class AutocompleteController

class AutocompleteController {
    
    /**
     * Returns a JSON list of autocomplete suggestions for data selectors.
     *
     * @param \Drupal\rules\Ui\RulesUiHandlerInterface $rules_ui_handler
     *   The UI handler.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The request object providing the autocomplete query parameter.
     * @param string $uuid
     *   The UUID of the expression in which the autocomplete is triggered. If the
     *   UUID is not provided all available variables from the end of the
     *   expression will be shown.
     *
     * @return \Symfony\Component\HttpFoundation\JsonResponse
     *   The JSON results.
     */
    public function autocomplete(RulesUiHandlerInterface $rules_ui_handler, Request $request, $uuid = NULL) {
        $component = $rules_ui_handler->getComponent();
        $nested_expression = $component->getExpression()
            ->getExpression($uuid);
        if ($nested_expression === FALSE) {
            // @todo We don't have a UUID when an expression is added. Just show all
            // variables available in that case. The correct solution would be to get
            // the form state of the expression form currently being added. That is
            // very complicated so we don't do it for now.
            $nested_expression = NULL;
        }
        $string = $request->query
            ->get('q');
        $results = $component->autocomplete($string, $nested_expression);
        return new JsonResponse($results);
    }

}