getAllowedProtocols example

public function validate($value, Constraint $constraint) {
    if (isset($value)) {
      try {
        /** @var \Drupal\Core\Url $url */
        $url = $value->getUrl();
      }
      // If the URL is malformed this constraint cannot check further.       catch (\InvalidArgumentException $e) {
        return;
      }
      // Disallow external URLs using untrusted protocols.       if ($url->isExternal() && !in_array(parse_url($url->getUri(), PHP_URL_SCHEME), UrlHelper::getAllowedProtocols())) {
        $this->context->addViolation($constraint->message, ['@uri' => $value->uri]);
      }
    }
  }

}