EntityTypeConstraintValidator.php

<?php

/**
 * @file
 * Contains \Drupal\Core\Validation\Plugin\Validation\Constraint\EntityTypeConstraintValidator.
 */
namespace Drupal\Core\Validation\Plugin\Validation\Constraint;

use Drupal\Core\Entity\EntityInterface;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates the EntityType constraint.
 */
class EntityTypeConstraintValidator extends ConstraintValidator {

  /**
   * Implements \Symfony\Component\Validator\ConstraintValidatorInterface::validate().
   */
  public function validate($typed_data, Constraint $constraint) {

    // If the entity is contained in a reference, unwrap it first.
    $entity = isset($typed_data) && !$typed_data instanceof EntityInterface ? $typed_data
      ->getValue() : FALSE;
    if (!empty($entity) && $entity
      ->entityType() != $constraint->type) {
      $this->context
        ->addViolation($constraint->message, array(
        '%type' => $constraint->type,
      ));
    }
  }

}