Observer

A behavioral design pattern, which lets you define a subscription mechanism to notify multiple objects about any events that happen to the object they’re observing.

• subscribe: Adds a listener function that will be called, when this observer emits a value/event.
• unsubscribe: Removes a listener function from the observer.
• emit: Executes all subscribed listeners with a given value.

type Listener<T = void> = (value: T) => void;

class Observer<T = void> {
  #listeners: Set<Listener<T>>;

  constructor() {
    this.#listeners = new Set<Listener<T>>();
    this.emit = this.emit.bind(this);
    this.subscribe = this.subscribe.bind(this);
    this.unsubscribe = this.unsubscribe.bind(this);
  }

  subscribe(listener: Listener<T>) {
    this.#listeners.add(listener);
    return () => this.unsubscribe(listener);
  }

  unsubscribe(listener: Listener<T>) {
    if (this.#listeners.has(listener)) {
      this.#listeners.delete(listener);
    }
  }

  emit(value: T) {
    this.#listeners.forEach(listener => {
      listener(value);
    });
  }
}