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'],
      ]));
    }
  }

}