How to set up multiple combine timer publishers? - swift

Question

Here I have a simple SwiftUI project which has two timer publishers which are set to fire every 1 second and 2 seconds respectively.
The expected behavior is that the first label would update every second and the second label would update every 2 seconds. However what actually happens is only the first label updates every second and the second label remains at 0 indefinitely.
I know it's possible to make multiple timers using Timer.scheduledTimer(withTimeInterval:) by simply making new variables of those timer instances but it doesn't seem to work the same way with these publishers.
How can I make both timers work?
import SwiftUI
struct ContentView: View {
#State private var counter1 = 0
#State private var counter2 = 0
let timer1 = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
let timer2 = Timer.publish(every: 2, on: .main, in: .common).autoconnect()
var body: some View {
VStack(spacing: 30) {
Text(String(counter1))
.frame(width: 50, height: 20)
Text(String(counter2))
.frame(width: 50, height: 20)
}
.padding()
.onReceive(timer1) {_ in
counter1 += 1
}
.onReceive(timer2) {_ in
counter2 += 2
}
}
}

Answer 1

In SwiftUI your view is a transitory struct. Ie the system remakes the views all the time and throws away the old ones. The View, therefore should not be holding the references to the timers (you will be making new timers and dropping old ones all the time). Make a viewModel and store the values there. In general don't store things on the view that are not in a property wrapper or passed in through the initializer.
final class ViewModel: ObservableObject {
#Published private(set) var first = 0
#Published private(set) var second = 0
private var subscriptions: Set<AnyCancellable> = []
func start() {
Timer.publish(every: 1, on: .main, in: .common)
.autoconnect()
.scan(0) { accumulated, _ in accumulated + 1 }
.assign(to: \.first, on: self)
.store(in: &subscriptions)
Timer.publish(every: 2, on: .main, in: .common)
.autoconnect()
.scan(0) { accumulated, _ in accumulated + 1 }
.assign(to: \.second, on: self)
.store(in: &subscriptions)
}
func stop() {
subscriptions.removeAll()
}
}
struct ContentView: View {
#StateObject private var viewModel = ViewModel()
var body: some View {
VStack {
Text(String(describing: viewModel.first))
Text(String(describing: viewModel.second))
}
.onAppear { viewModel.start() }
.onDisappear { viewModel.stop() }
}
}