class BookUninstallValidator

class BookUninstallValidator implements ModuleUninstallValidatorInterface {
    use StringTranslationTrait;
    
    /**
     * The book outline storage.
     *
     * @var \Drupal\book\BookOutlineStorageInterface
     */
    protected $bookOutlineStorage;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * Constructs a new BookUninstallValidator.
     *
     * @param \Drupal\book\BookOutlineStorageInterface $book_outline_storage
     *   The book outline storage.
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
     *   The string translation service.
     */
    public function __construct(BookOutlineStorageInterface $book_outline_storage, EntityTypeManagerInterface $entity_type_manager, TranslationInterface $string_translation) {
        $this->bookOutlineStorage = $book_outline_storage;
        $this->entityTypeManager = $entity_type_manager;
        $this->stringTranslation = $string_translation;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate($module) {
        $reasons = [];
        if ($module == 'book') {
            if ($this->hasBookOutlines()) {
                $reasons[] = $this->t('To uninstall Book, delete all content that is part of a book');
            }
            else {
                // The book node type is provided by the Book module. Prevent uninstall
                // if there are any nodes of that type.
                if ($this->hasBookNodes()) {
                    $reasons[] = $this->t('To uninstall Book, delete all content that has the Book content type');
                }
            }
        }
        return $reasons;
    }
    
    /**
     * Checks if there are any books in an outline.
     *
     * @return bool
     *   TRUE if there are books, FALSE if not.
     */
    protected function hasBookOutlines() {
        return $this->bookOutlineStorage
            ->hasBooks();
    }
    
    /**
     * Determines if there is any book nodes or not.
     *
     * @return bool
     *   TRUE if there are book nodes, FALSE otherwise.
     */
    protected function hasBookNodes() {
        $nodes = $this->entityTypeManager
            ->getStorage('node')
            ->getQuery()
            ->condition('type', 'book')
            ->accessCheck(FALSE)
            ->range(0, 1)
            ->execute();
        return !empty($nodes);
    }

}