Reactive Programming in iOS with Combine

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

Part 3: Combining Operators

15. Prepend

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Previous episode: 14. Introduction Next episode: 16. Append

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With transforming and filtering operators in your toolbelt, it’s time to move onto a complex, but useful, category of operators - Combining Operators. These operators let you combine events emitted by different publishers to create combinations of data to send to consumers.

example(of: "prepend(Output...)") {
  // 1
  let publisher = [3, 4].publisher

  // 2
  publisher
    .prepend(1, 2)
    //.prepend(-1, 0)
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

example(of: "prepend(Sequence)") {
  // 1
  let publisher = [5, 6, 7].publisher

  // 2
  publisher
    .prepend([3, 4])
    .prepend(Set(1...2))
    //.prepend(stride(from: 6, to: 11, by: 2))
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

example(of: "prepend(Publisher)") {
  // 1
  let publisher1 = [3, 4].publisher
  let publisher2 = [1, 2].publisher

  // 2
  publisher1
    .prepend(publisher2)
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

example(of: "prepend(Publisher) #2") {
  // 1
  let publisher1 = [3, 4].publisher
  let publisher2 = PassthroughSubject<Int, Never>()

  // 2
  publisher1
    .prepend(publisher2)
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)

  // 3
  publisher2.send(1)
  publisher2.send(2)
  //publisher2.send(completion: .finished)
}