book.module

<?php

/**
 * @file
 * Allows users to create and organize related content in an outline.
 */

use Drupal\Core\Entity\EntityInterface;
use Drupal\node\NodeTypeInterface;
use Drupal\Core\Language\Language;
use Drupal\entity\Entity\EntityDisplay;
use Drupal\Core\Template\Attribute;
use Drupal\menu_link\Entity\MenuLink;
use Drupal\menu_link\MenuLinkStorageController;
use Drupal\node\NodeInterface;

/**
 * Implements hook_help().
 */
function book_help($path, $arg) {
  switch ($path) {
    case 'admin/help#book':
      $output = '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Book module is used for creating structured, multi-page content, such as site resource guides, manuals, and wikis. It allows you to create content that has chapters, sections, subsections, or any similarly-tiered structure. For more information, see the <a href="!book">online documentation for the Book module</a>.', array('!book' => 'https://drupal.org/documentation/modules/book')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Adding and managing book content') . '</dt>';
      $output .= '<dd>' . t('You can assign separate permissions for <em>creating new books</em> as well as <em>creating</em>, <em>editing</em> and <em>deleting</em> book content. Users with the <em>Administer book outlines</em> permission can add <em>any</em> type of content to a book by selecting the appropriate book outline while editing the content. They can also view a list of all books, and edit and rearrange section titles on the <a href="!admin-book">Book administration page</a>.', array('!admin-book' => \Drupal::url('book.admin'))) . '</dd>';
      $output .= '<dt>' . t('Book navigation') . '</dt>';
      $output .= '<dd>' . t("Book pages have a default book-specific navigation block. This navigation block contains links that lead to the previous and next pages in the book, and to the level above the current page in the book's structure. This block can be enabled on the <a href='!admin-block'>Blocks administration page</a>. For book pages to show up in the book navigation, they must be added to a book outline.", array('!admin-block' => \Drupal::url('block.admin_display'))) . '</dd>';
      $output .= '<dt>' . t('Collaboration') . '</dt>';
      $output .= '<dd>' . t('Books can be created collaboratively, as they allow users with appropriate permissions to add pages into existing books, and add those pages to a custom table of contents menu.') . '</dd>';
      $output .= '<dt>' . t('Printing books') . '</dt>';
      $output .= '<dd>' . t("Users with the <em>View printer-friendly books</em> permission can select the <em>printer-friendly version</em> link visible at the bottom of a book page's content to generate a printer-friendly display of the page and all of its subsections.") . '</dd>';
      $output .= '</dl>';
      return $output;
    case 'admin/structure/book':
      return '<p>' . t('The book module offers a means to organize a collection of related content pages, collectively known as a book. When viewed, this content automatically displays links to adjacent book pages, providing a simple navigation system for creating and reviewing structured content.') . '</p>';
    case 'node/%/outline':
      return '<p>' . t('The outline feature allows you to include pages in the <a href="!book">Book hierarchy</a>, as well as move them within the hierarchy or to <a href="!book-admin">reorder an entire book</a>.', array('!book' => \Drupal::url('book.render'), '!book-admin' => \Drupal::url('book.admin'))) . '</p>';
  }
}

/**
 * Implements hook_entity_bundle_info().
 */
function book_entity_bundle_info() {
  $bundles['menu_link']['book-toc'] = array(
    'label' => t('Book'),
    'translatable' => FALSE,
  );
  return $bundles;
}

/**
 * Implements hook_TYPE_load().
 */
function book_menu_link_load($entities) {
  foreach ($entities as $entity) {
    // Change the bundle of menu links related to a book.
    if (strpos($entity->menu_name, 'book-toc-') === 0) {
      $entity->bundle = 'book-toc';
    }
  }
}

/**
 * Implements hook_theme().
 */
function book_theme() {
  return array(
    'book_navigation' => array(
      'variables' => array('book_link' => NULL),
      'template' => 'book-navigation',
    ),
    'book_export_html' => array(
      'variables' => array('title' => NULL, 'contents' => NULL, 'depth' => NULL),
      'template' => 'book-export-html',
    ),
    'book_admin_table' => array(
      'render element' => 'form',
      'file' => 'book.admin.inc',
    ),
    'book_all_books_block' => array(
      'render element' => 'book_menus',
      'template' => 'book-all-books-block',
    ),
    'book_node_export_html' => array(
      'variables' => array('node' => NULL, 'children' => NULL),
      'template' => 'book-node-export-html',
    ),
  );
}

