code: @Override
      public void notifyHandlableAdded(Featurable featurable)
      {
          if (featurable.hasFeature(Displayable.class))
          {
              final Displayable displayable = featurable.getFeature(Displayable.class);
              final Integer layer = getLayer(featurable);
              final Collection<Displayable> displayables = getLayer(layer);
              displayables.add(displayable);
              indexs.add(layer);
          }
      }
docs: /*
  HandlerListener