Timer-based example used during my Advanced SwiftUI workshop.

SwiftUITimers.swift

import SwiftUI

// Model + controller:

struct NamedTimer: Identifiable, Equatable {
    let id = UUID()
    var name: String
    var seconds = 0
}

final class TimerController: ObservableObject {
    @Published var timers: [NamedTimer]
    private lazy var systemTimer = makeSystemTimer()

    init() {
        timers = [NamedTimer(name: "First timer")]
        RunLoop.main.add(systemTimer, forMode: .common)
    }

    private func makeSystemTimer() -> Timer {
        Timer(
            timeInterval: 1,
            repeats: true,
            block: { [weak self] _ in
                self?.incrementTimers()
            }
        )
    }

    private func incrementTimers() {
        timers = timers.map { timer in
            var timer = timer
            timer.seconds += 1
            return timer
        }
    }
}

// List view:

struct TimerList: View {
    @StateObject private var controller = TimerController()

    var body: some View {
        NavigationView {
            List {
                ForEach(controller.timers) { timer in
                    NavigationLink(destination: {
                        TimerDetailsView(
                            viewModel: TimerDetailsViewModel(
                                timer: timer
                            )
                        )
                    }, label: {
                        TimerListRow(timer: timer)
                    })
                }

                Button("Add timer") {
                    controller.timers.append(NamedTimer(
                        name: "New timer"
                    ))
                }
            }
            .navigationTitle("Timers")
        }
    }
}

struct TimerListRow: View {
    var timer: NamedTimer

    var body: some View {
        HStack {
            Text(timer.name)
            Spacer()
            Text(String(timer.seconds))
                .bold()
        }
    }
}

// Detail view:

final class TimerDetailsViewModel: ObservableObject {
    @Published var timer: NamedTimer
    @Published var temporaryNotes = ""

    init(timer: NamedTimer) {
        self.timer = timer
    }

    func duplicateTimer() {
        // To be implemented
    }
}

struct TimerDetailsView: View {
    @ObservedObject var viewModel: TimerDetailsViewModel

    var body: some View {
        VStack(spacing: 20) {
            TextField("Timer name", text: $viewModel.timer.name)
                .textFieldStyle(.roundedBorder)
                .frame(maxWidth: 200)

            Text(String(viewModel.timer.seconds))
                .font(.largeTitle)

            VStack(alignment: .leading) {
                Text("Temporary notes:")
                TextEditor(text: $viewModel.temporaryNotes)
                    .cornerRadius(5)
            }
            .padding()
            .background(Color(.systemGroupedBackground))
        }
        .padding(.top, 20)
        .toolbar {
            Button("Duplicate") {
                viewModel.duplicateTimer()
            }
        }
        .navigationTitle("Timer details")
        .navigationBarTitleDisplayMode(.inline)
    }
}

// Preview:

#Preview {
    TimerList()
}