Swift：如何在后台运行计时器

Question

I have added an observer to my app that listens when the app goes into background mode. I did that through this command: NotificationCenter.default.addObserver(self, selector: #selector(enterBackground), name: UIApplication.willResignActiveNotification, object: nil)

Now I want my selector to run a timer.scheduled in the background. My code Looks like:

   @objc func enterBackground() {
       Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { (timer) in
           print(self.secondsCount)
           self.secondsCount -= 1
       }
   }

But the timer doesn´t print my print commands every second. Just once.

Does somebody know how to do that? Warm greetins!

AND NO! THIS ISN'T A DUPLICATE I READ A LOT OF ANSWERS AND QUESTIONS BUT I DID'T GET ANSWER FOR ME!

Answer 1

The short answer is that you can't have a timer running while your app is in the background. Your app gets "suspended" (stops getting CPU time) almost immediately after the user switches to another app. Apple does this both to conserve battery and to make the front-most app more responsive. Only a limited number of app types are allowed to run in the background for more than a few minutes (eg music players, turn-by-turn navigation apps, VoIP apps.)

If you need to track the amount of time elapsed since some event, you should save the Date at the time the event occurs. While your app is in the foreground, run a timer and when it goes off, subtract the current Date from the saved Date to get elapsed time.

When you get notified that you are going to the background, stop your timer.

When you return to the foreground, restart your timer so that it recalculates the amount of time elapsed.

PS This is a duplicate. The answer to this and all other "how can I run a timer while I'm in the background" questions is "You can't." I provided an individual answer rather than just closing it as a duplicate in order to explain why you can't solve your problem the way you are attempting to