gecko-dev/browser/devtools/shared/EventEmitter.jsm

var EXPORTED_SYMBOLS = ["EventEmitter"];

function EventEmitter() {
}

EventEmitter.prototype = {
  /**
   * Connect a listener.
   *
   * @param string aEvent
   *        The event name to which we're connecting.
   * @param function aListener
   *        Called when the event is fired.
   */
  on: function EventEmitter_on(aEvent, aListener) {
    if (!this._eventEmitterListeners)
      this._eventEmitterListeners = new Map();
    if (!this._eventEmitterListeners.has(aEvent)) {
      this._eventEmitterListeners.set(aEvent, []);
    }
    this._eventEmitterListeners.get(aEvent).push(aListener);
  },

  /**
   * Listen for the next time an event is fired.
   *
   * @param string aEvent
   *        The event name to which we're connecting.
   * @param function aListener
   *        Called when the event is fired.  Will be called at most one time.
   */
  once: function EventEmitter_once(aEvent, aListener) {
    let handler = function() {
      this.off(aEvent, handler);
      aListener();
    }.bind(this);
    this.on(aEvent, handler);
  },

  /**
   * Remove a previously-registered event listener.  Works for events
   * registered with either on or once.
   *
   * @param string aEvent
   *        The event name whose listener we're disconnecting.
   * @param function aListener
   *        The listener to remove.
   */
  off: function EventEmitter_off(aEvent, aListener) {
    if (!this._eventEmitterListeners)
      return;
    let listeners = this._eventEmitterListeners.get(aEvent);
    this._eventEmitterListeners.set(aEvent, listeners.filter(function(l) aListener != l));
  },

  /**
   * Emit an event.  All arguments to this method will
   * be sent to listner functions.
   */
  emit: function EventEmitter_emit(aEvent) {
    if (!this._eventEmitterListeners || !this._eventEmitterListeners.has(aEvent))
      return;

    let originalListeners = this._eventEmitterListeners.get(aEvent);
    for (let listener of this._eventEmitterListeners.get(aEvent)) {
      // If the object was destroyed during event emission, stop
      // emitting.
      if (!this._eventEmitterListeners) {
        break;
      }

      // If listeners were removed during emission, make sure the
      // event handler we're going to fire wasn't removed.
      if (originalListeners === this._eventEmitterListeners.get(aEvent) ||
          this._eventEmitterListeners.get(aEvent).some(function(l) l === listener)) {
        listener.apply(null, arguments);
      }
    }
  },
}