Prevent iOS from killing App after a few minutes

I'd like to prevent iOS from killing my app after a few minutes. I've read this thread here on SO: Prevent iOS from killing my app after 3 minutes . It says that if I have no backgroundtasks longer than 3 minutes my app wont be killed. Can someone verify that this is true? Because my background-task is not running longer than 3 minutes and even though my app gets killed after this time. My background-task is a timer that updates a widget. Heres some code:

self.backgroundTask = UIApplication.shared.beginBackgroundTask { [weak self] in
        self?.endBackgroundTask()
//endBackGroundTask looks like this
UIApplication.shared.endBackgroundTask(self.backgroundTask)
    self.backgroundTask = UIBackgroundTaskInvalid
//
    }
self.timer = Timer.scheduledTimer(timeInterval: 1, target: self,   selector: (#selector(self.updateTimer)), userInfo: nil, repeats: true)

// at the beginning of the class
var backgroundTask: UIBackgroundTaskIdentifier = UIBackgroundTaskInvalid

// in viewWillDisappear
self.timer.invalidate()
    if self.backgroundTask != UIBackgroundTaskInvalid {
        self.endBackgroundTask()

    }

Solution

You need to structure your app so that it doesn't require continual execution in the background. As I understand it, your app shows a count down timer and can show the same count down timer in a Today Widget. The approach I would use is follows:

Store the "end date" for the timer in user defaults to share with your widget
When your app is in the foreground, use a Timer to periodically update your UI
When your Widget is being displayed use a Timer in the widget to periodically update its UI
When your app moves to the background, schedule a local notification for the expiration time
When your app moves back to the foreground, you can cancel that scheduled notification if it hasn't yet fired.
Support app restoration for those cases where your app is legitimately terminated (e.g. due to memory pressure or being suspended for a long period)

If you do this then you never need to call beginBackgroundTask. If you do call beginBackgroundTask and don't call endBackgroundTask within 3 minutes of entering the background, then your app will be terminated, even if you aren't using any CPU.