Detect iOS app entering background

I'm working on a game for iOS coded in Swift. I've tried to find a way to detect when the app enters background mode or is interrupted for other reasons, for example a phone call but can't find anything. How do I do it?

You can add an observer to your view controller:

edit/update: Xcode 11 • Swift 5

iOS13 or later

UIScene.willDeactivateNotification

iOS12 or earlier

UIApplication.willResignActiveNotification

---

if #available(iOS 13.0, *) {
    NotificationCenter.default.addObserver(self, selector: #selector(willResignActive), name: UIScene.willDeactivateNotification, object: nil)
} else {
    NotificationCenter.default.addObserver(self, selector: #selector(willResignActive), name: UIApplication.willResignActiveNotification, object: nil)
}

and add a selector method to your view controller that will be executed when your app receives that notification:

@objc func willResignActive(_ notification: Notification) {
    // code to execute
}

In swift 5.x: To observe app enters background event, add this code to your viewDidLoad() method.

    let notificationCenter = NotificationCenter.default
    notificationCenter.addObserver(self, selector: #selector(appMovedToBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)

    @objc func appMovedToBackground() {
        // do whatever event you want
    }

you have to use UIApplication.didEnterBackgroundNotification. If you want to observe if app came to foreground event, use UIApplication.willEnterForegroundNotification

So, the full code will be:

override func viewDidLoad() {
    super.viewDidLoad()

    let notificationCenter = NotificationCenter.default
    notificationCenter.addObserver(self, selector: #selector(appMovedToBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
    
    notificationCenter.addObserver(self, selector: #selector(appCameToForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
    
    // Do any additional setup after loading the view.
}
 @objc func appMovedToBackground() {
    print("app enters background")
}

@objc func appCameToForeground() {
    print("app enters foreground")
}

Swift3

let notificationCenter = NotificationCenter.default
    notificationCenter.addObserver(self, selector: #selector(appMovedToBackground), name: Notification.Name.UIApplicationWillResignActive, object: nil)


func appMovedToBackground() {
    print("App moved to background!")
}

To detect the app enters background, you can check in the appDelegate.m find the application delegate method

> applicationDidEnterBackground

This method will get called, once the app enters background.

SwiftUI

From background

Text("Hello, World!")
.onReceive(NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)) { _ in
    print("To the foreground!")
}

To the background

Text("Hello, World!")
    .onReceive(NotificationCenter.default.publisher(for: UIApplication.willResignActiveNotification)) { _ in
        print("To the background!")
    }

For SwiftUI you can use:

YourView()
 .onReceive(NotificationCenter.default.publisher(for: UIScene.willDeactivateNotification)) { _ in
     //...
 }

Take a look at the delegate methods defined in your instance of UIApplicationDeletegate (called AppDelegate.m by default). Specifically the following would be useful:

- (void)applicationWillResignActive:(UIApplication *)application

> This method is called to let your app know that it is about to move from the active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the app and it begins the transition to the background state. An app in the inactive state continues to run but does not dispatch incoming events to responders.

Taken from the Apple Documentation - here