Timer in background is very slow

Question

I'm working with timer and I've try to make it works in background. On simulator this works fine but on my device (iOS 11) it's very slow: 1 seconde became 5 or 6 secondes...

This is my code for run application in background:

backgroundTaskIdentifier = UIApplication.shared.beginBackgroundTask(
    expirationHandler:
      {UIApplication.shared.endBackgroundTask(self.backgroundTaskIdentifier!)}
    )

Info.plist: Application does not run in background : NO

How can I make it works?

EDIT:

This is my timer code:

timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, 
  selector:#selector(ViewController.updateTimer), userInfo: nil, repeats: true)

EDIT 2:

func updateTimer () {
    var j = 0
    for _ in rows {
        if (rows[j]["Playing"] as! Bool == true ) {
            rows[j]["time"] = (rows[j]["time"] as! Double + 0.01) as AnyObject
        // print(rows[j]["time"]) - PRINT OUTPUT HERE
            rows[j]["lastTime"] = (rows[j]["lastTime"] as! Double + 0.01) as AnyObject
        }
        if (rows[j]["lastTime"] as! Double > 60.0) {
            min[j] += 1
            rows[j]["lastTime"] = 0.00 as AnyObject
        }
        j += 1
    }
}

I assume you are debugging out some values in the ViewController.updateTimer function some prints ... did you test your output also in the console? — AlexWoe89, Oct 09 '17 at 07:51
@AlexWoe89 yes i've print my output in console, and it's very very slow when the application run in background. It's only happened on my device and not on simulator... — pierreafranck, Oct 09 '17 at 07:58
Btw, you are not using 1 second as an interval, you are using 0,01 seconds: https://developer.apple.com/documentation/foundation/timer/1415941-scheduledtimer — Marco Pace, Oct 09 '17 at 07:58
@MarcoPace Yes, I know! So 0,01 sec become 0,05 sec or more in background. — pierreafranck, Oct 09 '17 at 08:02
Can you share the code in updateTimer to see how you debug it? — Robert D. Mogos, Oct 09 '17 at 08:11
Don't do background tasks with Timer. It will not work as you might expect. https://stackoverflow.com/questions/43755587/swift-timer-scheduledtimer-doesnt-work — Nishant Bhindi, Oct 09 '17 at 09:37
Background Tasks and Timers started from Background Tasks have limitations. One thing is that `NSTimer` may stop if the app goes to background. See [https://stackoverflow.com/a/42319410/1317117](Swift 3 - How to make timer work in background) or [https://stackoverflow.com/a/5187836/1317117](NSTimer on background IS working). It does not directly answer your question but it gives you some hints why it does not work as expected. A (temporary) suspended timer may lead to your effect. — andih, Oct 10 '17 at 03:38

score 0 · Answer 1 · answered Oct 09 '17 at 08:28

0

Instead of just printing your output, which might be executed from a different thread, print it from the main thread. That's why you might see the delay:

DispatchQueue.main.async {
  print(rows[j]["time"])
}

answered Oct 09 '17 at 08:28

Robert D. Mogos

900
7
14

score 0 · Answer 2 · answered Oct 09 '17 at 08:37

0

Your timer interval is too short. Timers have a resolution of 50-100 ms, so your interval (10 ms) can't get executed so fast. I think when your app is in the background this effect increases.

Take a look at this answer: https://stackoverflow.com/a/30983444/5613280

answered Oct 09 '17 at 08:37

Martin

151
10

Thanks for your asnwer. I've try with a delay of 50ms and same result... very slow in background – pierreafranck Oct 09 '17 at 08:47
Did you also try it with one full second? – Martin Oct 09 '17 at 08:54
Yes, I just try it, it works. But I need to be precise at 0.01 :'( – pierreafranck Oct 09 '17 at 09:05
Take a look at [this](https://stackoverflow.com/questions/20261129/better-alternative-for-nstimer), you would replace the Timer with DispatchAfter – Robert D. Mogos Oct 09 '17 at 09:16
I'm afraid that this is not possible, the reason for that is clearly explained in the post i linked above. Can you edit your answer and describe what you want to achieve with the timer? Maybe there is another working solution for this problem. – Martin Oct 09 '17 at 09:16

Timer in background is very slow

2 Answers2