Modifying user input in Swift - swift

Question

I'm trying to learn Swift and have gone through several tutorials, however, I seem to be going in circles and need some direction on how to solve this problem.
My goal is to create a decrementing timer, given a user-input (in seconds), in this case I've chosen a Stepper to -/+ the value. Then begin decrementing the timer on a button press, in this case "Decrement". The counter is displayed on the label.
This problem is super easy if I hard code the starting value, but what purpose would that serve for a UI Test. So, this was the "challenging" task I was able to think of to help understand how SwiftUI works.
The problem I'm encountering is the variable passed by the user is immutable. I have tried making copies of it or assigning it to other variables to manipulate but seem to be going in circles. A nudge in the right direction or a potential solution would go a long way.
struct ContentView: View {
#State private var timeInput: Int = 0
var timer = Timer()
var timeInputCopy: Int {
timeInput
}
var body: some View {
Stepper("Input", value: $timeInput, in: 0...150)
Button("Decrement", action: decrementFunction)
Label(String(timeInputCopy), image: "")
.labelStyle(TitleOnlyLabelStyle())
}
func decrementFunction() {
timer.invalidate()
timer = Timer.schedulerTimer(timeInterval: 1,
target: self,
selector: #selector(ContentView.timerClass),
userInfo: nil,
repeats: true)
}
func timerClass() {
timeInputCopy -= timeInputCopy
if (timeInputCopy == 0) {
timer.invalidate()
}
}
> Cannot assign to property: 'self' is immutable
> Mark method 'mutating' to make 'self' mutable
Attempting to auto-fix as Xcode recommends does not lead to a productive solution. I feel I am missing a core principle here.

Answer 1

As mentioned in my comments above:
timeInputCopy doesn't have a point -- it's not really a copy, it's just a computed property that returns timeInput
You won't have much luck with that form of Timer in SwiftUI with a selector. Instead, look at the Timer publisher.
Here's one solution:
import Combine
import SwiftUI
class TimerManager : ObservableObject {
#Published var timeRemaining = 0
private var cancellable : AnyCancellable?
func startTimer(initial: Int) {
timeRemaining = initial
cancellable = Timer.publish(every: 1, on: .main, in: .common)
.autoconnect()
.sink { _ in
self.timeRemaining -= 1
if self.timeRemaining == 0 {
self.cancellable?.cancel()
}
}
}
}
struct ContentView: View {
#StateObject private var timerManager = TimerManager()
#State private var stepperValue = 60
var body: some View {
Stepper("Input \(stepperValue)", value: $stepperValue, in: 0...150)
Button("Start") {
timerManager.startTimer(initial: stepperValue)
}
Label("\(timerManager.timeRemaining)", image: "")
.labelStyle(TitleOnlyLabelStyle())
}
}
This could all be done in the View, but using the ObservableObject gives a nice separation of managing the state of the timer vs the state of the UI.