✨cancellable timer

💾 程式

📗 參考：Paul ⟩ Counting down with a Timer

👥 相關：.onChange(), .onRecieve() events.

import SwiftUI

struct TimerView: View {
    
    // gives the user 100 seconds to start with
    @State private var timeRemaining = 100
    @State private var isTimerStopped = false
    @State private var isTimerCancelled = false
    
    // ⭐️ timer that fires once a second on the main thread.
    let timer = Timer
        // ⭐️ Timer.TimerPublisher
        .publish(
            every: 1,       // timer fires every 1 second.
            on: .main,      // run on the .main thread (UI thread).
            in: .common     // run on the .common run loop.
            // Run loop lets iOS handle running code while the user is actively
            // doing something, such as scrolling in a list
        )
        // ⭐️ connect automatically when a subscriber attaches
        .autoconnect()
    
    // ⭐️ detect whether app has gone background
    @Environment(\.scenePhase) private var scenePhase
    @State private var isActive = true
    
    var body: some View {
        VStack {
            timerView
            HStack {
                stopButton
                cancelButton
            }
        }
        .padding(8)
        .border(.secondary)
    }
    
    /// text for timer
    var text: some View {
        Text("Time: \(timeRemaining)")
            .font(.largeTitle)
            .foregroundColor(isTimerCancelled ? .secondary : .white)
            .padding(.horizontal, 20)
            .padding(.vertical, 5)
            .background(.pink.opacity(0.75))
            .clipShape(Capsule())
    }
    
    /// timer view
    var timerView: some View {
        text
            // ⭐️ connect `timer` automatically❓
            .onReceive(timer) { time in
                // ⭐️ if app goes background or timer stopped, stop count-down immediately.
                guard isActive && !isTimerStopped else { return }
                // ⭐️ count down
                if timeRemaining > 0 { timeRemaining -= 1 }
            }
            // ⭐️ mark the app inactive once it goes background.
            .onChange(of: scenePhase) { newPhase in
                isActive = (newPhase == .active)
            }
    }
    
    /// Stop/Resume button
    var stopButton: some View {
        Button {
            isTimerStopped.toggle()
        } label: {
            Text(isTimerStopped ? "Resume" : "Stop")
        }
        .disabled(isTimerCancelled)
    }
    
    /// cancel button
    var cancelButton: some View {
        Button {
            // ⭐️ cancel the timer
            timer               // Publishers.Autoconnect<Timer.TimerPublisher>
                .upstream       // Timer.TimerPublisher (ConnectablePublisher)
                .connect()      // allow to produce elements, return an instance (Cancellable)
                .cancel()       // ⭐️
            isTimerCancelled = true
        } label: {
            Text("Cancel")
        }
        .disabled(isTimerCancelled)
    }
}

📗 參考

• Paul ⟩

  • Counting down with a Timer
  • Triggering events repeatedly using a timer
• Combine ⟩ Replacing Foundation Timers with Timer Publishers ⭐️
• Ray ⟩ Combine: Asynchronous Programming with Swift, Ch. 11: Timers
• Sundell ⟩ Managing self and cancellable references when using Combine

📘 手冊

• Foundation ⟩ Task Management ⟩ Timer (class)

  • .publish(every:tolerance:on:in:options:) -> Timer.TimerPublisher
  • Timer.TimerPublisher (class)

    • .autoconnect() -> Publishers.Autoconnect<Timer.TimerPublisher>
• Combine ⟩ ConnectablePublisher (protocol) ⟩

  • .connect() ⭐️ - connects to the publisher, allowing it to produce elements, and returns an instance with which to cancel publishing.
• SwiftUI ⟩

  • Scenes ⟩ ScenePhase (enum)
  • State ⟩ EnvironmentValues (struct) ⟩ .scenePhase (instance property)
  • View ⟩ Input and Event Modifiers ⟩

    • .onReceive(_:perform:) - perform action when emitted data detected.
    • .onChange(of:perform:) - perform action when specific value changes.

👥 相關

• use @Environment values.
• use Timer.TimerPublisher (ConnectablePublisher) Publisher.