book-2.0.x-dev/tests/modules/book_breadcrumb_test/book_breadcrumb_test.module

<?php

/**
 * @file
 * Test module for testing the book module breadcrumb.
 */

use Drupal\Core\Access\AccessResultForbidden;
use Drupal\Core\Access\AccessResultInterface;
use Drupal\Core\Access\AccessResultNeutral;
use Drupal\Core\Session\AccountInterface;
use Drupal\node\NodeInterface;

/**
 * Implements hook_ENTITY_TYPE_access().
 */
function book_breadcrumb_test_node_access(NodeInterface $node, $operation, AccountInterface $account): AccessResultInterface {
  $config = \Drupal::config('book_breadcrumb_test.settings');
  if ($config->get('hide') && $node->getTitle() == "you can't see me" && $operation == 'view') {
    $access = new AccessResultForbidden();
  }
  else {
    $access = new AccessResultNeutral();
  }
  $access->addCacheableDependency($config);
  $access->addCacheableDependency($node);
  return $access;
}