class PathElement

class PathElement extends Textfield {
    
    /**
     * Do not convert the submitted value from the user-supplied path.
     */
    const CONVERT_NONE = 0;
    
    /**
     * Convert the submitted value into a route name and parameter pair.
     */
    const CONVERT_ROUTE = 1;
    
    /**
     * Convert the submitted value into a \Drupal\Core\Url value object.
     */
    const CONVERT_URL = 2;
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $info = parent::getInfo();
        $class = get_class($this);
        $info['#validate_path'] = TRUE;
        $info['#convert_path'] = self::CONVERT_ROUTE;
        $info['#element_validate'] = [
            [
                $class,
                'validateMatchedPath',
            ],
        ];
        return $info;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        return NULL;
    }
    
    /**
     * Form element validation handler for matched_path elements.
     *
     * Note that #maxlength is validated by _form_validate() already.
     *
     * This checks that the submitted value matches an active route.
     */
    public static function validateMatchedPath(&$element, FormStateInterface $form_state, &$complete_form) {
        if (!empty($element['#value']) && ($element['#validate_path'] || $element['#convert_path'] != self::CONVERT_NONE)) {
            
            /** @var \Drupal\Core\Url $url */
            if ($url = \Drupal::service('path.validator')->getUrlIfValid($element['#value'])) {
                if ($url->isExternal()) {
                    $form_state->setError($element, t('You cannot use an external URL, please enter a relative path.'));
                    return;
                }
                if ($element['#convert_path'] == self::CONVERT_NONE) {
                    // Url is valid, no conversion required.
                    return;
                }
                // We do the value conversion here whilst the Url object is in scope
                // after validation has occurred.
                if ($element['#convert_path'] == self::CONVERT_ROUTE) {
                    $form_state->setValueForElement($element, [
                        'route_name' => $url->getRouteName(),
                        'route_parameters' => $url->getRouteParameters(),
                    ]);
                    return;
                }
                elseif ($element['#convert_path'] == self::CONVERT_URL) {
                    $form_state->setValueForElement($element, $url);
                    return;
                }
            }
            $form_state->setError($element, t('This path does not exist or you do not have permission to link to %path.', [
                '%path' => $element['#value'],
            ]));
        }
    }

}