Reactive Programming in iOS with Combine

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

Part 1: Getting Started

03. Subscriber Operators and Subjects

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Previous episode: 02. Publishers and Subscribers Next episode: 04. Challenge: Create a Blackjack Dealer

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Although you ignored them in the previous example, the sink operator actually provides two closures: one to handle receiving a completion event, and one to handle receiving values. Let’s jump back into some code to see that in action.

example(of: "Just") {
  let just = Just("Hello world!")

  just
    .sink(
      receiveCompletion: {
        print("Received completion", $0)
    },
      receiveValue: {
        print("Received value", $0)
    })
    .store(in: &subscriptions)
}

example(of: "assign(to:on:)") {
  class SomeObject {
    var value: String = "" {
      didSet {
        print(value)
      }
    }
  }

  let object = SomeObject()
}

["Hello", "world!"].publisher
  .assign(to: \.value, on: object)
  .store(in: &subscriptions)

——— Example of: assign(to:on:) ———
Hello
world!

example(of: "PassthroughSubject") {
  let subject = PassthroughSubject<String, Never>()

  subject
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

subject.send("Hello")
subject.send("World")

——— Example of: PassthroughSubject ———
Hello
World

subject.send(completion: .finished)
subject.send("Still there?")

——— Example of: PassthroughSubject ———
Hello
World

example(of: "CurrentValueSubject") {
let subject = CurrentValueSubject<Int, Never>(0)

subject
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)
}

print(subject.value)

subject.send(1)
subject.send(2)

print(subject.value)

subject.send(completion: .finished)

——— Example of: CurrentValueSubject ———
0
0
1
2
2

.print()

——— Example of: CurrentValueSubject ———
receive subscription: (CurrentValueSubject)
request unlimited
receive value: (0)
0
0
receive value: (1)
1
receive value: (2)
2
2
receive finished

example(of: "Type erasure") {
  let subject = PassthroughSubject<Int, Never>()

  let publisher = subject.eraseToAnyPublisher()
}

publisher.send(0)

//  publisher.send(0)

publisher
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)

subject.send(0)