A repeating GCD timer that can run on a background queue

RepeatingTimer.swift

/// RepeatingTimer mimics the API of DispatchSourceTimer but in a way that prevents
/// crashes that occur from calling resume multiple times on a timer that is
/// already resumed (noted by https://github.com/SiftScience/sift-ios/issues/52
class RepeatingTimer {

    let timeInterval: TimeInterval
    
    init(timeInterval: TimeInterval) {
        self.timeInterval = timeInterval
    }
    
    private lazy var timer: DispatchSourceTimer = {
        let t = DispatchSource.makeTimerSource()
        t.schedule(deadline: .now() + self.timeInterval, repeating: self.timeInterval)
        t.setEventHandler(handler: { [weak self] in
            self?.eventHandler?()
        })
        return t
    }()

    var eventHandler: (() -> Void)?

    private enum State {
        case suspended
        case resumed
    }

    private var state: State = .suspended

    deinit {
        timer.setEventHandler {}
        timer.cancel()
        /*
         If the timer is suspended, calling cancel without resuming
         triggers a crash. This is documented here https://forums.developer.apple.com/thread/15902
         */
        resume()
        eventHandler = nil
    }

    func resume() {
        if state == .resumed {
            return
        }
        state = .resumed
        timer.resume()
    }

    func suspend() {
        if state == .suspended {
            return
        }
        state = .suspended
        timer.suspend()
    }
}

Hi @danielgalasko,
I have read your article in Medium with attention and was interested in using your class for a personal project. I tested it to display a background timer in a kind of game screen. This way, user can see elapsed time on the main screen and meanwhile, he can go through other VC without stopping timer.

However, a problem occurs :
For example, I store in CoreData the value for time elapsed (counter) :
var counter: Int16? var t = RepeatingTimer(timeInterval: 1)
In the viewDidLoad :
t.eventHandler = { self.counter! += 1 print("counter \(String(describing: self.counter!))") self.myEntity?.timeAsAttribute = self.counter ?? 0 self.save() DispatchQueue.main.async { self.timerField.text = String(describing: self.counter ?? 0) // label on my main screen } }
If I suspend() the timer, go to other VC, come back to my main screen, and resume() : everything is ok, my label (texfield) is actualized with my timeInterval.
But when, I go to other VC and come back without suspend() the timer, I can see my print func working, but label is "freezing" on the last known value before coming back, and my button play/Pause doesn't work... Unable to make it work correctly.
Any idea about what I did wrong ?
I'm juste like my label, I'm freezed on my func.

Hi @danielgalasko,

I'm trying to use this class but something weird is happening.

I used this bit of code from your Medium article, but nothing was printed to the console:

let t = RepeatingTimer(timeInterval: 3)
        t.eventHandler = {
            print("Timer Fired")
        }
        t.resume()

But, (now comes the weird part) if I do this, then "Timer Fired" gets printed every 3 seconds:

let t = RepeatingTimer(timeInterval: 3)
        t.eventHandler = {
            print("Timer Fired")
            if(false){   //I know this makes no sense, but it works. Go figure...
                t.suspend()
            }
        }
        t.resume()

Any ideas?

variable t should be a class variable instead of local. And it will work.

This is not working when app goes in background. Any idea on resolution? Please help.

This is not working when app goes in background. Any idea on resolution? Please help.

As far as I know, timers do not work in the background. What do you user your timer for?

If you have a stopwatch type of UI where your time needs to update when user reopens the app, try saving a timestamp when the app is backgrounded. Then, when app is foregrounded again by the user, calculate the diff in time and update your UI and restart your timer.

@iSachdeva & @Streebor, any solution for run a background timer?