SwiftUI: How to run a CountdownTimer in background

Question

How make CountdownTimer and how make let elapsed = public var . I want to make the variable constantly decrease life. For example, as in Tamagotchi - You need to constantly feed the animal so that it does not die. This timer should continuously run until it reaches 0. When the tamagotchi eats, the value is again added to the variable, which decreases over time.

class Stopwatch: ObservableObject {
    /// String to show in UI
    @Published private(set) var message = "Not running"
    /// Is the timer running?
    @Published private(set) var isRunning = false
    /// Time that we're counting from
    private var startTime: Date? { didSet { saveStartTime() } }
    /// The timer
    private var timer: AnyCancellable?
    init() {
        startTime = fetchStartTime()
        if startTime != nil {
            start()
        }
    }
}

---

extension Stopwatch {
    func start() {
        timer?.cancel()  // cancel timer if any
        if startTime == nil {
            startTime = Date()
        }
        message = ""
        timer = Timer
            .publish(every: 0.1, on: .main, in: .common)
            .autoconnect()
            .sink { [weak self] _ in
                guard
                    let self = self,
                    let startTime = self.startTime
                else { return }
                let now = Date()
                let elapsed = now.timeIntervalSince(startTime)
                guard elapsed < 60 else {
                    self.stop()
                    return
                }
                self.message = String(format: "%0.1f", elapsed)
            }
        isRunning = true
    }
    func stop() {
        timer?.cancel()
        timer = nil
        startTime = nil
        isRunning = false
        message = "Not running"
    }
}

---

private extension Stopwatch {
    func saveStartTime() {
        if let startTime = startTime {
            UserDefaults.standard.set(startTime, forKey: "startTime")
        } else {
            UserDefaults.standard.removeObject(forKey: "startTime")
        }
    }
    func fetchStartTime() -> Date? {
        UserDefaults.standard.object(forKey: "startTime") as? Date
    }
}

---