react-query/es/core/subscribable.js

Hooks for managing, caching and syncing asynchronous and remote data in React

export class Subscribable {
  constructor() {
    this.listeners = [];
    this.subscribe = this.subscribe.bind(this);
  }

  subscribe(listener) {
    this.listeners.push(listener);
    this.onSubscribe();
    return () => {
      this.listeners = this.listeners.filter(x => x !== listener);
      this.onUnsubscribe();
    };
  }

  hasListeners() {
    return this.listeners.length > 0;
  }

  onSubscribe() {// Do nothing
  }

  onUnsubscribe() {// Do nothing
  }

}