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());
  }

}