Publisher+Bind.swift

import Combine

extension Publisher {
  /// Binds a publisher to a subscriber with a resulting cancellable
  func bind<S: Subscriber>(to subscriber: S) -> AnyCancellable where S.Failure == Failure, S.Input == Output {
    let binding = Binding(subscriber)
    subscribe(binding)
    return AnyCancellable(binding.cancel)
  }
}

/// `Binding` is a simple wrapper for a Combine subscriber that can be cancelled.
private final class Binding<Input, Failure>: Subscriber where Failure: Error {
  private var upstream: AnySubscriber<Input, Failure>?
  private var subscription: Subscription?

  init<S: Subscriber>(_ subscriber: S) where S.Input == Input, S.Failure == Failure {
    upstream = AnySubscriber(subscriber)
  }

  func receive(subscription: Subscription) {
    self.subscription = subscription
    upstream!.receive(subscription: subscription)
  }

  func receive(_ input: Input) -> Subscribers.Demand {
    upstream!.receive(input)
  }

  func receive(completion: Subscribers.Completion<Failure>) {
    upstream!.receive(completion: completion)
  }

  func cancel() {
    subscription?.cancel()
    subscription = nil
    upstream = nil
  }
}