class TaxonomyTermHierarchyConstraintValidator

class TaxonomyTermHierarchyConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {

  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  private $entityTypeManager;

  /**
   * Creates a new TaxonomyTermHierarchyConstraintValidator instance.
   *
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
    $this->entityTypeManager = $entity_type_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container
      ->get('entity_type.manager'));
  }

  /**
   * {@inheritdoc}
   */
  public function validate($entity, Constraint $constraint) {
    $term_storage = $this->entityTypeManager
      ->getStorage($entity
      ->getEntityTypeId());
    assert($term_storage instanceof TermStorageInterface);

    // Newly created entities should be able to specify a parent.
    if ($entity && $entity
      ->isNew()) {
      return;
    }
    $is_pending_revision = !$entity
      ->isDefaultRevision();
    $pending_term_ids = $term_storage
      ->getTermIdsWithPendingRevisions();
    $ancestors = $term_storage
      ->loadAllParents($entity
      ->id());
    $ancestor_is_pending_revision = (bool) array_intersect_key($ancestors, array_flip($pending_term_ids));
    $new_parents = array_column($entity->parent
      ->getValue(), 'target_id');
    $original_parents = array_keys($term_storage
      ->loadParents($entity
      ->id())) ?: [
      0,
    ];
    if (($is_pending_revision || $ancestor_is_pending_revision) && $new_parents != $original_parents) {
      $this->context
        ->buildViolation($constraint->message)
        ->atPath('parent')
        ->addViolation();
    }
    $original = $term_storage
      ->loadUnchanged($entity
      ->id());
    if (($is_pending_revision || $ancestor_is_pending_revision) && !$entity->weight
      ->equals($original->weight)) {
      $this->context
        ->buildViolation($constraint->message)
        ->atPath('weight')
        ->addViolation();
    }
  }

}