LinkTypeConstraintValidator.php

<?php

namespace Drupal\link\Plugin\Validation\Constraint;

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

/**
 * Constraint validator for links receiving data allowed by its settings.
 */
class LinkTypeConstraintValidator extends ConstraintValidator {
    
    /**
     * {@inheritdoc}
     */
    public function validate($value, Constraint $constraint) : void {
        if (isset($value)) {
            $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) {
                $this->context
                    ->addViolation($constraint->message, [
                    '@uri' => $link_item->uri,
                ]);
            }
        }
    }

}