class BookOutlineForm

class BookOutlineForm extends ContentEntityForm {
    
    /**
     * The book being displayed.
     *
     * @var \Drupal\node\NodeInterface
     */
    protected $entity;
    
    /**
     * BookManager service.
     *
     * @var \Drupal\book\BookManagerInterface
     */
    protected $bookManager;
    
    /**
     * Constructs a BookOutlineForm object.
     *
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository.
     * @param \Drupal\book\BookManagerInterface $book_manager
     *   The BookManager service.
     * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
     *   The entity type bundle service.
     * @param \Drupal\Component\Datetime\TimeInterface $time
     *   The time service.
     */
    public function __construct(EntityRepositoryInterface $entity_repository, BookManagerInterface $book_manager, ?EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, ?TimeInterface $time = NULL) {
        parent::__construct($entity_repository, $entity_type_bundle_info, $time);
        $this->bookManager = $book_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity.repository'), $container->get('book.manager'), $container->get('entity_type.bundle.info'), $container->get('datetime.time'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getBaseFormId() {
        return NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form['#title'] = $this->entity
            ->label();
        if (!isset($this->entity->book)) {
            // The node is not part of any book yet - set default options.
            $this->entity->book = $this->bookManager
                ->getLinkDefaults($this->entity
                ->id());
        }
        else {
            $this->entity->book['original_bid'] = $this->entity->book['bid'];
        }
        // Find the depth limit for the parent select.
        if (!isset($this->entity->book['parent_depth_limit'])) {
            $this->entity->book['parent_depth_limit'] = $this->bookManager
                ->getParentDepthLimit($this->entity->book);
        }
        $form = $this->bookManager
            ->addFormElements($form, $form_state, $this->entity, $this->currentUser(), FALSE);
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function actions(array $form, FormStateInterface $form_state) {
        $actions = parent::actions($form, $form_state);
        $actions['submit']['#value'] = $this->entity->book['original_bid'] ? $this->t('Update book outline') : $this->t('Add to book outline');
        $actions['delete']['#title'] = $this->t('Remove from book outline');
        $actions['delete']['#url'] = new Url('entity.node.book_remove_form', [
            'node' => $this->entity->book['nid'],
        ]);
        $actions['delete']['#access'] = $this->bookManager
            ->checkNodeIsRemovable($this->entity);
        return $actions;
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        $form_state->setRedirect('entity.node.canonical', [
            'node' => $this->entity
                ->id(),
        ]);
        $book_link = $form_state->getValue('book');
        if (!$book_link['bid']) {
            $this->messenger()
                ->addStatus($this->t('No changes were made'));
            return;
        }
        $this->entity->book = $book_link;
        if ($this->bookManager
            ->updateOutline($this->entity)) {
            if (isset($this->entity->book['parent_mismatch']) && $this->entity->book['parent_mismatch']) {
                // This will usually only happen when JS is disabled.
                $this->messenger()
                    ->addStatus($this->t('The post has been added to the selected book. You may now position it relative to other pages.'));
                $form_state->setRedirectUrl($this->entity
                    ->toUrl('book-outline-form'));
            }
            else {
                $this->messenger()
                    ->addStatus($this->t('The book outline has been updated.'));
            }
        }
        else {
            $this->messenger()
                ->addError($this->t('There was an error adding the post to the book.'));
        }
    }

}