Swift 3 Timer not firing

Question

I've been trying to utilize Timer in Swift and I've simplified it town to the following:

func startTimer () {
    timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(ViewController.test), userInfo: nil, repeats: true)
}
func test () {
    print("FIRED")
}

I would like to call this from another function and have verified the startTimer function works, but the timer doesn't fire. Does this have something to do with the RunLoop? I'm fairly new to coding so any explanation would be appreciated.

Solution

Good Practice: In startTimer() check that the timer has not already been created and make the assignment. In stopTimer() check that the timer exists before calling invalidate, and set the timer back to nil.

Also, for your selector make sure you have the @objc prefix. You should be able to get a working timer with the code provided. Happy coding!

class SomeClass {
    var timer: Timer?

    func startTimer() {
        guard timer == nil else { return }
        timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(test), userInfo: nil, repeats: true)
    }

    func stopTimer() {
        guard timer != nil else { return }
        timer?.invalidate()
        timer = nil
    }

    @objc func test() {

    }
}

Comments

[Alex Blair]

One last thing, I don't think ViewController.test is required in your selector parameter, test should work just fine.

[R.P. Carson]

why does checking to see if its nil matter before you set it to nil

[Alex Blair]

It prevents calling invalidate() on a potentially nil object. For instance, lets say you call stopTimer() in multiple functions. It fires once, invalidates the timer, and is set back to nil. If stopTimer() is called again in succession, without reinitializing the timer, you would receive a fatal error. This is because the timer has already been deallocated. Both guard statements are in place to keep starting and stopping the timer in sync.

[R.P. Carson]

but it uses optional chaining and if timer is nil, invalidate wont be called

[Alex Blair]

That's true! @R.P.Carson.

[Tom Spee]

What if startTimer() has a completion block depending on the test() code?

[Alex Blair]

Could you be more specific?