Question 69340 in IOS Swift Question posted in
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How to fire one publisher after another? – Ios swift

I have two publishers, one that gathers messages based on a state, and the second which is a Timer. I want these to fire in order – so first gather data, then start a timer. How can I do this? This is my current code:

let messagesPublisher = OnboardingStateLogic.publisher(
  forState: state,
  nextState: nextState
)

messagesPublisher
  .sink { completion in 
    print("completed")
  } receiveValue: { [weak self] messages in
    messages.forEach { message in
      self?.queue.enqueue(message)
    }
  }

timer = Timer
  .publish(every: 2, on: .main, in: .default)
  .autoconnect()
  .sink { _ in
    self.dequeueMessages()
  }
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2

Answers


  1. Chosen as BEST ANSWER
    0

    I ended up using Publishers.Zip. This only publishes when both upstream publishers have outputted a value:

    messagesPublisher.sink(
  receiveCompletion: { _ in },
  receiveValue: { [weak self] message in
   self?.queue.enqueue(message)
  }
).store(in: &cancellables)

let delayPublisher = Timer
  .publish(every: 2, on: .main, in: .default)
  .autoconnect()
  .eraseToAnyPublisher()
  .setFailureType(to: Error.self)

let delayedValuesPublisher = Publishers.Zip(messagesPublisher, delayPublisher).eraseToAnyPublisher()

delayedValuesPublisher.sink(
  receiveCompletion: { completion in
    switch completion {
    case .failure(let error):
      print("something went wrong", error)
    case .finished:
      break
    }
  },
  receiveValue: { [weak self] _ in
    self?.dequeueMessages()
  }
).store(in: &cancellables)

  2. 0

    You can use the flatMap operator to transform a publisher or value into a new publisher in the Combine chain. For example:

    class ViewModel : ObservableObject {
    var cancellable : AnyCancellable?
    
    init() {
        cancellable = Just(1).flatMap { _ in
            Timer
              .publish(every: 2, on: .main, in: .default)
              .autoconnect()
        }.sink { value in
            print("Publisher fired: (value)")
        }
    }
}

    You would replace my trivial Just(1) publisher with your messagesPublisher

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