class FormElement

abstract class FormElement extends RenderElement implements FormElementInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        return NULL;
    }
    
    /**
     * #process callback for #pattern form element property.
     *
     * @param array $element
     *   An associative array containing the properties and children of the
     *   generic input element.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $complete_form
     *   The complete form structure.
     *
     * @return array
     *   The processed element.
     */
    public static function processPattern(&$element, FormStateInterface $form_state, &$complete_form) {
        if (isset($element['#pattern']) && !isset($element['#attributes']['pattern'])) {
            $element['#attributes']['pattern'] = $element['#pattern'];
            $element['#element_validate'][] = [
                get_called_class(),
                'validatePattern',
            ];
        }
        return $element;
    }
    
    /**
     * #element_validate callback for #pattern form element property.
     *
     * @param $element
     *   An associative array containing the properties and children of the
     *   generic form element.
     * @param $form_state
     *   The current state of the form.
     * @param array $complete_form
     *   The complete form structure.
     */
    public static function validatePattern(&$element, FormStateInterface $form_state, &$complete_form) {
        if ($element['#value'] !== '') {
            // The pattern must match the entire string and should have the same
            // behavior as the RegExp object in ECMA 262.
            // - Use bracket-style delimiters to avoid introducing a special delimiter
            //   character like '/' that would have to be escaped.
            // - Put in brackets so that the pattern can't interfere with what's
            //   prepended and appended.
            $pattern = '{^(?:' . $element['#pattern'] . ')$}';
            if (!preg_match($pattern, $element['#value'])) {
                $form_state->setError($element, t('%name field is not in the right format.', [
                    '%name' => $element['#title'],
                ]));
            }
        }
    }
    
    /**
     * Adds autocomplete functionality to elements.
     *
     * This sets up autocomplete functionality for elements with an
     * #autocomplete_route_name property, using the #autocomplete_route_parameters
     * property if present.
     *
     * For example, suppose your autocomplete route name is
     * 'mymodule.autocomplete' and its path is
     * '/mymodule/autocomplete/{a}/{b}'. In a form array, you would create a text
     * field with properties:
     * @code
     * '#autocomplete_route_name' => 'mymodule.autocomplete',
     * '#autocomplete_route_parameters' => array('a' => $some_key, 'b' => $some_id),
     * @endcode
     * If the user types "keywords" in that field, the full path called would be:
     * 'mymodule_autocomplete/$some_key/$some_id?q=keywords'
     *
     * @param array $element
     *   The form element to process. Properties used:
     *   - #autocomplete_route_name: A route to be used as callback URL by the
     *     autocomplete JavaScript library.
     *   - #autocomplete_route_parameters: The parameters to be used in
     *     conjunction with the route name.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $complete_form
     *   The complete form structure.
     *
     * @return array
     *   The form element.
     */
    public static function processAutocomplete(&$element, FormStateInterface $form_state, &$complete_form) {
        $url = NULL;
        $access = FALSE;
        if (!empty($element['#autocomplete_route_name'])) {
            $parameters = isset($element['#autocomplete_route_parameters']) ? $element['#autocomplete_route_parameters'] : [];
            $url = Url::fromRoute($element['#autocomplete_route_name'], $parameters)->toString(TRUE);
            
            /** @var \Drupal\Core\Access\AccessManagerInterface $access_manager */
            $access_manager = \Drupal::service('access_manager');
            $access = $access_manager->checkNamedRoute($element['#autocomplete_route_name'], $parameters, \Drupal::currentUser(), TRUE);
        }
        if ($access) {
            $metadata = BubbleableMetadata::createFromRenderArray($element);
            if ($access->isAllowed()) {
                $element['#attributes']['class'][] = 'form-autocomplete';
                $metadata->addAttachments([
                    'library' => [
                        'core/drupal.autocomplete',
                    ],
                ]);
                // Provide a data attribute for the JavaScript behavior to bind to.
                $element['#attributes']['data-autocomplete-path'] = $url->getGeneratedUrl();
                $metadata = $metadata->merge($url);
            }
            $metadata->merge(BubbleableMetadata::createFromObject($access))
                ->applyTo($element);
        }
        return $element;
    }

}