SwiftUI: How do you restart a timer after cancelling it?

Question

I have a navigation view consisting of the main ContentView and a TimerView. The TimerView has a timer which correctly increments and also corr

Answer 1

Having a variable inside a View which isn't attached to a @State variable or an @ObservedObject model doesn't seem to have very well defined documentation - but the general rule is that everything without a @State/@ObservedObject annotation is essentially throwaway - especially as TimerView is a struct and won't be preserved over re-renders.

Timer.TimerPublisher is a class - so essentially this would boil down to either two use cases

• If its transient (ie: resets when the view is removed from screen and reloaded) then put the timer in a @State (be aware of strong retention with self captures - again documentation is thin on how onReceive stores its connections)
• If its non-transient (ie: preserved over the view loading/unloading) then it needs to go into an external model (or a global var) and brought in via the init call for TimerView

That you have @State var secondsElapsed makes me think it should be transient

Answer 2

if you have a countdown timer, you define it something like this:

let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

...and use it on a Text field for e.g.:

    Text("Punkte: \(Punkte.formatnumber())")
        .onReceive(timer) { input in
                            if time < 1 { gameOver() } else { self.time -= TimerToggle ? 1 : 0  }
                    }

so you only have to set the Toggle in the action of the button:

Button(action: { TimerToggle.toggle() }, label: {
                    Text("Pause")
                })

Much simpler and works in SwiftUI 5.2

Answer 3

To my knowledge (and historically), Timer cannot be restarted after cancellation or invalidation.

What I do is just re-declare the timer.

Here is an example with some added personal preferences:

struct TimerView: View {
    @State var secondsElapsed = 0
    @State var timer: Timer.TimerPublisher = Timer.publish (every: 1, on: .main, in: .common)
    var body: some View {
        VStack {
            Text("\(self.secondsElapsed) seconds elapsed")
            Button("Stop timer",
                   action: {
                    self.cancelTimer() // Just to be a little more DRY/maintenance-friendly, but personal preference.
            })
        }.onAppear(perform: {
            self.instantiateTimer() // You could also consider an optional self.timer variable.
            self.timer.connect() // This allows you to manually connect where you want with greater efficiency, if you don't always want to autostart.
        }).onDisappear(perform: {
            // I don't know your exact flow, but you said you want the timer to restart upon return navigation.  
            // So, I am just assuming you want to stop the timer when you navigate out.  
            // But, you can also easily remove this closure.
            // Also, this may not run as you would intuit.
            self.cancelTimer()
        }).onReceive(timer) { _ in
            self.secondsElapsed += 1
        }
    }

    func instantiateTimer() {
        self.timer = Timer.publish (every: 1, on: .main, in: .common)
        return
    }

    func cancelTimer() {
        self.timer.connect().cancel()
        return
    }
}