/**
 * Implements hook_permission().
 */
function book_permission() {
  return array(
    'administer book outlines' => array(
      'title' => t('Administer book outlines'),
    ),
    'create new books' => array(
      'title' => t('Create new books'),
    ),
    'add content to books' => array(
      'title' => t('Add content and child pages to books'),
    ),
    'access printer-friendly version' => array(
      'title' => t('View printer-friendly books'),
      'description' => t('View a book page and all of its sub-pages as a single document for ease of printing. Can be performance heavy.'),
    ),
  );
}

/**
 * Implements hook_entity_info().
 */
function book_entity_info(&$entity_info) {
  $entity_info['node']['controllers']['form']['book_outline'] = '\Drupal\book\Form\BookOutlineForm';
}

/**
 * Adds relevant book links to the node's links.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The book page node to add links to.
 * @param $view_mode
 *   The view mode of the node.
 */
function book_node_view_link(NodeInterface $node, $view_mode) {
  $links = array();
  $account = \Drupal::currentUser();

  if (isset($node->book['depth'])) {
    if ($view_mode == 'full' && node_is_page($node)) {
      $child_type = \Drupal::config('book.settings')->get('child_type');
      if (($account->hasPermission('add content to books') || $account->hasPermission('administer book outlines')) && node_access('create', $child_type) && $node->isPublished() && $node->book['depth'] < MENU_MAX_DEPTH) {
        $links['book_add_child'] = array(
          'title' => t('Add child page'),
          'href' => 'node/add/' . $child_type,
          'query' => array('parent' => $node->book['mlid']),
        );
      }

      if ($account->hasPermission('access printer-friendly version')) {
        $links['book_printer'] = array(
          'title' => t('Printer-friendly version'),
          'href' => 'book/export/html/' . $node->id(),
          'attributes' => array('title' => t('Show a printer-friendly version of this book page and its sub-pages.'))
        );
      }
    }
  }

  if (!empty($links)) {
    $node->content['links']['book'] = array(
      '#theme' => 'links__node__book',
      '#links' => $links,
      '#attributes' => array('class' => array('links', 'inline')),
    );
  }
}

/**
 * Implements hook_menu().
 */
function book_menu() {
  $items['admin/structure/book'] = array(
    'title' => 'Books',
    'description' => "Manage your site's book outlines.",
    'route_name' => 'book.admin',
  );
  $items['book'] = array(
    'title' => 'Books',
    'route_name' => 'book.render',
    'type' => MENU_SUGGESTED_ITEM,
  );
  $items['node/%node/outline/remove'] = array(
    'title' => 'Remove from outline',
    'route_name' => 'book.remove',
  );

  return $items;
}

/**
 * Access callback: Determines if the outline tab is accessible.
 *
 * Path:
 * - admin/structure/book/%node
 * - node/%node/outline
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node whose outline tab is to be viewed.
 *
 * @see book_menu()
 */
function _book_outline_access(EntityInterface $node) {
  return \Drupal::currentUser()->hasPermission('administer book outlines') && node_access('view', $node);
}

/**
 * Access callback: Determines if the user can remove nodes from the outline.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node to remove from the outline.
 *
 * @see book_menu()
 */
function _book_outline_remove_access(EntityInterface $node) {
  return \Drupal::service('book.manager')->checkNodeIsRemovable($node)
    && _book_outline_access($node);
}

/**
 * Implements hook_admin_paths().
 */
function book_admin_paths() {
  if (\Drupal::config('node.settings')->get('use_admin_theme')) {
    $paths = array(
      'node/*/outline' => TRUE,
      'node/*/outline/remove' => TRUE,
    );
    return $paths;
  }
}

/**
 * Returns an array of all books.
 *
 * @todo Remove in favor of BookManager Service. http://drupal.org/node/1963894
 *
 * This list may be used for generating a list of all the books, or for building
 * the options for a form select.
 *
 * @return
 *   An array of all books.
 */
function book_get_books() {
  return \Drupal::service('book.manager')->getAllBooks();
}

