Observer Pattern

Observer.swift

protocol Observer {
  func update()
}
class ConcreteObserver: Observer {
  var id: String
  init(id: String) {
    self.id = id
  }
  func update() {
    print(id + " observered the subject's state was changed.")
  }
}

class TextField {
  var observers: [Observer] = []
  var state: String {
    didSet {
      notify()
    }
  }
  init(state: String) {
    self.state = state
  }
  func attach(observer: Observer) {
    observers.append(observer)
  }
  func notify() {
    for observer in observers {
      observer.update()
    }
  }
}

//usage
let textField = TextField(state: "initial state")
let observerA = ConcreteObserver(id: "A")
let observerB = ConcreteObserver(id: "B")
textField.attach(observer: observerA)
textField.attach(observer: observerB)
textField.state = "new state"