Countdown timer not running - swift

Question

Code not running when returning from background. If I suspend the app, close it out completely and re-open it calculates the time perfectly.
How can I get it to run when user leaves the app and returns.
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
if let lastTimerStartTime = UserDefaults.standard.value(forKey: "lastTimerStartTime") as? Date,
let totalTime = UserDefaults.standard.value(forKey: "totalTime") as? Double {
let timeSinceStartTime = Date() - lastTimerStartTime
if timeSinceStartTime < totalTime {
seconds = totalTime - timeSinceStartTime
runTimer()
} else {
timeLabel.text = timeString(time: TimeInterval(seconds))
}
} else {
timeLabel.text = timeString(time: TimeInterval(seconds))
}
}
#IBAction func startBtn(_ sender: Any) {
runTimer()
}
func runTimer() {
timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: (#selector(ViewController.updateTimer)), userInfo: nil, repeats: true)
UserDefaults.standard.set(Date(), forKey: "lastTimerStartTime")
UserDefaults.standard.set(seconds, forKey: "totalTime")
}
#objc func updateTimer(){
seconds -= 1
timeLabel.text = timeString(time: TimeInterval(seconds))
}

Answer 1

You need to add code to listen for suspend and resume events. (UIApplication.willResignActiveNotification and UIApplication. didBecomeActiveNotification).
When you get notified that you are going to resign being the active app, you’ll need to save the amount time remaining (or time elapsed) whatever makes sense.
You don’t know if your app will be resumed or killed, so you will need to save the info to UserDefaults, and then pick it up either when handing a didBecomeActiveNotification or when your app gets launched again.
If your app is moved the background and then brought back to the front without being terminated, your viewDidAppear method won’t get called because your view controller’s view never left the screen.