Create a Timer Publisher using Swift Combine

问题:

I've been watching the Data Flow Through SwiftUI WWDC talk. They have a slide with a sample code where they use a Timer publisher that gets connected to a SwiftUI View, and updates the UI with the time.

I'm working on some code where I want to do the exact same thing, but can't figure out how this PodcastPlayer.currentTimePublisher is implemented, and then hooked to the UI struct. I have also watched all the videos about Combine.

How can I achieve this?

The sample code:

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
    }
  }
}

回答1:

Here you have an example of a Combine timer. I am using a global, but of course you should use whatever is applicable to your scenario (environmentObject, State, etc).

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
    }
  }
}

回答2:

Using ObservableObject

to Create a Timer Publisher using Swift Combine

class TimeCounter: ObservableObject {
    @Published var time = 0

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

That's it! now you just need to observe for changes:

struct ContentView: View {
    @ObservedObject var timeCounter = TimeCounter()

    var body: some View {
        Text("\(timeCounter.time)")
    }
}

回答3:

I implemented a Combine timer with a new feature allowing you to switch between different intervals.

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)
    }

}

To start the timer, simply subscribe to the publisher property.

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

You can switch to a new timer with a different interval by changing the interval property.

combineTimer.interval = someNewInterval

回答4:

I believe that that was left as an exercise for the reader.

Please note that that isn't a timer publisher per se but a publisher of the current time in the podcast i.e. it isn't published periodically, only when the currentTime changes.

There is a property wrapper called @Published which is supposed to handle this but I haven't yet tried it: https://developer.apple.com/documentation/combine/published