class BookRemoveForm

class BookRemoveForm extends ConfirmFormBase {
    
    /**
     * The book manager.
     *
     * @var \Drupal\book\BookManagerInterface
     */
    protected $bookManager;
    
    /**
     * The node representing the book.
     *
     * @var \Drupal\node\NodeInterface
     */
    protected $node;
    
    /**
     * Constructs a BookRemoveForm object.
     *
     * @param \Drupal\book\BookManagerInterface $book_manager
     *   The book manager.
     */
    public function __construct(BookManagerInterface $book_manager) {
        $this->bookManager = $book_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('book.manager'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'book_remove_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, NodeInterface $node = NULL) {
        $this->node = $node;
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDescription() {
        $title = [
            '%title' => $this->node
                ->label(),
        ];
        if ($this->node->book['has_children']) {
            return $this->t('%title has associated child pages, which will be relocated automatically to maintain their connection to the book. To recreate the hierarchy (as it was before removing this page), %title may be added again using the Outline tab, and each of its former child pages will need to be relocated manually.', $title);
        }
        else {
            return $this->t('%title may be added to hierarchy again using the Outline tab.', $title);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConfirmText() {
        return $this->t('Remove');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getQuestion() {
        return $this->t('Are you sure you want to remove %title from the book hierarchy?', [
            '%title' => $this->node
                ->label(),
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCancelUrl() {
        return $this->node
            ->toUrl();
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        if ($this->bookManager
            ->checkNodeIsRemovable($this->node)) {
            $this->bookManager
                ->deleteFromBook($this->node
                ->id());
            $this->messenger()
                ->addStatus($this->t('The post has been removed from the book.'));
        }
        $form_state->setRedirectUrl($this->getCancelUrl());
    }

}