summaryrefslogtreecommitdiffstats
path: root/core/modules/book/book.module
blob: 59f1e45c7d594f03d0c0a77be624dcfe148b58b8 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
<?php

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

use Drupal\Core\Entity\EntityInterface;
use Drupal\entity\Plugin\Core\Entity\EntityDisplay;
use Drupal\Core\Template\Attribute;
use Drupal\menu_link\Plugin\Core\Entity\MenuLink;
use Drupal\menu_link\MenuLinkStorageController;

/**
 * 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 online handbook entry for <a href="@book">Book module</a>.', array('@book' => 'http://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</em>, <em>editing</em>, and <em>deleting</em> book content, as well as <em>adding content to books</em>, and <em>creating new books</em>. 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' => url('admin/structure/book'))) . '</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' => url('admin/structure/block'))) . '</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' => url('book'), '@book-admin' => url('admin/structure/book'))) . '</p>';
  }
}

/**
 * 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',
    ),
    '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.'),
    ),
  );
}

/**
 * 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(EntityInterface $node, $view_mode) {
  $links = array();

  if (isset($node->book['depth'])) {
    if ($view_mode == 'full' && node_is_page($node)) {
      $child_type = config('book.settings')->get('child_type');
      if ((user_access('add content to books') || user_access('administer book outlines')) && node_access('create', $child_type) && $node->status == 1 && $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 (user_access('access printer-friendly version')) {
        $links['book_printer'] = array(
          'title' => t('Printer-friendly version'),
          'href' => 'book/export/html/' . $node->nid,
          '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['admin/structure/book/list'] = array(
    'title' => 'List',
    'type' => MENU_DEFAULT_LOCAL_TASK,
  );
  $items['admin/structure/book/settings'] = array(
    'title' => 'Settings',
    'route_name' => 'book_settings',
    'access arguments' => array('administer site configuration'),
    'type' => MENU_LOCAL_TASK,
    'weight' => 100,
  );
  $items['admin/structure/book/%node'] = array(
    'title' => 'Re-order book pages and change titles',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('book_admin_edit', 3),
    'access callback' => '_book_outline_access',
    'access arguments' => array(3),
    'type' => MENU_CALLBACK,
    'file' => 'book.admin.inc',
  );
  $items['book'] = array(
    'title' => 'Books',
    'route_name' => 'book_render',
    'type' => MENU_SUGGESTED_ITEM,
  );
  $items['book/export/%/%node'] = array(
    'page callback' => 'book_export',
    'page arguments' => array(2, 3),
    'access callback' => 'book_export_access',
    'access arguments' => array(3),
    'type' => MENU_CALLBACK,
    'file' => 'book.pages.inc',
  );
  $items['node/%node/outline'] = array(
    'title' => 'Outline',
    'page callback' => 'book_outline',
    'page arguments' => array(1),
    'access callback' => '_book_outline_access',
    'access arguments' => array(1),
    'type' => MENU_LOCAL_TASK,
    'weight' => 2,
    'file' => 'book.pages.inc',
  );
  $items['node/%node/outline/remove'] = array(
    'title' => 'Remove from outline',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('book_remove_form', 1),
    'access callback' => '_book_outline_remove_access',
    'access arguments' => array(1),
    'file' => 'book.pages.inc',
  );

  return $items;
}

/**
 * Access callback: Determines if the book export page is accessible.
 *
 * @param \Drupal\node\Plugin\Core\Entity\EntityInterface $node
 *   The node whose export page is to be viewed.
 */
function book_export_access(EntityInterface $node) {
  return user_access('access printer-friendly version') && node_access('view', $node);
}

/**
 * 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 user_access('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 $node) {
  return _book_node_is_removable($node) && _book_outline_access($node);
}

/**
 * Determines if a node can be removed from the book.
 *
 * A node can be removed from a book if it is actually in a book and it either
 * is not a top-level page or is a top-level page with no children.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node to remove from the outline.
 */
function _book_node_is_removable(EntityInterface $node) {
  return (!empty($node->book['bid']) && (($node->book['bid'] != $node->nid) || !$node->book['has_children']));
}

