How to pause Timer() when the app goes into the background

Question

So I am stuck I am building a bingo app and I want the app to stop the timer when the app goes into the background. my code looks like this

var timer = Timer()

func ViewDidLoad(){
    // other stuff
    timer = Timer.scheduledTimer(timeInterval: 5, target: self, selector: #selector(numberPicker), userInfo: nil, repeats: true)
}

In other posts I've read about this I've seen people recommend defining timer as a weak variable. When I try this My app crashes when it try's to make the timer.

Answer 1

A modern solution is a DispatchSourceTimer which can be suspended and resumed.

The two notification observers use also modern swifty block based API, no @objc needed.

let timer = DispatchSource.makeTimerSource(queue: DispatchQueue.global())

override func viewDidLoad() {
    super.viewDidLoad()
    timer.schedule(deadline: .now() + .seconds(5), repeating: 5.0)
    timer.setEventHandler {
        print("Timer fired")
    }

    NotificationCenter.default.addObserver(forName: UIApplication.didEnterBackgroundNotification, object: nil, queue: .main) { [weak self] _ in
        self?.timer.suspend()
    }
    NotificationCenter.default.addObserver(forName: UIApplication.didBecomeActiveNotification, object: nil, queue: .main) { [weak self] _ in
        self?.timer.resume()
    }

    timer.activate()
}