/**
 * A simple pub-sub implementation for dispatching events by key.
 */
export default class PubSub {
  constructor() {
    this.subscribersByTopic = {};
  }

  /**
   * Links the given function to the given topic.
   *
   * Subsequent calls to `broadcast` with this topic will result in
   * this function being called.
   *
   * Returns a subscription object with `destroy` method that unsubscribes.
   */
  subscribe(topic, callback) {
    if (!Array.isArray(this.subscribersByTopic[topic])) {
      this.subscribersByTopic[topic] = [];
    }

    this.subscribersByTopic[topic].push(callback);

    return {
      destroy: () => {
        this.unsubscribe(topic, callback);
      },
    };
  }

  /**
   * Unlinks the given function from the given topic.
   *
   * Note that you must pass the same function reference as you passed
   * to `subscribe`.
   */
  unsubscribe(topic, callback) {
    const idx = this.subscribersByTopic[topic].indexOf(callback);

    if (idx !== -1) {
      this.subscribersByTopic[topic].splice(idx, 1);

      if (this.subscribersByTopic[topic].length === 0) {
        delete this.subscribersByTopic[topic];
      }
    }
  }

  /**
   * Calls all functions linked to the given topic and passes `payload`
   * as the argument.
   */
  broadcast(topic, payload) {
    if (Array.isArray(this.subscribersByTopic[topic])) {
      this.subscribersByTopic[topic].forEach((callback) => {
        callback(payload);
      });
    }
  }
}

export const globalPubsub = new PubSub();