/**
 * Implements hook_admin_paths().
 */
function book_admin_paths() {
  if (variable_get('node_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() {
  $all_books = &drupal_static(__FUNCTION__);

  if (!isset($all_books)) {
    $all_books = array();
    $nids = db_query("SELECT DISTINCT(bid) FROM {book}")->fetchCol();

    if ($nids) {
      $query = db_select('book', 'b', array('fetch' => PDO::FETCH_ASSOC));
      $query->join('node', 'n', 'b.nid = n.nid');
      $query->join('menu_links', 'ml', 'b.mlid = ml.mlid');
      $query->addField('n', 'type', 'type');
      $query->addField('n', 'title', 'title');
      $query->fields('b');
      $query->fields('ml');
      $query->condition('n.nid', $nids, 'IN');
      $query->condition('n.status', 1);
      $query->orderBy('ml.weight');
      $query->orderBy('ml.link_title');
      $query->addTag('node_access');
      $result2 = $query->execute();
      foreach ($result2 as $link) {
        $link['href'] = $link['link_path'];
        $link['options'] = unserialize($link['options']);
        $all_books[$link['bid']] = $link;
      }
    }
  }

  return $all_books;
}

/**
 * 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();
  $access = user_access('administer book outlines');
  if (!$access) {
    if (user_access('add content to books') && ((!empty($node->book['mlid']) && !empty($node->nid)) || book_type_is_allowed($node->type))) {
      // Already in the book hierarchy, or this node type is allowed.
      $access = TRUE;
    }
  }

  if ($access) {
    _book_add_form_elements($form, $form_state, $node);
    // 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') . '/book.admin.css'),
      ),
    );
  }
}

/**
 * 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;
}

/**
 * Builds the parent selection form element for the node form or outline tab.
 *
 * This function is also called when generating a new set of options during the
 * Ajax callback, so an array is returned that can be used to replace an
 * existing form element.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   A parent selection form element.
 */
function _book_parent_select($book_link) {
  if (config('menu.settings')->get('override_parent_selector')) {
    return array();
  }
  // Offer a message or a drop-down to choose a different parent page.
  $form = array(
    '#type' => 'hidden',
    '#value' => -1,
    '#prefix' => '<div id="edit-book-plid-wrapper">',
    '#suffix' => '</div>',
  );

  if ($book_link['nid'] === $book_link['bid']) {
    // This is a book - at the top level.
    if ($book_link['original_bid'] === $book_link['bid']) {
      $form['#prefix'] .= '<em>' . t('This is the top-level page in this book.') . '</em>';
    }
    else {
      $form['#prefix'] .= '<em>' . t('This will be the top-level page in this book.') . '</em>';
    }
  }
  elseif (!$book_link['bid']) {
    $form['#prefix'] .= '<em>' . t('No book selected.') . '</em>';
  }
  else {
    $form = array(
      '#type' => 'select',
      '#title' => t('Parent item'),
      '#default_value' => $book_link['plid'],
      '#description' => t('The parent page in the book. The maximum depth for a book and all child pages is !maxdepth. Some pages in the selected book may not be available as parents if selecting them would exceed this limit.', array('!maxdepth' => MENU_MAX_DEPTH)),
      '#options' => book_toc($book_link['bid'], $book_link['parent_depth_limit'], array($book_link['mlid'])),
      '#attributes' => array('class' => array('book-title-select')),
      '#prefix' => '<div id="edit-book-plid-wrapper">',
      '#suffix' => '</div>',
    );
  }

  return $form;
}

/**
 * Builds the common elements of the book form for the node and outline forms.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node whose form is being viewed.
 */
function _book_add_form_elements(&$form, &$form_state, EntityInterface $node) {
  // If the form is being processed during the Ajax callback of our book bid
  // dropdown, then $form_state will hold the value that was selected.
  if (isset($form_state['values']['book'])) {
    $node->book = $form_state['values']['book'];
  }

