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On Xcode 15.0.1, I’m trying to use SwiftData to create a StopwatchItem in my stopwatch app, which has a timer property. I get the errors:

1. Instance method ‘setValue(forKey:to:)’ requires that ‘Timer’ conform to ‘PersistentModel’

2. Instance method ‘getValue(forKey:)’ requires that ‘Timer’ conform to ‘PersistentModel’

I know this means something along the lines of “Cannot save Timer to SwiftData”, but I’m not sure how I can resolve this problem I’m facing.

My stopwatch app would have the ability to create and control multiple stopwatches, which includes global control (Start all, stop all, reset all etc.) and the ability to save stopwatch sets to refer to at a later time.

Solutions I tried

1. ChatGPT 4o suggested solution: Define a separate StopwatchController which has the functions start(), stop(), reset() and contains a timer property.
Why is this not ideal: I find that I cannot globally control the stopwatches this way

2. Defining stopwatch control functions in StopwatchItemView, which is shown in ContentView in the form of

forEach(stopwatches) { stopwatch in
    StopwatchItemView(stopwatch: stopwatch)
}

Why this is not ideal: Unable to globally control stopwatches, harder to persist timer status/progress when app is quit

Code

Note: Including ContentView is not relevant as it still features the boilerplate code.

StopwatchController.swift

import Foundation
import SwiftData
import Combine

@Model
class StopwatchSet: Identifiable, ObservableObject {
    var id = UUID()
    var label: String
    var stopwatches: [StopwatchItem]
    
    init(id: UUID = UUID(), label: String, stopwatches: [StopwatchItem]) {
        self.id = id
        self.label = label
        self.stopwatches = stopwatches
    }
}

@Model
class StopwatchItem: Identifiable, ObservableObject {
    var id = UUID()
    var isRunning: Bool = false
    var label: String
    var timeElapsed: Double = 0.0
    var laps: [Lap] = []
    private var timer: Timer?
    
    init(id: UUID = UUID(), label: String, timeElapsed: Double) {
        self.id = id
        self.label = label
        self.timeElapsed = timeElapsed
    }
    
    func start() {
        isRunning = true
        timer = Timer.scheduledTimer(withTimeInterval: 0.01, repeats: true) { [weak self] _ in
            self?.timeElapsed += 0.01
        }
    }
    
    func stop() {
        isRunning = false
        timer?.invalidate()
        timer = nil
    }
    
    func reset() {
        stop()
        timeElapsed = 0.0
    }
}

@Model
class Lap: Identifiable, ObservableObject {
    var id = UUID()
    var label: String
    var timeElapsed: String
    
    init(label: String, timeElapsed: String) {
        self.label = label
        self.timeElapsed = timeElapsed
    }
}

StopwatchItemView.swift

import SwiftUI
import SwiftData

struct StopwatchItemView: View {
    
    @State var stopwatch: StopwatchItem
    
    var body: some View {
        VStack {
            Text(formatTime(stopwatch: stopwatch, input: stopwatch.timeElapsed))
        }
    }
}

#Preview {
    StopwatchItemView(stopwatch: StopwatchItem(label: "Stopwatch", timeElapsed: 0.0))
}