Reactive Programming in iOS with Combine

Feb 4 2021 · Swift 5.3, macOS 11.0, Xcode 12.2

Part 3: Combining Operators

19. Operator Examples

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Previous episode: 18. More Combining Operators Next episode: 20. Challenge: Advanced Combining

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Let’s take a look at the switchToLatest operator. First, define 3 PassthroughSubject publishers inside an example(of) stub - these will be the publishers that our main publisher - called publishers - will emit. Note that publishers’ value type is PassthroughSubject<Int, Never>, which means it will emit publishers as its value type.

example(of: "switchToLatest") {
  // 1
  let publisher1 = PassthroughSubject<Int, Never>()
  let publisher2 = PassthroughSubject<Int, Never>()
  let publisher3 = PassthroughSubject<Int, Never>()

  // 2
  let publishers = PassthroughSubject<PassthroughSubject<Int, Never>, Never>()

  // 3
  publishers
    .switchToLatest()
    .sink(receiveCompletion: { _ in print("Completed!") },
          receiveValue: { print($0) })
    .store(in: &subscriptions)

  // 4
  publishers.send(publisher1)
  publisher1.send(1)
  publisher1.send(2)

  // 5
  publishers.send(publisher2)
  publisher1.send(3)
  publisher2.send(4)
  publisher2.send(5)

  // 6
  publishers.send(publisher3)
  publisher2.send(6)
  publisher3.send(7)
  publisher3.send(8)
  publisher3.send(9)

  // 7
  publisher3.send(completion: .finished)
  publishers.send(completion: .finished)
}

example(of: "merge(with:)") {
  // 1
  let publisher1 = PassthroughSubject<Int, Never>()
  let publisher2 = PassthroughSubject<Int, Never>()

  // 2
  publisher1
    .merge(with: publisher2)
    .sink(receiveCompletion: { _ in print("Completed") },
          receiveValue: { print($0) })
    .store(in: &subscriptions)


  // 3
  publisher1.send(1)
  publisher1.send(2)

  publisher2.send(3)

  publisher1.send(4)

  publisher2.send(5)

  // 4
  publisher1.send(completion: .finished)
  publisher2.send(completion: .finished)
}

example(of: "combineLatest") {
  // 1
  let publisher1 = PassthroughSubject<Int, Never>()
  let publisher2 = PassthroughSubject<String, Never>()

  // 2
  publisher1
    .combineLatest(publisher2)
    .sink(receiveCompletion: { _ in print("Completed") },
          receiveValue: { print("P1: \($0), P2: \($1)") })
    .store(in: &subscriptions)

  // 3
  publisher1.send(1)
  publisher1.send(2)

  publisher2.send("a")
  publisher2.send("b")

  publisher1.send(3)

  publisher2.send("c")

  // 4
  publisher1.send(completion: .finished)
  publisher2.send(completion: .finished)
}

example(of: "zip") {
  // 1
  let publisher1 = PassthroughSubject<Int, Never>()
  let publisher2 = PassthroughSubject<String, Never>()

  // 2
  publisher1
      .zip(publisher2)
      .sink(receiveCompletion: { _ in print("Completed") },
            receiveValue: { print("P1: \($0), P2: \($1)") })
      .store(in: &subscriptions)

  // 3
  publisher1.send(1)
  publisher1.send(2)
  publisher2.send("a")
  publisher2.send("b")
  publisher1.send(3)
  publisher2.send("c")
  publisher2.send("d")

  // 4
  publisher1.send(completion: .finished)
  publisher2.send(completion: .finished)
}