  $form['book'] = array(
    '#type' => 'details',
    '#title' => t('Book outline'),
    '#weight' => 10,
    '#collapsed' => TRUE,
    '#group' => 'advanced',
    '#attributes' => array(
      'class' => array('book-outline-form'),
    ),
    '#attached' => array(
      'library' => array(array('book', 'drupal.book')),
    ),
    '#tree' => TRUE,
  );
  foreach (array('menu_name', 'mlid', 'nid', 'router_path', 'has_children', 'options', 'module', 'original_bid', 'parent_depth_limit') as $key) {
    $form['book'][$key] = array(
      '#type' => 'value',
      '#value' => $node->book[$key],
    );
  }

  $form['book']['plid'] = _book_parent_select($node->book);

  // @see _book_admin_table_tree(). The weight may be larger than 15.
  $form['book']['weight'] = array(
    '#type' => 'weight',
    '#title' => t('Weight'),
    '#default_value' => $node->book['weight'],
    '#delta' => max(15, abs($node->book['weight'])),
    '#weight' => 5,
    '#description' => t('Pages at a given level are ordered first by weight and then by title.'),
  );
  $options = array();
  $nid = isset($node->nid) ? $node->nid : 'new';

  if (isset($node->nid) && ($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->nid] = $node->label();
  }
  else {
    foreach (book_get_books() as $book) {
      $options[$book['nid']] = $book['title'];
    }
  }

  if (user_access('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 => t('- Create a new book -')) + $options;
  }
  if (!$node->book['mlid']) {
    // The node is not currently in the hierarchy.
    $options = array(0 => t('- None -')) + $options;
  }

  // Add a drop-down to select the destination book.
  $form['book']['bid'] = array(
    '#type' => 'select',
    '#title' => t('Book'),
    '#default_value' => $node->book['bid'],
    '#options' => $options,
    '#access' => (bool) $options,
    '#description' => 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',
    ),
  );
}

/**
 * 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'];
}

/**
 * Handles additions and updates to the book outline.
 *
 * This common helper function performs all additions and updates to the book
 * outline through node addition, node editing, node deletion, or the outline
 * tab.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node that is being saved, added, deleted, or moved.
 *
 * @return
 *   TRUE if the menu link was saved; FALSE otherwise.
 */
function _book_update_outline(EntityInterface $node) {
  if (empty($node->book['bid'])) {
    return FALSE;
  }
  $new = empty($node->book['mlid']);

  $node->book['link_path'] = 'node/' . $node->nid;
  $node->book['link_title'] = $node->label();
  $node->book['parent_mismatch'] = FALSE; // The normal case.

  if ($node->book['bid'] == $node->nid) {
    $node->book['plid'] = 0;
    $node->book['menu_name'] = book_menu_name($node->nid);
  }
  else {
    // Check in case the parent is not is this book; the book takes precedence.
    if (!empty($node->book['plid'])) {
      $parent = db_query("SELECT * FROM {book} WHERE mlid = :mlid", array(
        ':mlid' => $node->book['plid'],
      ))->fetchAssoc();
    }
    if (empty($node->book['plid']) || !$parent || $parent['bid'] != $node->book['bid']) {
      $node->book['plid'] = db_query("SELECT mlid FROM {book} WHERE nid = :nid", array(
        ':nid' => $node->book['bid'],
      ))->fetchField();
      $node->book['parent_mismatch'] = TRUE; // Likely when JS is disabled.
    }
  }

  $node->book = entity_create('menu_link', $node->book);
  if ($node->book->save()) {
    if ($new) {
      // Insert new.
      db_insert('book')
        ->fields(array(
          'nid' => $node->nid,
          'mlid' => $node->book['mlid'],
          'bid' => $node->book['bid'],
        ))
        ->execute();
      // Reset the cache of stored books.
      drupal_static_reset('book_get_books');
    }
    else {
      if ($node->book['bid'] != db_query("SELECT bid FROM {book} WHERE nid = :nid", array(
          ':nid' => $node->nid,
        ))->fetchField()) {
        // Update the bid for this page and all children.
        book_update_bid($node->book);
        // Reset the cache of stored books.
        drupal_static_reset('book_get_books');
      }
    }

    return TRUE;
  }

  // Failed to save the menu link.
  return FALSE;
}

/**
 * Updates the book ID of a page and its children when it moves to a new book.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 */
function book_update_bid($book_link) {
  $query = db_select('menu_links');
  $query->addField('menu_links', 'mlid');
  for ($i = 1; $i <= MENU_MAX_DEPTH && $book_link["p$i"]; $i++) {
    $query->condition("p$i", $book_link["p$i"]);
  }
  $mlids = $query->execute()->fetchCol();

  if ($mlids) {
    db_update('book')
      ->fields(array('bid' => $book_link['bid']))
      ->condition('mlid', $mlids, 'IN')
      ->execute();
  }
}