/**
 * Implements hook_form_BASE_FORM_ID_alter() for node_form().
 *
 * Adds the book form element to the node form.
 *
 * @see book_pick_book_nojs_submit()
 */
function book_form_node_form_alter(&$form, &$form_state, $form_id) {
  $node = $form_state['controller']->getEntity();
  $account = \Drupal::currentUser();
  $access = $account->hasPermission('administer book outlines');
  if (!$access) {
    if ($account->hasPermission('add content to books') && ((!empty($node->book['mlid']) && !$node->isNew()) || book_type_is_allowed($node->getType()))) {
      // Already in the book hierarchy, or this node type is allowed.
      $access = TRUE;
    }
  }

  if ($access) {
    $form = \Drupal::service('book.manager')->addFormElements($form, $form_state, $node, $account);
    // Since the "Book" dropdown can't trigger a form submission when
    // JavaScript is disabled, add a submit button to do that. book.admin.css hides
    // this button when JavaScript is enabled.
    $form['book']['pick-book'] = array(
      '#type' => 'submit',
      '#value' => t('Change book (update list of parents)'),
      '#submit' => array('book_pick_book_nojs_submit'),
      '#weight' => 20,
      '#attached' => array(
        'css' => array(drupal_get_path('module', 'book') . '/css/book.admin.css'),
      ),
    );
    $form['#entity_builders'][] = 'book_node_builder';
  }
}

/**
 * Entity form builder to add the book information to the node.
 *
 * @todo: Remove this in favor of an entity field.
 */
function book_node_builder($entity_type, $entity, &$form, &$form_state) {
  $entity->book = $form_state['values']['book'];
}

/**
 * Form submission handler for node_form().
 *
 * This handler is run when JavaScript is disabled. It triggers the form to
 * rebuild so that the "Parent item" options are changed to reflect the newly
 * selected book. When JavaScript is enabled, the submit button that triggers
 * this handler is hidden, and the "Book" dropdown directly triggers the
 * book_form_update() Ajax callback instead.
 *
 * @see book_form_update()
 * @see book_form_node_form_alter()
 */
function book_pick_book_nojs_submit($form, &$form_state) {
  $node = $form_state['controller']->getEntity();
  $node->book = $form_state['values']['book'];
  $form_state['rebuild'] = TRUE;
}

/**
 * Renders a new parent page select element when the book selection changes.
 *
 * This function is called via Ajax when the selected book is changed on a node
 * or book outline form.
 *
 * @return
 *   The rendered parent page select element.
 */
function book_form_update($form, $form_state) {
  return $form['book']['plid'];
}

/**
 * Gets the book menu tree for a page and returns it as a linear array.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   A linear array of menu links in the order that the links are shown in the
 *   menu, so the previous and next pages are the elements before and after the
 *   element corresponding to the current node. The children of the current node
 *   (if any) will come immediately after it in the array, and links will only
 *   be fetched as deep as one level deeper than $book_link.
 */
function book_get_flat_menu($book_link) {
  $flat = &drupal_static(__FUNCTION__, array());

  if (!isset($flat[$book_link['mlid']])) {
    // Call menu_tree_all_data() to take advantage of the menu system's caching.
    $tree = menu_tree_all_data($book_link['menu_name'], $book_link, $book_link['depth'] + 1);
    $flat[$book_link['mlid']] = array();
    _book_flatten_menu($tree, $flat[$book_link['mlid']]);
  }

  return $flat[$book_link['mlid']];
}

/**
 * Recursively converts a tree of menu links to a flat array.
 *
 * @param $tree
 *   A tree of menu links in an array.
 * @param $flat
 *   A flat array of the menu links from $tree, passed by reference.
 *
 * @see book_get_flat_menu().
 */
function _book_flatten_menu($tree, &$flat) {
  foreach ($tree as $data) {
    if (!$data['link']['hidden']) {
      $flat[$data['link']['mlid']] = $data['link'];
      if ($data['below']) {
        _book_flatten_menu($data['below'], $flat);
      }
    }
  }
}

/**
 * Fetches the menu link for the previous page of the book.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   A fully loaded menu link for the page before the one represented in
 *   $book_link.
 */
function book_prev($book_link) {
  // If the parent is zero, we are at the start of a book.
  if ($book_link['plid'] == 0) {
    return NULL;
  }
  $flat = book_get_flat_menu($book_link);
  // Assigning the array to $flat resets the array pointer for use with each().
  $curr = NULL;
  do {
    $prev = $curr;
    list($key, $curr) = each($flat);
  } while ($key && $key != $book_link['mlid']);

  if ($key == $book_link['mlid']) {
    // The previous page in the book may be a child of the previous visible link.
    if ($prev['depth'] == $book_link['depth'] && $prev['has_children']) {
      // The subtree will have only one link at the top level - get its data.
      $tree = book_menu_subtree_data($prev);
      $data = array_shift($tree);
      // The link of interest is the last child - iterate to find the deepest one.
      while ($data['below']) {
        $data = end($data['below']);
      }

