public class Token {

  public typealias DisposeAction = () -> Void

  private let action: DisposeAction
  private var isDisposed: Bool = false

  public init(action: @escaping DisposeAction) { self.action = action }

  private func innerDispose() {
    guard !isDisposed else { return }
    isDisposed = true
    action()
  }

  public func dispose() { innerDispose() }
  deinit { innerDispose() }
}

public class Pipe<Element> {

  public let signal = Signal<Element>()

  public func dispatch(_ value: Element) {
    signal.dispatch(value)
  }

  public init() {}
}

public class Signal<Element> {

  public typealias Callback = (Element) -> Void

  private var callbacks = [:] as [Int: Callback]
  private var nextID = 0

  fileprivate func dispatch(_ value: Element) {
    callbacks.forEach { $1(value) }
  }

  public func onUpdate(_ callback: @escaping Callback) -> Token {

    let id = nextID
    nextID += 1
    callbacks[id] = callback

    return Token { [weak self] in self?.callbacks[id] = nil }
  }
}