UserNameValidator.php

<?php

declare (strict_types=1);
namespace Drupal\user;

use Drupal\Core\Validation\BasicRecursiveValidatorFactory;
use Drupal\Core\Validation\ConstraintManager;
use Symfony\Component\Validator\ConstraintViolationListInterface;

/**
 * Provides a username validator.
 *
 * This validator re-uses the UserName constraint plugin but does not require a
 * User entity.
 */
class UserNameValidator {
    public function __construct(BasicRecursiveValidatorFactory $validatorFactory, ConstraintManager $constraintManager) {
    }
    
    /**
     * Validates a user name.
     *
     * @return \Symfony\Component\Validator\ConstraintViolationListInterface
     *   The list of constraint violations.
     */
    public function validateName(string $name) : ConstraintViolationListInterface {
        $validator = $this->validatorFactory
            ->createValidator();
        $constraint = $this->constraintManager
            ->create('UserName', []);
        return $validator->validate($name, $constraint);
    }

}