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 correctly stops when I call self.timer.upstream.connect().cancel().

However when I go back to ContentView and then navigate to TimerView again, I want the timer to start counting again however this does not happen. secondsElapsed does reset to 0 but the timer doesn't run.

import SwiftUI

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: TimerView()) {
                Text("Go to Timer View")
            }
        }
    }
}

struct TimerView: View {
    @State var secondsElapsed = 0
    var timer = Timer.publish (every: 1, on: .main, in: .common).autoconnect()
    var body: some View {
        VStack {
            Text("\(self.secondsElapsed) seconds elapsed")
            Button("Stop timer",
                   action: {
                    self.timer.upstream.connect().cancel()
            })
        }.onReceive(timer) { _ in
            self.secondsElapsed += 1
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Answer 1

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

What I do is just re-declare the timer.

Update 12/2021:

The previous code was from SwiftUI 1, which had some "oddities," especially around lifecycle. So, I updated the code to how I would do it in SwiftUI 3.

For this code, you must import combine to declare a Cancellable type.

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)
    @State var connectedTimer: Cancellable? = nil
    
    var body: some View {
        VStack {
            Text("\(self.secondsElapsed) seconds elapsed")
            Button("Stop Timer", action: {
                self.cancelTimer()
            })
            Button("Continue Timer", action: {
                self.instantiateTimer()
            })
            Button("Restart Timer", action: {
                self.restartTimer()
            })
        }.onAppear {
            self.instantiateTimer()
        }.onDisappear {
            self.cancelTimer()
        }.onReceive(timer) { _ in
            self.secondsElapsed += 1
        }
    }
    
    func instantiateTimer() {
        self.timer = Timer.publish(every: 1, on: .main, in: .common)
        self.connectedTimer = self.timer.connect()
        return
    }
    
    func cancelTimer() {
        self.connectedTimer?.cancel()
        return
    }
    
    func resetCounter() {
        self.secondsElapsed = 0
        return
    }
    
    func restartTimer() {
        self.secondsElapsed = 0
        self.cancelTimer()
        self.instantiateTimer()
        return
    }
}