class CommentBreadcrumbBuilder

class CommentBreadcrumbBuilder implements BreadcrumbBuilderInterface {
  use StringTranslationTrait;
  
  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;
  
  /**
   * Constructs the CommentBreadcrumbBuilder.
   *
   * @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 function applies(RouteMatchInterface $route_match) {
    return $route_match->getRouteName() == 'comment.reply' && $route_match->getParameter('entity');
  }
  
  /**
   * {@inheritdoc}
   */
  public function build(RouteMatchInterface $route_match) {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addCacheContexts([
      'route',
    ]);
    $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));
    $entity = $route_match->getParameter('entity');
    $breadcrumb->addLink(new Link($entity->label(), $entity->toUrl()));
    $breadcrumb->addCacheableDependency($entity);
    if (($pid = $route_match->getParameter('pid')) && $comment = $this->entityTypeManager
      ->getStorage('comment')
      ->load($pid)) {
      /** @var \Drupal\comment\CommentInterface $comment */
      $breadcrumb->addCacheableDependency($comment);
      // Display link to parent comment.
      // @todo Clean-up permalink in https://www.drupal.org/node/2198041
      $breadcrumb->addLink(new Link($comment->getSubject(), $comment->toUrl()));
    }
    return $breadcrumb;
  }

}