Keeping timers running when app enters background - ios

Question

I am making an app that relies on a timer that fires every minute to change a label from "x minutes left" to "(x-1) minutes left. Basically, it decrements the number every minute. I need the timer to function even when I close the app (not entirely, just press the home screen and leave it running in the background), so that when the user comes back to the app after leaving it in the background for 5 minutes, the label will say "(x-5) minutes left".
Right now, when I run it in the simulator it works perfectly, but when I run it on my phone it does not work. If I have the app open on my phone, it works, but if the app is running in the background, the label never decrements. Could this be due to differences in the way the simulator and actual iPhone handle multitasking? If so, how can I change my code so that the timer will still update the label every minute so that the correct number is displayed when the user reopens the app?
Here is my timer setup:
var individualTaskTimer = NSTimer()
func createTimerForTopTask(){
individualTaskTimer = NSTimer.scheduledTimerWithTimeInterval(tasks[0].minutes * 60, target: self, selector: "deleteTopTask", userInfo: nil, repeats: true)
}

Answer 1

As mentioned in Ewan Mellor's answer, you will not be able to rely on a timer while the app is in the background. So you will need to adjust as necessary when your app returns to the foreground.
Upon first reading the documentation, it might seem like viewWillAppear and viewWillDisappear (or viewDidDisappear) are the correct places to handle this. However, they do not get called when the app moves to/from the background.
Instead, you can make use of two notifications, UIApplicationWillResignActiveNotification and UIApplicationDidBecomeActiveNotification. The first notification will be sent to your app when it is about to go into the background. The second notification will be sent to your app when it is about to return to the foreground.
So in viewWillAppear you can register for the notifications as follows:
override func viewWillAppear(animated: Bool) {
// some other code
NSNotificationCenter.defaultCenter().addObserver(self, selector: "activeAgain", name: "UIApplicationDidBecomeActiveNotification", object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: "goingAway", name: "UIApplicationWillResignActiveNotification", object: nil)
}
where activeAgain and goingAway are two functions you've written to enable and disable the timer. So based on the code snippet in your question, they would look something like this:
func activeAgain() {
let newTime = // calculate how much time is left (in seconds)
individualTaskTimer = NSTimer.scheduledTimerWithTimeInterval(newTime, target: self, selector: "deleteTopTask", userInfo: nil, repeats: true)
}
func goingAway() {
individualTaskTimer.invalidate()
}
Note that you need to unregister for the notifications when you switch away from this view. Doing this in viewWillDisappear is probably a good spot.

Answer 2

You can't do this. iOS will suspend your app after a short while when it goes into the background. Just update your label with the correct time when the app comes back to the foreground.