code: public boolean addListener(ListenerController listenerController) {
  		if (listenerController != null && !listeners.containsKey(listenerController)) {
  			List<HandlerRegistration> registrations = new ArrayList<HandlerRegistration>();
  			registrations.add(eventWidget.addMouseDownHandler(listenerController));
  			registrations.add(eventWidget.addMouseMoveHandler(listenerController));
  			registrations.add(eventWidget.addMouseOutHandler(listenerController));
  			registrations.add(eventWidget.addMouseOverHandler(listenerController));
  			registrations.add(eventWidget.addMouseUpHandler(listenerController));
  			registrations.add(eventWidget.addMouseWheelHandler(listenerController));
  			listeners.put(listenerController, registrations);
  			return true;
  		}
  		return false;
  	}
docs: Add a new listener controller on this widget. These listeners passively listen to mouse events on the map. They
  do not interfere with these events.
  
  @param listenerController
  The new listener controller to add.
  @return Returns true if addition was successful, false otherwise.
  @since 1.8.0