Reactive Programming in iOS with Combine

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

Part 2: Transforming & Filtering Operators

07. Transforming Operators

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Previous episode: 06. Introduction Next episode: 08. More Transforming Operators

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The first type of operators we’ll focus on are Transforming operators. This style of operator is the most common, and is used to manipulate values coming from publishers into a format that is suitable for subscribers downstream. As we discuss this type of operator, you’ll see many similarities (and differences!) with operators from the Swift standard library such as map and flatMap.

example(of: "collect") {
  ["A", "B", "C", "D", "E"].publisher
    .sink(receiveCompletion: { print($0) },
          receiveValue: { print($0) })
    .store(in: &subscriptions)
}

["A", "B", "C", "D", "E"].publisher
  .collect()
  .sink(receiveCompletion: { print($0) },
        receiveValue: { print($0) })
  .store(in: &subscriptions)

example(of: "map") {
  // 1
  let formatter = NumberFormatter()
  formatter.numberStyle = .spellOut

  // 2
  [123, 4, 56].publisher
    // 3
    .map {
      formatter.string(for: NSNumber(integerLiteral: $0)) ?? ""
    }
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

example(of: "replaceNil") {
  // 1
  ["A", nil, "C"].publisher
    .replaceNil(with: "-") // 2
    .sink(receiveValue: { print($0) }) // 3
    .store(in: &subscriptions)
}

example(of: "replaceNil") {
  ["A", nil, "C"].publisher
    .replaceNil(with: "-")
    .map { $0! }
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

example(of: "replaceEmpty(with:1)") {
  // 1
  let empty = Empty<Int, Never>()

  // 2
  empty
    .replaceEmpty(with: 1)
    .sink(receiveCompletion: { print($0) },
          receiveValue: { print($0) })
    .store(in: &subscriptions)
}