How to make an action repeat using Timer

Swift version: 5.10

Paul Hudson @twostraws September 19th 2019

Timers are a great way to run code on a repeating basis, and iOS has the Timer class to handle it for you. First, create a property of the type Timer?. For example:

var gameTimer: Timer?

You can then create that timer in somewhere like viewDidLoad() and tell it to execute every five seconds, like this:

gameTimer = Timer.scheduledTimer(timeInterval: 5, target: self, selector: #selector(runTimedCode), userInfo: nil, repeats: true)

The runTimedCode selector means that the timer will call a method named runTimedCode() every five seconds until the timer is terminated, so you'll need to replace that method name with whatever you want to call – and don’t forget to mark it using @objc.

Important note: because your object has a property to store the timer, and the timer calls a method on the object, you have a strong reference cycle that means neither object can be freed. To fix this, make sure you invalidate the timer when you're done with it, such as when your view is about to disappear:

gameTimer?.invalidate()

Alternatively, you can create timers that execute a closure rather than calling a method. For example, this creates a timer that executes a closure every second, and inside the closure a random number between 1 and 20 is selected:

Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { timer in
    let randomNumber = Int.random(in: 1...20)
    print("Number: \(randomNumber)")

    if randomNumber == 10 {
        timer.invalidate()
    }
}

As you can see, the closure is given a reference to the active timer, and can invalidate it at will – in our case, that’s when the random number is 10.