iOS: Can I detect if the device screen is on/off?

Question

How can I check if the screen is on or off in iOS? I'd like to know if the screen is currently on, even my app is in the background. I'd like to have an event listener on this. thanks.

Answer 1

In Swift 3 you can do:

override func viewDidLoad() {
        super.viewDidLoad()
        // Observer UIApplicationDidBecomeActive,UIApplicationDidEnterBackground
        NotificationCenter.default.addObserver(
                self,
                selector: #selector(MyViewController.applicationDidBecomeActive(notification:)),
                name: NSNotification.Name.UIApplicationDidBecomeActive,
                object: nil)

        NotificationCenter.default.addObserver(
                self,
                selector: #selector(MyViewController.applicationDidEnterBackground(notification:)),
                name:NSNotification.Name.UIApplicationDidEnterBackground,
                object: nil)
}

func applicationDidBecomeActive(notification: NSNotification) {
  // here my app did become active
}
func applicationDidEnterBackground(notification: NSNotification) {
  // here my app did enter background
}

You can find more details in the official guide.

---

Details from the actual sources:

extension NSNotification.Name {
    // These notifications are sent out after the equivalent delegate message is called
    @available(iOS 4.0, *)
    public static let UIApplicationDidEnterBackground: NSNotification.Name
    @available(iOS 4.0, *)
    public static let UIApplicationWillEnterForeground: NSNotification.Name
    public static let UIApplicationDidFinishLaunching: NSNotification.Name
    public static let UIApplicationDidBecomeActive: NSNotification.Name
    public static let UIApplicationWillResignActive: NSNotification.Name
    public static let UIApplicationDidReceiveMemoryWarning: NSNotification.Name
    public static let UIApplicationWillTerminate: NSNotification.Name
    public static let UIApplicationSignificantTimeChange: NSNotification.Name
    public static let UIApplicationWillChangeStatusBarOrientation: NSNotification.Name // userInfo contains NSNumber with new orientation
    public static let UIApplicationDidChangeStatusBarOrientation: NSNotification.Name // userInfo contains NSNumber with old orientation
    // userInfo dictionary key for status bar orientation
    public static let UIApplicationWillChangeStatusBarFrame: NSNotification.Name // userInfo contains NSValue with new frame
    public static let UIApplicationDidChangeStatusBarFrame: NSNotification.Name // userInfo contains NSValue with old frame
    // userInfo dictionary key for status bar frame
    @available(iOS 7.0, *)
    public static let UIApplicationBackgroundRefreshStatusDidChange: NSNotification.Name
    @available(iOS 4.0, *)
    public static let UIApplicationProtectedDataWillBecomeUnavailable: NSNotification.Name
    @available(iOS 4.0, *)
    public static let UIApplicationProtectedDataDidBecomeAvailable: NSNotification.Name   
    // Key in options dict passed to application:[will | did]FinishLaunchingWithOptions and info for UIApplicationDidFinishLaunchingNotification    
    // This notification is posted after the user takes a screenshot (for example by pressing both the home and lock screen buttons)
    @available(iOS 7.0, *)
    public static let UIApplicationUserDidTakeScreenshot: NSNotification.Name
}