Notification mechanism

Pipe.swift

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 }
  }
}

Example of usage:

  let producer = Pipe<Int>()

  let signal1 = producer.signal
  let signal2 = producer.signal
  
  signal1.onUpdate { value in print("signal 1 callback 1: \(value)") }
  signal1.onUpdate { value in print("signal 1 callback 2: \(value)") }
  
  let token = signal2.onUpdate { value in print("signal 2: \(value)") }
  
  (1...3).forEach(producer.dispatch)
  
  token.dispose()
  producer.dispatch(666)

will print:

signal 1 callback 1: 1
signal 1 callback 2: 1
signal 2: 1
signal 1 callback 1: 2
signal 1 callback 2: 2
signal 2: 2
signal 1 callback 1: 3
signal 1 callback 2: 3
signal 2: 3
signal 1 callback 1: 666
signal 1 callback 2: 666