【Swift/Combine】Futureの使い方！コールバックの解消

final class Future<Output, Failure> where Failure : Error

func getPublisher() -> Future<String, Never> {
    return Future() { promise in
        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            promise(.success("成功したよ"))
        }
    }
}

@State  var cancellable:AnyCancellable?
var body: some View {
  Text("Hello")
      .onAppear {
          cancellable = getPublisher().sink { str in
              print(str)
          }
      }
}

let publisher = Future<String, Never>() { promise in
    print("In Future")
    DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
        promise(.success("成功したよ"))
    }
}

print("Out Future")

let cancellable = publisher.sink { str in
    print("Sink Future")
    print(str)
}

In Future
Out Future
// 2秒後
Sink Future
成功したよ

let publisher = Deferred {
    Future<String, Never>() { promise in
        print("In Future")
        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            promise(.success("成功したよ"))
        }
    }
}

print("Out Future")

let cancellable = publisher.sink { str in
    print("Sink Future")
    print(str)
}

Out Future
In Future
// 2秒後
Sink Future
成功したよ