Observer

Observer is a behavioral design pattern also known as Publish-Subscribe which allows one object to notify muliple objects about events or state changes.

interface Publisher {
  subscribe(observer: Observer): void
  unsubscribe(observer: Observer): void
  notify(): void
}

interface Observer {
  update(publisher: Publisher): void
  getId(): string
}

class Timer implements Publisher {
  private observers: Observer[] = [];
  time = 0

  subscribe(observer: Observer): void {
    const isExist = this.observers.filter(existObserver => existObserver.getId() === observer.getId())[0];

    if (isExist) {
      console.log('Observer has been subscribed already.');
      return
    }

    this.observers.push(observer);
  };

  //unSubscribe an observer from the Publisher.
  unsubscribe(observer: Observer): void {
    const observerIndex = this.observers.findIndex(existObserver => existObserver.getId() === observer.getId());
    if (observerIndex === -1) {
      console.log('Nonexistent observer.');
      return
    }

    this.observers.splice(observerIndex, 1);
  };

  // Notify all observers about an event.
  notify(): void {
    for (const observer of this.observers) {
      observer.update(this);
    }
  };

  tick() {
    this.time++
    this.notify()
  }
}

class DigitalClock implements Observer {
  private timer: Timer
  private displayTime = 0
  private id: string


  constructor(timer: Timer) {
    this.timer = timer
    this.id = "randomId1"
    this.timer.subscribe(this)
    this.setDisplayTime(this.timer.time)
  }

  getId() {
    return this.id
  }

  setDisplayTime(time: number){
    this.displayTime = time
  }

  show(){
    console.log("Digital Clock:", this.displayTime)
  }

  update(timer: Timer): void {
    this.setDisplayTime(timer.time)
    this.show()
  }

  unsubscribe(){
    this.timer.unsubscribe(this)
  }
}

class AnalogClock implements Observer {
  private timer: Timer
  private displayTime = 0 
  private id: string


  constructor(timer: Timer) {
    this.timer = timer
    this.id = "randomId2"
    this.timer.subscribe(this)
    this.setDisplayTime(this.timer.time)
  }

  getId() {
    return this.id
  }

  setDisplayTime(time: number){
    this.displayTime = time
  }

  show(){
    console.log("Analog Clock:", this.displayTime)
  }

  update(timer: Timer): void {
    this.setDisplayTime(timer.time)
    this.show()
  }

  unsubscribe(){
    this.timer.unsubscribe(this)
  }
}


const timer = new Timer()

new DigitalClock(timer)
new AnalogClock(timer)

timer.tick()
timer.tick()
timer.tick()