code: private void addBehaviors()
      {
          if (this.uiField.getFieldConfiguration().getField().hasEvents(EventType.UI_FIELD_UPDATE)) {
              final List<EventDefinition> events =
                              this.uiField.getFieldConfiguration().getField().getEvents(EventType.UI_FIELD_UPDATE);
              String eventName = "change";
              for (final EventDefinition event : events) {
                  eventName = event.getProperty("Event") == null ? "change" : event.getProperty("Event");
              }
              add(new AjaxFieldUpdateBehavior(eventName, Model.of(this.uiField), false) {
  
                  /** The Constant serialVersionUID. */
                  private static final long serialVersionUID = 1L;
  
                  @Override
                  protected void onSubmit(final AjaxRequestTarget _target)
                  {
                      // add the previous value as a parameter
                      final DropDownOption option = (DropDownOption) getDefaultModelObject();
                      try {
                          if (option != null) {
                              Context.getThreadContext().getParameters().put(
                                              getFieldConfig().getName() + "_eFapsPrevious",
                                              new String[] { option.getValue() });
                          }
                      } catch (final EFapsException e) {
                          DropDownField.LOG.error("EFapsException", e);
                      }
                      super.onSubmit(_target);
                      DropDownField.this.converted = false;
                      updateModel();
                      DropDownField.this.converted = false;
                  }
              });
          }
          add(new AttributeAppender("style", "text-align:" + getFieldConfig().getAlign(), ";"));
          // add the width only if not in a table
          if (getFieldConfig().hasProperty(UIFormFieldProperty.WIDTH) && !getFieldConfig().isTableField()) {
              add(new AttributeAppender("style", "width:" + getFieldConfig().getWidth(), ";"));
          }
      }
docs: Adds the behaviors.