      return $data['link'];
    }
    else {
      return $prev;
    }
  }
}

/**
 * Fetches the menu link for the next page of the book.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   A fully loaded menu link for the page after the one represented in
 *   $book_link.
 */
function book_next($book_link) {
  $flat = book_get_flat_menu($book_link);
  // Assigning the array to $flat resets the array pointer for use with each().
  do {
    list($key, $curr) = each($flat);
  }
  while ($key && $key != $book_link['mlid']);

  if ($key == $book_link['mlid']) {
    return current($flat);
  }
}

/**
 * Formats the menu links for the child pages of the current page.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   HTML for the links to the child pages of the current page.
 */
function book_children($book_link) {
  $flat = book_get_flat_menu($book_link);

  $children = array();

  if ($book_link['has_children']) {
    // Walk through the array until we find the current page.
    do {
      $link = array_shift($flat);
    }
    while ($link && ($link['mlid'] != $book_link['mlid']));
    // Continue though the array and collect the links whose parent is this page.
    while (($link = array_shift($flat)) && $link['plid'] == $book_link['mlid']) {
      $data['link'] = $link;
      $data['below'] = '';
      $children[] = $data;
    }
  }

  if ($children) {
    $elements = menu_tree_output($children);
    return drupal_render($elements);
  }
  return '';
}

/**
 * Implements hook_node_load().
 */
function book_node_load($nodes, $types) {
  $result = db_query("SELECT * FROM {book} b INNER JOIN {menu_links} ml ON b.mlid = ml.mlid WHERE b.nid IN (:nids)", array(':nids' =>  array_keys($nodes)), array('fetch' => PDO::FETCH_ASSOC));
  foreach ($result as $record) {
    $nodes[$record['nid']]->book = $record;
    $nodes[$record['nid']]->book['href'] = $record['link_path'];
    $nodes[$record['nid']]->book['title'] = $record['link_title'];
    $nodes[$record['nid']]->book['options'] = unserialize($record['options']);
  }
}

/**
 * Implements hook_node_view().
 */
function book_node_view(EntityInterface $node, EntityDisplay $display, $view_mode) {
  if ($view_mode == 'full') {
    if (!empty($node->book['bid']) && empty($node->in_preview)) {
      $book_navigation = array( '#theme' => 'book_navigation', '#book_link' => $node->book);
      $node->content['book_navigation'] = array(
        '#markup' => drupal_render($book_navigation),
        '#weight' => 100,
        '#attached' => array(
          'css' => array(
            drupal_get_path('module', 'book') . '/css/book.theme.css',
          ),
        ),
      );
    }
  }

  if ($view_mode != 'rss') {
    book_node_view_link($node, $view_mode);
  }
}

/**
 * Implements hook_page_alter().
 *
 * Adds the book menu to the list of menus used to build the active trail when
 * viewing a book page.
 */
function book_page_alter(&$page) {
  if (($node = menu_get_object()) && !empty($node->book['bid'])) {
    $active_menus = menu_get_active_menu_names();
    $active_menus[] = $node->book['menu_name'];
    menu_set_active_menu_names($active_menus);
  }
}

/**
 * Implements hook_node_presave().
 */
function book_node_presave(EntityInterface $node) {
  // Always save a revision for non-administrators.
  if (!empty($node->book['bid']) && !\Drupal::currentUser()->hasPermission('administer nodes')) {
    $node->setNewRevision();
  }
  // Make sure a new node gets a new menu link.
  if ($node->isNew()) {
    $node->book['mlid'] = NULL;
  }
}

/**
 * Implements hook_node_insert().
 */
function book_node_insert(EntityInterface $node) {
  $book_manager = \Drupal::service('book.manager');
  if (!empty($node->book['bid'])) {
    if ($node->book['bid'] == 'new') {
      // New nodes that are their own book.
      $node->book['bid'] = $node->id();
    }
    $node->book['nid'] = $node->id();
    $node->book['menu_name'] = $book_manager->createMenuName($node->book['bid']);
    $book_manager->updateOutline($node);
  }
}

/**
 * Implements hook_node_update().
 */
function book_node_update(EntityInterface $node) {
  $book_manager = \Drupal::service('book.manager');
  if (!empty($node->book['bid'])) {
    if ($node->book['bid'] == 'new') {
      // New nodes that are their own book.
      $node->book['bid'] = $node->id();
    }
    $node->book['nid'] = $node->id();
    $node->book['menu_name'] = $book_manager->createMenuName($node->book['bid']);
    $book_manager->updateOutline($node);
  }
}

