.set(true, forKey: "DarkDefault")
        userDefaults.set(false, forKey: "LightDefault")



    } else {

        //enable light mode
        DarkisOn = false

        userDefaults.set(false, forKey: "DarkDefault")
        userDefaults.set(true, forKey: "LightDefault")
    }

}



}



class DarkModeViewController: UIViewController {



func set(for viewController: UIViewController) {



    viewController.view.backgroundColor = UIColor(red: 0.1, green: 0.1, blue: 0.1, alpha: 1.0)
        viewController.navigationController?.navigationBar.titleTextAttributes = [NSAttributedStringKey.foregroundColor: UIColor.white]
    viewController.navigationController?.navigationBar.tintColor =     UIColor.white
    viewController.navigationController?.navigationBar.barStyle =     UIBarStyle.black
    viewController.tabBarController?.tabBar.barStyle = UIBarStyle.black






}
static let instance = DarkModeViewController()
}

and then what I do is call the function in each one of the view controllers to see what it looks like, but I need to be able to access the bool value on if the switch is on or off and if it is then have it do that function, otherwise to just keep things the same. If you have any further questions, please let me know, I know some of this might not make to much sense.

Answer 1

UPDATE: This question (and therefore, this answer) was written before iOS 13 was announced, therefore it does not use iOS 13 specific APIs.

---

I'd solve this using Notifications (NSNotificationCenter APIs).

The idea is to notify your view controllers in real-time when the dark mode is enabled and when it is disabled, so they can also adapt to the change in real time. You don't need to check the status of the switch or anything like that.

Start by creating two notifications (you can also do it with one only and pass in the desired theme in the userInfo dictionary, but in this case it's easier to create two notifications, since you need to cast and what-not with Swift).

NotificationsName+Extensions.swift:

import Foundation

extension Notification.Name {
    static let darkModeEnabled = Notification.Name("com.yourApp.notifications.darkModeEnabled")
    static let darkModeDisabled = Notification.Name("com.yourApp.notifications.darkModeDisabled")
}

On all your "themable" view controllers, listen to these notifications:

    override func viewDidLoad() {
        super.viewDidLoad()

        // Add Observers
        NotificationCenter.default.addObserver(self, selector: #selector(darkModeEnabled(_:)), name: .darkModeEnabled, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(darkModeDisabled(_:)), name: .darkModeDisabled, object: nil)
    }

Don't forget to remove them in deinit, since sending notifications to invalid objects raises an exception:

deinit {
    NotificationCenter.default.removeObserver(self, name: .darkModeEnabled, object: nil)
    NotificationCenter.default.removeObserver(self, name: .darkModeDisabled, object: nil)
}

In your "themable" view controllers, implement darkModeEnabled(_:) and darkModeDisabled(_:):

@objc private func darkModeEnabled(_ notification: Notification) {
    // Write your dark mode code here
}

@objc private func darkModeDisabled(_ notification: Notification) {
    // Write your non-dark mode code here
}

Finally, toggling your switch will trigger either notification:

@IBAction func darkModeSwitched(_ sender: Any) {

    if darkModeSwitchOutlet.isOn == true {
        userDefaults.set(true, forKey: "darkModeEnabled")

        // Post the notification to let all current view controllers that the app has changed to dark mode, and they should theme themselves to reflect this change.
        NotificationCenter.default.post(name: .darkModeEnabled, object: nil)

    } else {

        userDefaults.set(false, forKey: "darkModeEnabled")

        // Post the notification to let all current view controllers that the app has changed to non-dark mode, and they should theme themselves to reflect this change.
        NotificationCenter.default.post(name: .darkModeDisabled, object: nil)
    }

}

With this, all your view controllers will be notified in real time when the "theme" changes and they will react accordingly. Do note that you need to take measures to show the right mode when the app launches, but I'm sure you are doing that since you are using UserDefaults and presumably checking them. Also worth mentioning NSNotificationCenter is not thread-safe, although it shouldn't matter since this all UI code that should go in the main thread anyway.

For more information, you can check the NSNotificationCenter documentation.

Note: This code is built upon what OP had. It can be simplified (you don't need to keep track of both "light" and "dark" states for example, just one).