How to properly measure elapsed time in background in Swift - swift

Question

I have this two functions that measure the elapsed time when the phone is locked or the app is in background:
func saveTimeInBackground(){
startMeasureTime = Int(Date.timeIntervalSinceReferenceDate)
}
func timeOnAppActivated(){
stopMeasureTime = Int(Date.timeIntervalSinceReferenceDate)
elapsedTime = stopMeasureTime - startMeasureTime
seconds = seconds - elapsedTime + 2
if seconds > 0 {
timerLbl.text = "time: \(seconds)"
} else {
seconds = 0
timerLbl.text = "time: \(seconds)"
}
}
and then in the viewDidLoad() i have observers that are trigger the functions when the app becomes active/inactive:
NotificationCenter.default.addObserver(self, selector: #selector(saveTimeInBackground), name: Notification.Name.UIApplicationWillResignActive, object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(timeOnAppActivated), name: Notification.Name.UIApplicationDidBecomeActive, object: nil)
The problem is that when the app becomes active there are 2 seconds (approximately) of difference so i've added 2 seconds and it seems to work fine, but only if the elapsed time is > 15 seconds.
If i lock the phone and immediately unlock it the there are like 5 or more seconds that are missing. For example, if there are 50 seconds left, when i lock and immediately unlock it there are like 42 seconds left.
Can anyone please explain, what i am doing wrong?
Edit: The logic of the app is this:
It starts a match between 2 players with 60 seconds for a game. The problem is that when one of the players locks the phone the app stop to measure the time. This way if the player1 has 10 seconds left to make a move, the player2 still has 50 seconds left. I'm looking for a reliable way to calculate the time even if the player locks the phone or put the app in background.
Edit 2: I think i figured out what the problem is: I think the issue has to do with the fact that the “seconds” are Int, and the Date not and when it gets converted it’s rounded up. I didn't tested it, but when i ahve the solution i'll post the answer. Thanks all for your time!

Answer 1

You're relying on exact timing of notifications that aren't guaranteed to have any exact timing. There's no guarantee about when, exactly, either of those notifications will arrive, and there's nothing you can do about that. Even your two-second fix is, as you say, approximate. It'll probably be different on different models of iPhone or even at different times on the same iPhone, depending how busy iOS is when you check.
What's more, when you go into the background, you can't be certain that you'll stay there. Once in the background, iOS might decide to terminate your app at any time.
I'm not sure what the goal is here but I think you'll need to reconsider what you want to do and see if there's some other approach. Your current two-second hack will, at best, spawn a bunch of other hacks (like the 15 second threshold you mention) without ever being especially accurate. And then it'll probably all break in the next iOS update when some iOS change causes the timing to change.

Answer 2

I would use Date object to track game time.
func gameStart() {
gameStartDate = Date()
}
func timeOnAppActivated() {
let secondsLeft = 60 - abs(gameStartDate?.timeIntervalSinceNow ?? 0)
if secondsLeft > 0 {
timerLbl.text = "time: \(secondsLeft)"
} else {
timerLbl.text = "time: 0"
}
}

Answer 3

Ok, like I mention in the edit 2 of the question:
The first issue was because "seconds" is a Int and then it almost always gains or lose when converting it from Double.
But the main problem was that i had to invalidate the timer when the app enter in background and i didn't.
So now with invalidating the timer when the app gets the notification that will enter background and then starting it when it enter foreground everything works fine.

Answer 4

To test this properly call those methods on button click. It may be coz of delay in releasing some resources in background.