on(eventName, callback) {
    this.eventHandlers[eventName] = this.eventHandlers[eventName] || [];
    this.eventHandlers[eventName].push(callback);
    return callback;
  }
Listen to specific events
@param {string} eventName The name of the event to listen to
@param {EventHandlerCallbackType} callback Code to execute on event
@returns {EventHandlerCallbackType}