Binder.swift

import Combine

/// Subscriber that weakly references its target with a closure.
/// The target is sent to the action when it exists, so the caller can avoid
/// the [weak self] dance.
///
/// ```
/// let binder = Binder(self) { $0.doSomething($1) )
/// ```
///
final class Binder<Input>: Subscriber {
  private var handler: ((Input) -> Void)?
  private var subscription: Subscription?

  init<Target: AnyObject>(_ target: Target, action: @escaping (Target, Input) -> Void) {
    handler = { [weak target] input in
      if let target = target {
        action(target, input)
      }
    }
  }

  func receive(subscription: Subscription) {
    self.subscription = subscription
    subscription.request(.unlimited)
  }

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

  func receive(completion _: Subscribers.Completion<Never>) {
    subscription = nil
    handler = nil
  }

  func cancel() {
    subscription!.cancel()
    subscription = nil
    handler = nil
  }
}