Intermediate Combine

Apr 13 2021 · Swift 5.3, macOS 11.1, Xcode 12.2

Part 1: Intermediate Combine

02. Sharing Resources

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Previous episode: 01. Networking with Combine Next episode: 03. Managing Backpressure

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In Combine, Publishers are usually structs, and therefore passed by value throughout your code, copying the publisher each time it passed around. Once it recieves a subscription, those individual publishers will do the only thing they can - start its work and deliver values.

example(of: "shared") {

let shared = URLSession.shared
  .dataTaskPublisher(for: rwUrl)
  .map(\.data)
  .print("shared")
  .share()
print("subscribing first")

shared.sink(
  receiveCompletion: { _ in },
  receiveValue: { print("subscription1 received: '\($0)'") }
)
.store(in: &subscriptions)
print("subscribing second")

var subscription2 = shared.sink(
  receiveCompletion: { _ in },
  receiveValue: { print("subscription2 received: '\($0)'") }
)
.store(in: &subscriptions)
var subscription2: AnyCancellable? = nil

DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
  print("subscribing second")

  subscription2 = shared.sink(
    receiveCompletion: { print("subscription2 completion \($0)") },
    receiveValue: { print("subscription2 received: '\($0)'") }
  )
}
// 1
example(of: "multicast") {
let subject = PassthroughSubject<Data, URLError>() // 2
// 2
let multicasted = URLSession.shared
  .dataTaskPublisher(for: rwUrl)
  .map(\.data)
  .print("shared")
  .multicast(subject: subject)
// 3
multicasted
  .sink(
    receiveCompletion: { _ in },
    receiveValue: { print("subscription1 received: '\($0)'") }
  )

multicasted
  .sink(
    receiveCompletion: { _ in },
    receiveValue: { print("subscription2 received: '\($0)'") }
  )
// 4
multicasted.connect()
// 5
subject.send(Data())