BookManager::addFormElements

public BookManager::addFormElements(array $form, FormStateInterface $form_state, NodeInterface $node, AccountInterface $account, $collapsed = TRUE)

public function addFormElements(array $form, FormStateInterface $form_state, NodeInterface $node, AccountInterface $account, $collapsed = TRUE) {
  // If the form is being processed during the Ajax callback of our book bid
  // dropdown, then $form_state will hold the value that was selected.
  if ($form_state->hasValue('book')) {
    $node->book = $form_state->getValue('book');
  }
  $form['book'] = array(
    '#type' => 'details',
    '#title' => $this->t('Book outline'),
    '#weight' => 10,
    '#open' => !$collapsed,
    '#group' => 'advanced',
    '#attributes' => array(
      'class' => array('book-outline-form'),
    ),
    '#attached' => array(
      'library' => array('book/drupal.book'),
    ),
    '#tree' => TRUE,
  );
  foreach (array('nid', 'has_children', 'original_bid', 'parent_depth_limit') as $key) {
    $form['book'][$key] = array(
      '#type' => 'value',
      '#value' => $node->book[$key],
    );
  }

  $form['book']['pid'] = $this->addParentSelectFormElements($node->book);

  // @see \Drupal\book\Form\BookAdminEditForm::bookAdminTableTree(). The
  // weight may be larger than 15.
  $form['book']['weight'] = array(
    '#type' => 'weight',
    '#title' => $this->t('Weight'),
    '#default_value' => $node->book['weight'],
    '#delta' => max(15, abs($node->book['weight'])),
    '#weight' => 5,
    '#description' => $this->t('Pages at a given level are ordered first by weight and then by title.'),
  );
  $options = array();
  $nid = !$node->isNew() ? $node->id() : 'new';
  if ($node->id() && ($nid == $node->book['original_bid']) && ($node->book['parent_depth_limit'] == 0)) {
    // This is the top level node in a maximum depth book and thus cannot be
    // moved.
    $options[$node->id()] = $node->label();
  }
  else {
    foreach ($this->getAllBooks() as $book) {
      $options[$book['nid']] = $book['title'];
    }
  }

  if ($account->hasPermission('create new books') && ($nid == 'new' || ($nid != $node->book['original_bid']))) {
    // The node can become a new book, if it is not one already.
    $options = array($nid => $this->t('- Create a new book -')) + $options;
  }
  if (!$node->book['bid']) {
    // The node is not currently in the hierarchy.
    $options = array(0 => $this->t('- None -')) + $options;
  }

  // Add a drop-down to select the destination book.
  $form['book']['bid'] = array(
    '#type' => 'select',
    '#title' => $this->t('Book'),
    '#default_value' => $node->book['bid'],
    '#options' => $options,
    '#access' => (bool) $options,
    '#description' => $this->t('Your page will be a part of the selected book.'),
    '#weight' => -5,
    '#attributes' => array('class' => array('book-title-select')),
    '#ajax' => array(
      'callback' => 'book_form_update',
      'wrapper' => 'edit-book-plid-wrapper',
      'effect' => 'fade',
      'speed' => 'fast',
    ),
  );
  return $form;
}