index.ts

interface Subject {
  attach(observer: Observer): void;

  detach(observer: Observer): void;

  notify(): void;
}

class ConcreteSubject implements Subject {
  public state: number;
  private observers: Observer[] = [];

  public attach(observer: Observer): void {
    console.log('Subject: Attached an observer.');
    this.observers.push(observer);
  }

  public detach(observer: Observer): void {
    const observerIndex = this.observers.indexOf(observer);
    if (observerIndex === -1) {
      return console.log('Subject: Nonexistent observer.');
    }

    this.observers.splice(observerIndex, 1);
    console.log('Subject: Detached an observer.');
  }

  public notify(): void {
    console.log('Subject: Notifying observers...');
    for (const observer of this.observers) {
      observer.update(this);
    }
  }

  public someBusinessLogic(): void {
    console.log('\nSubject: I\'m doing something important.');
    this.state = Math.floor(Math.random() * (10 + 1));

    console.log(`Subject: My state has just changed to: ${this.state}`);
    this.notify();
  }
}

interface Observer {
  update(subject: Subject): void;
}

class ConcreteObserverA implements Observer {
  public update(subject: Subject): void {
    if (subject instanceof ConcreteSubject && subject.state < 3) {
      console.log('ConcreteObserverA: Reacted to the event.');
    }
  }
}

class ConcreteObserverB implements Observer {
  public update(subject: Subject): void {
    if (subject instanceof ConcreteSubject && (subject.state === 0 || subject.state >= 2)) {
      console.log('ConcreteObserverB: Reacted to the event.');
    }
  }
}

function clientCodeContext() {
  const subject = new ConcreteSubject();

  const observer1 = new ConcreteObserverA();
  subject.attach(observer1);

  const observer2 = new ConcreteObserverB();
  subject.attach(observer2);

  subject.someBusinessLogic();
  subject.someBusinessLogic();

  subject.detach(observer2);

  subject.someBusinessLogic();
}

export function observer() {
  clientCodeContext()
}