/**
 * 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 '';
}

/**
 * Generates the corresponding menu name from a book ID.
 *
 * @param $bid
 *   The book ID for which to make a menu name.
 *
 * @return
 *   The menu name.
 */
function book_menu_name($bid) {
  return 'book-toc-' . $bid;
}

/**
 * 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)) {
      $node->content['book_navigation'] = array(
        '#markup' => theme('book_navigation', array('book_link' => $node->book)),
        '#weight' => 100,
      );
    }
  }

  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']) && !user_access('administer nodes')) {
    $node->setNewRevision();
  }
  // Make sure a new node gets a new menu link.
  if (empty($node->nid)) {
    $node->book['mlid'] = NULL;
  }
}

/**
 * Implements hook_node_insert().
 */
function book_node_insert(EntityInterface $node) {
  if (!empty($node->book['bid'])) {
    if ($node->book['bid'] == 'new') {
      // New nodes that are their own book.
      $node->book['bid'] = $node->nid;
    }
    $node->book['nid'] = $node->nid;
    $node->book['menu_name'] = book_menu_name($node->book['bid']);
    _book_update_outline($node);
  }
}

/**
 * Implements hook_node_update().
 */
function book_node_update(EntityInterface $node) {
  if (!empty($node->book['bid'])) {
    if ($node->book['bid'] == 'new') {
      // New nodes that are their own book.
      $node->book['bid'] = $node->nid;
    }
    $node->book['nid'] = $node->nid;
    $node->book['menu_name'] = book_menu_name($node->book['bid']);
    _book_update_outline($node);
  }
}

/**
 * Implements hook_node_predelete().
 */
function book_node_predelete(EntityInterface $node) {
  if (!empty($node->book['bid'])) {
    if ($node->nid == $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->nid);
        $child_node->book['bid'] = $child_node->nid;
        _book_update_outline($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().
 */
function book_node_prepare(EntityInterface $node) {
  // Prepare defaults for the add/edit form.
  if (empty($node->book) && (user_access('add content to books') || user_access('administer book outlines'))) {
    $node->book = array();

    if (empty($node->nid) && isset($_GET['parent']) && is_numeric($_GET['parent'])) {
      // Handle "Add child page" links:
      $parent = book_link_load($_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.
    $node->book += _book_link_defaults(!empty($node->nid) ? $node->nid : 'new');
  }
  else {
    if (isset($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'])) {
    $node->book['parent_depth_limit'] = _book_parent_depth_limit($node->book);
  }
}

/**
 * Finds the depth limit for items in the parent select.
 *
 * @param $book_link
 *   A fully loaded menu link that is part of the book hierarchy.
 *
 * @return
 *   The depth limit for items in the parent select.
 */
function _book_parent_depth_limit($book_link) {
  return MENU_MAX_DEPTH - 1 - (($book_link['mlid'] && $book_link['has_children']) ? entity_get_controller('menu_link')->findChildrenRelativeDepth($book_link) : 0);
}

/**
 * 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,
    );
  }
}

/**
 * Returns an array with default values for a book page's menu link.
 *
 * @param $nid
 *   The ID of the node whose menu link is being created.
 *
 * @return
 *   The default values for the menu link.
 */
function _book_link_defaults($nid) {
  return array('original_bid' => 0, 'menu_name' => '', 'nid' => $nid, 'bid' => 0, 'router_path' => 'node/%', 'plid' => 0, 'mlid' => 0, 'has_children' => 0, 'weight' => 0, 'module' => 'book', 'options' => array());
}

