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How to use a timer with SwiftUI

Paul Hudson    @twostraws   

Fully updated for Xcode 11.4

If you want to run some code regularly, perhaps to make a countdown timer or similar, you should use Timer and the onReceive() modifier.

For example, this code creates a timer publisher that fires every second, updating a label with the current time:

struct ContentView: View {
    @State var currentDate = Date()
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

    var body: some View {
        Text("\(currentDate)")
            .onReceive(timer) { input in
                self.currentDate = input
            }
    }
}

It’s important to use .main for the runloop option, because our timer will update the user interface. As for the .common mode, that allows the timer to run alongside other common events – for example, if the text was in a scroll view that was moving.

As you can see, the onReceive() closure gets passed in some input containing the current date. In the code above we assign that straight to currentDate, but you could use it to calculate how much time has passed since a previous date.

If you specifically wanted to create a countdown timer or stopwatch, you should create some state to track how much time remains, then subtract from that when the timer fires.

For example, we could create a countdown timer that shows time remaining in a label, like this:

struct ContentView: View {
    @State var timeRemaining = 10
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

    var body: some View {
        Text("\(timeRemaining)")
            .onReceive(timer) { _ in
                if self.timeRemaining > 0 {
                    self.timeRemaining -= 1
                }
            }
    }
}
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