Swift Timer.scheduledTimer() doesn't work

Question

I have two views in my swift app. I am performing a segue as below.

ViewController.swift -----------------> GameViewController.swift

When loading the GameViewController an value array also passed to GameViewController.swift from ViewController.swift

A timer should be initialized in GameViewController.swift

I tried to initialize a timer and call a method through it, but it doesn't work.

Followings are my code snippets.

ViewController.swift

func signIn(difficultyLvl:String){
    let username = usernameTxt.text
    let password = passwordTxt.text

    let url = URL(string: "http://192.168.1.106/speed/scoreBoardController.php?username="+username!+"&password="+password!+"&action=SIGNIN")

    let task = URLSession.shared.dataTask(with: url!) {(data, response, error) in
        let isPassed = String(data: data!, encoding:.utf8)?.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)

        var gameViewControllerParams = [Int: [String: String]]()
        gameViewControllerParams[0] = ["userId" : isPassed!]
        gameViewControllerParams[1] = ["difficultyLvl" : difficultyLvl]

        if(isPassed != "null"){
            self.performSegue(withIdentifier: "gotoGame", sender: gameViewControllerParams)
        }
    }

    task.resume()
}

GameViewController.swift

class GameViewController: UIViewController {

    var gameViewControllerParams = [Int: [String: String]]()

    override func viewDidLoad() {
        super.viewDidLoad()

        let _ = Timer.scheduledTimer(timeInterval: 1.0, target:self, selector: #selector(self.setCalculationLs), userInfo:nil,repeats: true)
    }

    func setCalculationLs(){
        print("Timing")
    }

}

Answer 1

Timers don't work on background queues (without some sleight of hand involving creating run loops or manually scheduling it on an existing run loop). But you should never initiate any UI update from anything other than the main queue, anyway.

So, since you're calling performSegue from a URLSession completion closure (which runs on a background queue), it's actually running viewDidLoad from the background queue, too. Thus the attempt to schedule the timer is failing. To get around this, you have to manually dispatch the performSegue code to the main queue:

let task = URLSession.shared.dataTask(with: url!) { data, response, error in
    ...

    if isPassed != "null" {
        DispatchQueue.main.async {
            self.performSegue(withIdentifier: "gotoGame", sender: ...)
        }
    }
}

If you're ever unsure whether some code is running on the main queue or not, refer to the documentation. Or you can use a dispatch precondition:

dispatchPrecondition(condition: .onQueue(.main))

That way it will (in debug builds) stop the app if you've accidentally invoked the code from a background queue.

---

Unrelated to your current problem, but as an aside, to avoid a strong reference cycle between the timer and the view controller, you generally want to keep a reference to the timer so that you can invalidate it when the view disappears (e.g. create timer in viewDidAppear and remove it in viewDidDisappear). Otherwise you can end up retaining the GameViewController after it was dismissed, e.g.:

class GameViewController: UIViewController {

    weak var timer: Timer?

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        timer = Timer.scheduledTimer(timeInterval: 1.0, target:self, selector: #selector(setCalculationLs(_:)), userInfo: nil, repeats: true)
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)

        timer?.invalidate()
    }

    @objc func setCalculationLs(_ timer: Timer) {
        print("Tick")
    }
}

Or in iOS 10 or later, you can use the block-based variant with weak reference to self, and invalidate in deinit:

class GameViewController: UIViewController {

    weak var timer: Timer?

    override func viewDidLoad() {
        super.viewDidLoad()

        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] timer in
            self?.setCalculationLs()
        }
    }

    deinit {
        timer?.invalidate()
    }

    func setCalculationLs() {
        print("Tick")
    }

}