LinkTypeConstraintValidator.php

<?php

namespace Drupal\link\Plugin\Validation\Constraint;

use Drupal\link\LinkItemInterface;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedValueException;

/**
 * Constraint validator for links receiving data allowed by its settings.
 */
class LinkTypeConstraintValidator extends ConstraintValidator {
  
  /**
   * {@inheritdoc}
   */
  public function validate($value, Constraint $constraint) : void {
    if (!$value instanceof LinkItemInterface) {
      throw new UnexpectedValueException($value, LinkItemInterface::class);
    }
    if ($value->isEmpty()) {
      return;
    }
    $uri_is_valid = TRUE;
    /** @var \Drupal\link\LinkItemInterface $link_item */
    $link_item = $value;
    $link_type = $link_item->getFieldDefinition()
      ->getSetting('link_type');
    // Try to resolve the given URI to a URL. It may fail if it's schemeless.
    try {
      $url = $link_item->getUrl();
    } catch (\InvalidArgumentException) {
      $uri_is_valid = FALSE;
    }
    // If the link field doesn't support both internal and external links,
    // check whether the URL (a resolved URI) is in fact violating either
    // restriction.
    if ($uri_is_valid && $link_type !== LinkItemInterface::LINK_GENERIC) {
      if (!($link_type & LinkItemInterface::LINK_EXTERNAL) && $url->isExternal()) {
        $uri_is_valid = FALSE;
      }
      if (!($link_type & LinkItemInterface::LINK_INTERNAL) && !$url->isExternal()) {
        $uri_is_valid = FALSE;
      }
    }
    if (!$uri_is_valid) {
      $this->context
        ->buildViolation($constraint->message, [
        '@uri' => $link_item->uri,
      ])
        ->atPath('uri')
        ->addViolation();
    }
  }

}