使用 Swift Combine 创建一个 Timer Publisher

Question

我一直在看Data Flow Through SwiftUI WWDC talk。他们有一张带有示例代码的幻灯片，其中他们使用连接到 SwiftUI 视图的 Timer 发布者，并随时间更新 UI。

我正在编写一些我想做完全相同的事情的代码，但无法弄清楚这个PodcastPlayer.currentTimePublisher 是如何实现的，然后连接到 UI 结构。我也看过所有关于 Combine 的视频。

我怎样才能做到这一点？

示例代码：

struct PlayerView : View {
  let episode: Episode
  @State private var isPlaying: Bool = true
  @State private var currentTime: TimeInterval = 0.0

  var body: some View {
    VStack { // ...
      Text("\(playhead, formatter: currentTimeFormatter)")
    }
    .onReceive(PodcastPlayer.currentTimePublisher) { newCurrentTime in
      self.currentTime = newCurrentTime
    }
  }
}

Answer 1

这里有一个组合计时器的示例。我使用的是全局变量，但您当然应该使用适用于您的场景的任何变量（环境对象、状态等）。

import SwiftUI
import Combine

class MyTimer {
    let currentTimePublisher = Timer.TimerPublisher(interval: 1.0, runLoop: .main, mode: .default)
    let cancellable: AnyCancellable?

    init() {
        self.cancellable = currentTimePublisher.connect() as? AnyCancellable
    }

    deinit {
        self.cancellable?.cancel()
    }
}

let timer = MyTimer()

struct Clock : View {
  @State private var currentTime: Date = Date()

  var body: some View {
    VStack {
      Text("\(currentTime)")
    }
    .onReceive(timer.currentTimePublisher) { newCurrentTime in
      self.currentTime = newCurrentTime
    }
  }
}

Answer 2

使用ObservableObject

使用 Swift Combine 创建一个 Timer Publisher

class TimeCounter: ObservableObject {
    @Published var time = 0
    
    lazy var timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { _ in self.time += 1 }
    init() { timer.fire() }
}

就是这样！现在你只需要观察变化：

struct ContentView: View {
    @StateObject var timeCounter = TimeCounter()
    
    var body: some View {
        Text("\(timeCounter.time)")
    }
}

Answer 3

我实现了一个带有新功能的组合计时器，允许您在不同的时间间隔之间切换。

class CombineTimer {

    private let intervalSubject: CurrentValueSubject<TimeInterval, Never>

    var interval: TimeInterval {
        get {
            intervalSubject.value
        }
        set {
            intervalSubject.send(newValue)
        }
    }

    var publisher: AnyPublisher<Date, Never> {
        intervalSubject
            .map {
                Timer.TimerPublisher(interval: $0, runLoop: .main, mode: .default).autoconnect()
            }
            .switchToLatest()
            .eraseToAnyPublisher()
    }

    init(interval: TimeInterval = 1.0) {
        intervalSubject = CurrentValueSubject<TimeInterval, Never>(interval)
    }

}

要启动计时器，只需订阅publisher 属性。

SomeView()
    .onReceive(combineTimer.publisher) { date in
        // ...
    }

您可以通过更改interval 属性切换到具有不同间隔的新计时器。

combineTimer.interval = someNewInterval

Answer 4

从 0 到 9 运行的计时器。

struct PlayerView : View {

    @State private var currentTime: TimeInterval = 0.0  
    @ObservedObject var player = PodcastPlayer()        
    var body: some View {      
        Text("\(Int(currentTime))")
            .font(.largeTitle)
            .onReceive(player.$currentTimePublisher.filter { $0 < 10.0 }) { newCurrentTime in
                self.currentTime = newCurrentTime
        }
    }
}
class PodcastPlayer: ObservableObject {

    @Published var currentTimePublisher: TimeInterval = 0.0     
    init() {
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
            self.currentTimePublisher += 1
        }
    }
}