/**
 * Implements hook_node_predelete().
 */
function book_node_predelete(EntityInterface $node) {
  if (!empty($node->book['bid'])) {
    if ($node->id() == $node->book['bid']) {
      // Handle deletion of a top-level post.
      $result = db_query("SELECT b.nid FROM {menu_links} ml INNER JOIN {book} b on b.mlid = ml.mlid WHERE ml.plid = :plid", array(
        ':plid' => $node->book['mlid']
      ));
      foreach ($result as $child) {
        $child_node = node_load($child->id());
        $child_node->book['bid'] = $child_node->id();
        \Drupal::service('book.manager')->updateOutline($child_node);
      }
    }
    menu_link_delete($node->book['mlid']);
    db_delete('book')
      ->condition('mlid', $node->book['mlid'])
      ->execute();
    drupal_static_reset('book_get_books');
  }
}

/**
 * Implements hook_node_prepare_form().
 */
function book_node_prepare_form(NodeInterface $node, $form_display, $operation, array &$form_state) {
  // Get BookManager service
  $book_manager = \Drupal::service('book.manager');

  // Prepare defaults for the add/edit form.
  $account = \Drupal::currentUser();
  if (empty($node->book) && ($account->hasPermission('add content to books') || $account->hasPermission('administer book outlines'))) {
    $node->book = array();

    $query = \Drupal::request()->query;
    if ($node->isNew() && !is_null($query->get('parent')) && is_numeric($query->get('parent'))) {
      // Handle "Add child page" links:
      $parent = book_link_load($query->get('parent'));

      if ($parent && $parent['access']) {
        $node->book['bid'] = $parent['bid'];
        $node->book['plid'] = $parent['mlid'];
        $node->book['menu_name'] = $parent['menu_name'];
      }
    }
    // Set defaults.
    $node_ref = !$node->isNew() ? $node->id() : 'new';
    $node->book += $book_manager->getLinkDefaults($node_ref);
  }
  else {
    if (isset($node->book['bid']) && !isset($node->book['original_bid'])) {
      $node->book['original_bid'] = $node->book['bid'];
    }
  }
  // Find the depth limit for the parent select.
  if (isset($node->book['bid']) && !isset($node->book['parent_depth_limit'])) {
    $node->book['parent_depth_limit'] = $book_manager->getParentDepthLimit($node->book);
  }
}

/**
 * Implements hook_form_FORM_ID_alter() for node_delete_confirm().
 *
 * Alters the confirm form for a single node deletion.
 *
 * @see node_delete_confirm()
 */
function book_form_node_delete_confirm_alter(&$form, $form_state) {
  $node = node_load($form['nid']['#value']);

  if (isset($node->book) && $node->book['has_children']) {
    $form['book_warning'] = array(
      '#markup' => '<p>' . t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.', array('%title' => $node->label())) . '</p>',
      '#weight' => -10,
    );
  }
}

/**
 * Implements hook_preprocess_HOOK() for block templates.
 */
function book_preprocess_block(&$variables) {
  if ($variables['configuration']['module'] == 'book') {
    $variables['attributes']['role'] = 'navigation';
  }
}

/**
 * Prepares variables for book listing block templates.
 *
 * Default template: book-all-books-block.html.twig.
 *
 * All non-renderable elements are removed so that the template has full access
 * to the structured data but can also simply iterate over all elements and
 * render them (as in the default template).
 *
 * @param array $variables
 *   An associative array containing the following key:
 *   - book_menus: An associative array containing renderable menu links for all
 *     book menus.
 */
function template_preprocess_book_all_books_block(&$variables) {
  // Remove all non-renderable elements.
  $elements = $variables['book_menus'];
  $variables['book_menus'] = array();
  foreach (element_children($elements) as $index) {
    $variables['book_menus'][$index] = $elements[$index];
  }
}

/**
 * Prepares variables for book navigation templates.
 *
 * Default template: book-navigation.html.twig.
 *
 * @param array $variables
 *   An associative array containing the following key:
 *   - book_link: An associative array of book link properties.
 *     Properties used: bid, link_title, depth, plid, mlid.
 */
function template_preprocess_book_navigation(&$variables) {
  $book_link = $variables['book_link'];

  // Provide extra variables for themers. Not needed by default.
  $variables['book_id'] = $book_link['bid'];
  $variables['book_title'] = check_plain($book_link['link_title']);
  $variables['book_url'] = 'node/' . $book_link['bid'];
  $variables['current_depth'] = $book_link['depth'];
  $variables['tree'] = '';

  if ($book_link['mlid']) {
    $variables['tree'] = book_children($book_link);

