BlockContentGetDependencyEvent example

// Add the entity as a cacheable dependency because access will at least be     // determined by whether the block is reusable.     $access->addCacheableDependency($entity);
    if ($entity->isReusable() === FALSE && $access->isForbidden() !== TRUE) {
      if (!$entity instanceof DependentAccessInterface) {
        throw new \LogicException("Non-reusable block entities must implement \Drupal\block_content\Access\DependentAccessInterface for access control.");
      }
      $dependency = $entity->getAccessDependency();
      if (empty($dependency)) {
        // If an access dependency has not been set let modules set one.         $event = new BlockContentGetDependencyEvent($entity);
        $this->eventDispatcher->dispatch($event, BlockContentEvents::BLOCK_CONTENT_GET_DEPENDENCY);
        $dependency = $event->getAccessDependency();
        if (empty($dependency)) {
          return AccessResult::forbidden("Non-reusable blocks must set an access dependency for access control.");
        }
      }
      /** @var \Drupal\Core\Entity\EntityInterface $dependency */
      $access = $access->andIf($dependency->access($operation, $account, TRUE));
    }
    return $access;
  }