iOS NSNotificationCenter to check whether the app came from background to foreground

Question

I have a situation in which i have to intialize an object everytime when it comes from background to foreground and that should be using the NSNotificationCenter not with ap

Answer 1

Swift 5

Subscribe to Notification -

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    NotificationCenter.default.addObserver(
      self,
      selector: #selector(applicationWillEnterForeground(_:)),
      name: UIApplication.willEnterForegroundNotification,
      object: nil)
}

Remove subscription -

override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        NotificationCenter.default.removeObserver(self)
} 

Function to be called -

@objc func applicationWillEnterForeground(_ notification: NSNotification) {
       self.volumeSlider.value = AVAudioSession.sharedInstance().outputVolume
    }