Swift 4 Timer Crashes with NSException

Question

I\'ve been searching for a way to use a Timer in Swift 4 and looked at this article. I test out my code in xcode and when the the timer first ticks (in this case after 10 se

Answer 1

As Andrea said, you should instantiate the timer in viewDidLoad. Also the documentation says:

The selector should have the following signature: timerFireMethod: (including a colon to indicate that the method takes an argument).

And don't forget to disable this timer in, for example, viewDidDisappear. You can't invalidate it in deinit because the repeating timer keeps a strong reference to its target, and your deinit will not get called as long as the timer is running. And if you remove it in viewDidDisappear, you might want to create the timer in viewDidAppear.

Thus, resulting in something like:

class ViewController: UIViewController {

    weak var timer: Timer?

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

        timer = Timer.scheduledTimer(timeInterval: 10, target: self, selector: #selector(tick(_:)), userInfo: nil, repeats: true)
    }

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

        timer?.invalidate()
    }

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

}

Or you can use the block-based rendition, with [weak self] pattern, and the timer won't keep a strong reference to the view controller, in which case you can use deinit:

class ViewController: UIViewController {

    var timer: Timer?

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

        timer = Timer.scheduledTimer(withTimeInterval: 10, repeats: true) { [weak self] timer in  // the `[weak self] reference is only needed if you reference `self` in the closure
            print("tick")
        }
    }

    deinit {
        timer?.invalidate()
    }

}