 /**
 * Implements hook_preprocess_HOOK() for block.html.twig.
 */
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) {
  drupal_add_css(drupal_get_path('module', 'book') . '/book.theme.css');
  $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);

    if ($prev = book_prev($book_link)) {
      $prev_href = url($prev['href']);
      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']);
      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']);
      drupal_add_html_head_link(array('rel' => 'next', 'href' => $next_href));
      $variables['next_url'] = $next_href;
      $variables['next_title'] = check_plain($next['title']);
    }
  }

  $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] = '';
    }
  }
}

/**
 * Recursively processes and formats menu items for book_toc().
 *
 * This helper function recursively modifies the table of contents array for
 * each item in the menu tree, ignoring items in the exclude array or at a depth
 * greater than the limit. Truncates titles over thirty characters and appends
 * an indentation string incremented by depth.
 *
 * @param $tree
 *   The data structure of the book's menu tree. Includes hidden links.
 * @param $indent
 *   A string appended to each menu item title. Increments by '--' per depth
 *   level.
 * @param $toc
 *   Reference to the table of contents array. This is modified in place, so the
 *   function does not have a return value.
 * @param $exclude
 *   Optional array of menu link ID values. Any link whose menu link ID is in
 *   this array will be excluded (along with its children).
 * @param $depth_limit
 *   Any link deeper than this value will be excluded (along with its children).
 */
function _book_toc_recurse($tree, $indent, &$toc, $exclude, $depth_limit) {
  foreach ($tree as $data) {
    if ($data['link']['depth'] > $depth_limit) {
      // Don't iterate through any links on this level.
      break;
    }

    if (!in_array($data['link']['mlid'], $exclude)) {
      $toc[$data['link']['mlid']] = $indent . ' ' . truncate_utf8($data['link']['title'], 30, TRUE, TRUE);
      if ($data['below']) {
        _book_toc_recurse($data['below'], $indent . '--', $toc, $exclude, $depth_limit);
      }
    }
  }
}

/**
 * Returns an array of book pages in table of contents order.
 *
 * @param $bid
 *   The ID of the book whose pages are to be listed.
 * @param $depth_limit
 *   Any link deeper than this value will be excluded (along with its children).
 * @param $exclude
 *   (optional) An array of menu link ID values. Any link whose menu link ID is
 *   in this array will be excluded (along with its children). Defaults to an
 *   empty array.
 *
 * @return
 *   An array of (menu link ID, title) pairs for use as options for selecting a
 *   book page.
 */
function book_toc($bid, $depth_limit, $exclude = array()) {
  $tree = menu_tree_all_data(book_menu_name($bid));
  $toc = array();
  _book_toc_recurse($tree, '', $toc, $exclude, $depth_limit);

  return $toc;
}

/**
 * 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_RTL);
  $variables['head'] = drupal_get_html_head();

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

/**
 * Traverses the book tree to build printable or exportable output.
 *
 * During the traversal, the $visit_func() callback is applied to each node and
 * is called recursively for each child of the node (in weight, title order).
 *
 * @param $tree
 *   A subtree of the book menu hierarchy, rooted at the current page.
 * @param $visit_func
 *   A function callback to be called upon visiting a node in the tree.
 *
 * @return
 *   The output generated in visiting each node.
 */
function book_export_traverse($tree, $visit_func) {
  $output = '';

  foreach ($tree as $data) {
    // Note- access checking is already performed when building the tree.
    if ($node = node_load($data['link']['nid'])) {
      $children = '';

      if ($data['below']) {
        $children = book_export_traverse($data['below'], $visit_func);
      }

      if (!empty($visit_func)) {
        $output .= call_user_func($visit_func, $node, $children);
      }
      else {
        // Use the default function.
        $output .= book_node_export($node, $children);
      }
    }
  }

  return $output;
}

/**
 * Generates printer-friendly HTML for a node.
 *
 * @param \Drupal\Core\Entity\EntityInterface $node
 *   The node that will be output.
 * @param $children
 *   (optional) All the rendered child nodes within the current node. Defaults
 *   to an empty string.
 *
 * @return
 *   The HTML generated for the given node.
 *
 * @see book_export_traverse()
 */
function book_node_export(EntityInterface $node, $children = '') {
  $build = node_view($node, 'print');
  unset($build['#theme']);
  // @todo Rendering should happen in the template using render().
  $node->rendered = drupal_render($build);

  return theme('book_node_export_html', array('node' => $node, 'children' => $children));
}

