Invalidating a Timer in SwiftUI doesn't work

Question

I have a Timer object within my view:

@State var timer: Timer?

Inside the view's body, I start the timer when the value of popup changes to true, and I invalidate the timer when viewModel.present changes to true.

var body: some View {
  /* ... */
  .onChange(of: viewModel.popup) { popup in guard popup else { return }; setupTimer() }
  .onChange(of: viewModel.present) { present in guard present else { return }; resetTimer() }
}

In setupTimer(), I create the timer instance:

private func setupTimer() {
  guard timer == nil else { return }
  timer = Timer.scheduledTimer(withTimeInterval: 8.0, repeats: false, block: { _ in
    // This code won't stop running!
  })
}

Next, in resetTimer(), I invalidate and remove the timer:

private func resetTimer() {
  timer?.invalidate()
  timer = nil
  // ...
}

But the timer continues to run. I'm not sure why timer?.invaludate() fails to work; I've used breakpoints to assure that

setupTimer() is indeed only called once such that only a single timer is running (as per the answer here)
self.timer is non-nil when invalidating within resetTimer (as per the answer here)

Any ideas on what is going wrong here? I'm running this program in macOS 12.2.1.

Edit: Ok, so after some investigation, I've seemed to narrow down the problem. It seems as though the timer is successfully invalidated when setupTimer() is called within a tap gesture or action by the user. However, if setupTimer() is called "automatically" within a view (i.e. via onAppear, onChange, etc.) then it cannot be invalidated. So the new question I have is: why is this? Why can I not call setupTimer() within onAppear/onChange and cancel the timer?

Your understanding about State wrapper is not 100% correct, State cannot and did not design for tracking a reference type! Your code should not and must work, because State doing its job. You should change your approach for using observed object. — ios coder, Mar 19 '22 at 02:30
@swiftPunk Odd though, the code seems to work in some cases even when the State wrapper is used; so, I'm not sure this is the root of the issue. — Ben Myers, Mar 19 '22 at 08:36
State wrapper is for value type designed! StateObject wrapper is for reference type! — ios coder, Mar 19 '22 at 14:03

workingdog support Ukraine · Answer 1 · 2022-03-19T08:59:51.390

There could be some other code (that you are not showing) affecting the timer, or maybe it does not work well on macOS 12.2.1. However, here is a simple test that works well for me, on macos 12.3 (the only one I have) using xcode 13.3, targets ios 15 and macCatalyst 12. Does this code work for you?

struct ContentView: View {
    @State var timer: Timer?
    
    var body: some View {
        VStack (spacing: 55) {
            Button(action: {setupTimer()}) {
                Text("start")
            }
            Button(action: {resetTimer()}) {
                Text("reset")
            }
        }.frame(width: 333, height: 333)
        .onAppear {
           setupTimer()
        }
    }
    
    private func setupTimer() {
        guard timer == nil else { return }
        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true, block: { _ in
            // This code stop running when timer invalidate
            print("----> timer running")
        })
    }
    
    private func resetTimer() {
        timer?.invalidate()
        timer = nil
    }
}

Note that in your code, after resetTimer() is called any change in viewModel.popup may trigger setupTimer() again, and so the timer starts again since it is nil.

Thanks for the answer! Yes, your code works. I edited my original question with some additional findings. — Ben Myers, Mar 19 '22 at 08:37
updated my answer with `.onAppear`, and it still works well for me. Does this not work for you? — workingdog support Ukraine, Mar 19 '22 at 09:00

Invalidating a Timer in SwiftUI doesn't work

1 Answers1