    $build = array();

    if ($prev = book_prev($book_link)) {
      $prev_href = url($prev['href']);
      $build['#attached']['drupal_add_html_head_link'][][] = array(
        'rel' => 'prev',
        'href' => $prev_href,
      );
      $variables['prev_url'] = $prev_href;
      $variables['prev_title'] = check_plain($prev['title']);
    }

    if ($book_link['plid'] && $parent = book_link_load($book_link['plid'])) {
      $parent_href = url($parent['link_path']);
      $build['#attached']['drupal_add_html_head_link'][][] = array(
        'rel' => 'up',
        'href' => $parent_href,
      );
      $variables['parent_url'] = $parent_href;
      $variables['parent_title'] = check_plain($parent['title']);
    }

    if ($next = book_next($book_link)) {
      $next_href = url($next['href']);
      $build['#attached']['drupal_add_html_head_link'][][] = array(
        'rel' => 'next',
        'href' => $next_href,
      );
      $variables['next_url'] = $next_href;
      $variables['next_title'] = check_plain($next['title']);
    }
  }

  if (!empty($build)) {
    drupal_render($build);
  }

  $variables['has_links'] = FALSE;
  // Link variables to filter for values and set state of the flag variable.
  $links = array('prev_url', 'prev_title', 'parent_url', 'parent_title', 'next_url', 'next_title');
  foreach ($links as $link) {
    if (isset($variables[$link])) {
      // Flag when there is a value.
      $variables['has_links'] = TRUE;
    }
    else {
      // Set empty to prevent notices.
      $variables[$link] = '';
    }
  }
}

/**
 * Prepares variables for book export templates.
 *
 * Default template: book-export-html.html.twig.
 *
 * @param array $variables
 *   An associative array containing:
 *   - title: The title of the book.
 *   - contents: Output of each book page.
 *   - depth: The max depth of the book.
 */
function template_preprocess_book_export_html(&$variables) {
  global $base_url;
  $language_interface = language(Language::TYPE_INTERFACE);

  $variables['title'] = check_plain($variables['title']);
  $variables['base_url'] = $base_url;
  $variables['language'] = $language_interface;
  $variables['language_rtl'] = ($language_interface->direction == Language::DIRECTION_RTL);
  $variables['head'] = drupal_get_html_head();

  // HTML element attributes.
  $attributes = array();
  $attributes['lang'] = $language_interface->id;
  $attributes['dir'] = $language_interface->direction ? 'rtl' : 'ltr';
  $variables['html_attributes'] = new Attribute($attributes);
}

/**
 * Prepares variables for single node export templates.
 *
 * Default template: book-node-export-html.html.twig.
 *
 * @param array $variables
 *   An associative array containing the following keys:
 *   - node: The node that will be output.
 *   - children: All the rendered child nodes within the current node. Defaults
 *     to an empty string.
 */
function template_preprocess_book_node_export_html(&$variables) {
  $variables['depth'] = $variables['node']->book['depth'];
  $variables['title'] = check_plain($variables['node']->label());
  $variables['content'] = $variables['node']->rendered;
}

/**
 * Determines if a given node type is in the list of types allowed for books.
 *
 * @param string $type
 *   A node type.
 *
 * @return bool
 *   A Boolean TRUE if the node type can be included in books; otherwise, FALSE.
 */
function book_type_is_allowed($type) {
  return in_array($type, \Drupal::config('book.settings')->get('allowed_types'));
}

/**
 * Implements hook_node_type_update().
 *
 * Updates book.settings configuration object if the machine-readable name of a
 * node type is changed.
 */
function book_node_type_update(NodeTypeInterface $type) {
  if ($type->getOriginalId() != $type->id()) {
    $config = \Drupal::config('book.settings');
    // Update the list of node types that are allowed to be added to books.
    $allowed_types = $config->get('allowed_types');
    $old_key = array_search($type->getOriginalId(), $allowed_types);

    if ($old_key !== FALSE) {
      $allowed_types[$old_key] = $type->id();
      // Ensure that the allowed_types array is sorted consistently.
      // @see BookSettingsForm::submitForm()
      sort($allowed_types);
      $config->set('allowed_types', $allowed_types);
    }

    // Update the setting for the "Add child page" link.
    if ($config->get('child_type') == $type->getOriginalId()) {
      $config->set('child_type', $type->id());
    }
    $config->save();
  }