code: @Override
      public void registerAttributeListener(String attributeName, AttributeListener attributeListener) {
          attributeListeners.putIfAbsent(attributeName, new ArrayList<AttributeListener>());
          List<AttributeListener> listeners = attributeListeners.get(attributeName);
          synchronized (listeners) {
              listeners.add(attributeListener);
          }
      }
docs: Registers an attribute listener that gets notified in case the attribute
  changes. This is used for on change subscriptions.
  
  @param attributeName     the attribute name as defined in the Franca model
  to subscribe to.
  @param attributeListener the listener to add.