Observer(옵저버) 패턴

옵저버 패턴(Observer Pattern)은 옵저버(관찰자)들이 관찰하고 있는 대상자의 상태가 변화가 있을 때마다 대상자는 직접 목록의 각 관찰자들에게 통지하고, 관찰자들은 알림을 받아 조치를 취하는 행동 패턴이다.

 

 

interface Observer {

  update(subject: Subject): void;

}

class ConcreteObserever implements Observer {

  constructor(private id: number) {}

  public update(subject: Subject): void {

    console.log(

      `Observer ${this.id} updated, new state: ${subject.getState()}`

    );

  }

}

interface Subject {

  addObserver(observer: Observer): void;

  removeObserver(observer: Observer): void;

  notifyObservers(): void;

  getState(): number;

  setState(state: number): void;

}

class ConcreteSubject implements Subject {

  private observers: Observer[] = [];

  private state: number = 0;

  public addObserver(observer: Observer): void {

    const isExists = this.observers.includes(observer);

    if (isExists) {

      return console.log("Observer already exists");

    }

    this.observers.push(observer);

    console.log("Observer Added Successfully");

  }

  public removeObserver(observer: Observer): void {

    const observerIndex = this.observers.indexOf(observer);

    if (observerIndex === -1) {

      return console.log("Observer Does not Exist");

    }

    this.observers.splice(observerIndex, 1);

    console.log("Observer was successfully removed");

  }

  public notifyObservers(): void {

    this.observers.forEach((observer) => observer.update(this));

  }

  public getState(): number {

    return this.state;

  }

  public setState(state: number): void {

    this.state = state;

    console.log("Setting State ....");

    this.notifyObservers();

  }

}

// client code

const subject = new ConcreteSubject();

const observer1 = new ConcreteObserever(1);

subject.addObserver(observer1);

const observer2 = new ConcreteObserever(2);

subject.addObserver(observer2);

subject.setState(123);