NSTimer not firing after home button is pressed

Question

II have an NSTimer set to fire each second. Using the Simulator this will call the eachSecond() method correctly after I navigate away from the app by pressing the home button. However, on a real iPhone 6, once I navigate away eachSeconds is not called again until the app is open again.

Why might this behave differently on a real device? Is there anyway to ensure it will fire each second in this use case?

Its a fitness app, so needs to record duration when the phone locks or user navigates away momentarily.

Thanks,

lazy var timer = NSTimer()

fun init() {
   timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: #selector(RunViewController.eachSecond(_:)), userInfo: nil, repeats: true)
}

func eachSecond() {
 // update UI etc.
}

Answer 1

An NSTimer will not fire when your app is suspended. Background execution is different when running under XCode; background execution limits do not apply.

In order to determine how much time has elapsed while your app is suspended, you can store a timestamp in applicationDidEnterBackground and calculate the elapsed time based on the difference between the saved timestamp and current time in applicationWillEnterForeground

Answer 2

You need to add the timer to the main run loop.

func init() {
    timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: #selector(RunViewController.eachSecond(_:)), userInfo: nil, repeats: true)

    NSRunLoop.mainRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)
}