swift - How to detect from what ViewController application enter background?

Question

In my swift app I need to know from what screen my application entered background. Im trying to use NotificationCenter this way:

class MainViewController: UIViewController{
   override func viewDidLoad() {
        super.viewDidLoad()
        let notificationCenter = NotificationCenter.default
        notificationCenter.addObserver(self, selector: #selector(appMovedToBackgroundMain), name: UIApplication.didEnterBackgroundNotification, object: nil)
    }

    @objc func appMovedToBackgroundMain() {
        print("main - App moved to Background!")
    }
}

class InitViewController: UIViewController{
       override func viewDidLoad() {
            super.viewDidLoad()
            let notificationCenter = NotificationCenter.default
            notificationCenter.addObserver(self, selector: #selector(appMovedToBackgroundInit), name: UIApplication.didEnterBackgroundNotification, object: nil)
        }

        @objc func appMovedToBackgroundInit() {
            print("init - App moved to Background!")
        }
    }

and when I'm press Home button at the MainViewController I got in Xcode's console these lines:

init - App moved to Background!
main - App moved to Background!

and I expected only one line there - main - App moved to Background!. How can I reach this?

Just remove the observer that is not in use. After the Init View is done it should remove the observer. You'll end up with only Main view added as an observer. Voila! — Haroun Hajem, Mar 22 '22 at 21:43

Gustavo Vollbrecht · Accepted Answer · 2019-01-29T12:33:36.147

3

On AppDelegate Methods: applicationDidEnterBackground or applicationWillEnterForeground, you can get the top most UIViewController. It is well explained on this question: Get top most UIViewController

edited Jan 29 '19 at 12:33

answered Jan 29 '19 at 12:27

Gustavo Vollbrecht

3,188
2
19
37

score 1 · Answer 2 · answered Jan 29 '19 at 11:42

1

You can use following function:

 func applicationWillEnterForeground(_ application: UIApplication) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
}

Here you can check which controller is on the top in your navigation controller's controller.

print(self.navigationController.topViewController)

answered Jan 29 '19 at 11:42

Mahak Mittal

121
1
10

should I put this function to AppDelegate? then I have an error that AppDelegate has no NavigationController and I still don't know what ViewController was at the screen – nastassia Jan 29 '19 at 11:56
Yes put it in App Delegate – Mahak Mittal Jan 29 '19 at 11:58
then I have an error that AppDelegate has no NavigationController and I still don't know what ViewController was at the screen – nastassia Jan 29 '19 at 11:58
What is your navigation stack? How you are loading controllers? – Mahak Mittal Jan 29 '19 at 11:59
Is there any navigation controller which you are using in App Delegate then use that instance. – Mahak Mittal Jan 29 '19 at 12:00
I have `TabBar` with `NavigationController` on each tab, and at `AppDelegate` don't declare any `NavigationController` – nastassia Jan 29 '19 at 12:03

score -2 · Answer 3 · answered Jan 29 '19 at 12:39

-2

When application enter in background state below method will call.

func applicationWillEnterForeground(_ application: UIApplication) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
}

answered Jan 29 '19 at 12:39

Parth

634
8
15

function `applicationWillEnterForeground ` belongs to UIApplicationDelegate and it is not accessible from ViewController – gondo May 27 '20 at 08:05

swift - How to detect from what ViewController application enter background?

3 Answers3