/**
 * 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, 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($type) {
  if (!empty($type->old_type) && $type->old_type != $type->type) {
    $config = 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->old_type, $allowed_types);
    if ($old_key !== FALSE) {
      // Replace the old machine-readable name with the new machine-readable
      // name.
      $allowed_types[$old_key] = $type->type;
      // Ensure that the allowed_types array is sorted consistently.
      // @see book_admin_settings_submit()
      sort($allowed_types);
      $config->set('allowed_types', $allowed_types);
    }

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

/**
 * Gets a book menu link by its menu link ID.
 *
 * Like menu_link_load(), but adds additional data from the {book} table.
 *
 * Do not call when loading a node, since this function may call node_load().
 *
 * @param $mlid
 *   The menu link ID of the menu item.
 *
 * @return
 *   A menu link, with the link translated for rendering and data added from the
 *   {book} table. FALSE if there is an error.
 */
function book_link_load($mlid) {
  return entity_load('menu_link', $mlid);
}

/**
 * Gets the data representing a subtree of the book hierarchy.
 *
 * The root of the subtree will be the link passed as a parameter, so the
 * returned tree will contain this item and all its descendents in the menu
 * tree.
 *
 * @param $link
 *   A fully loaded menu link.
 *
 * @return
 *   An subtree of menu links in an array, in the order they should be rendered.
 */
function book_menu_subtree_data($link) {
  $tree = &drupal_static(__FUNCTION__, array());

  // Generate a cache ID (cid) specific for this $menu_name and $link.
  $cid = 'links:' . $link['menu_name'] . ':subtree-cid:' . $link['mlid'];

  if (!isset($tree[$cid])) {
    $cache = cache('menu')->get($cid);

    if ($cache && isset($cache->data)) {
      // If the cache entry exists, it will just be the cid for the actual data.
      // This avoids duplication of large amounts of data.
      $cache = cache('menu')->get($cache->data);

      if ($cache && isset($cache->data)) {
        $data = $cache->data;
      }
    }

    // If the subtree data was not in the cache, $data will be NULL.
    if (!isset($data)) {
      $query = db_select('menu_links', 'ml', array('fetch' => PDO::FETCH_ASSOC));
      $query->join('menu_router', 'm', 'm.path = ml.router_path');
      $query->join('book', 'b', 'ml.mlid = b.mlid');
      $query->fields('b');
      $query->fields('m', array('load_functions', 'to_arg_functions', 'access_callback', 'access_arguments', 'page_callback', 'page_arguments', 'title', 'title_callback', 'title_arguments', 'type'));
      $query->fields('ml');
      $query->condition('menu_name', $link['menu_name']);
      for ($i = 1; $i <= MENU_MAX_DEPTH && $link["p$i"]; ++$i) {
        $query->condition("p$i", $link["p$i"]);
      }
      for ($i = 1; $i <= MENU_MAX_DEPTH; ++$i) {
        $query->orderBy("p$i");
      }
      $links = array();
      foreach ($query->execute() as $item) {
        $links[] = $item;
      }
      $data['tree'] = menu_tree_data($links, array(), $link['depth']);
      $data['node_links'] = array();
      menu_tree_collect_node_links($data['tree'], $data['node_links']);
      // Compute the real cid for book subtree data.
      $tree_cid = 'links:' . $item['menu_name'] . ':subtree-data:' . hash('sha256', serialize($data));
      // Cache the data, if it is not already in the cache.

      if (!cache('menu')->get($tree_cid)) {
        cache('menu')->set($tree_cid, $data);
      }
      // Cache the cid of the (shared) data using the menu and item-specific cid.
      cache('menu')->set($cid, $tree_cid);
    }
    // Check access for the current user to each item in the tree.
    menu_tree_check_access($data['tree'], $data['node_links']);
    $tree[$cid] = $data['tree'];
  }

  return $tree[$cid];
}

/**
 * Implements hook_library_info().
 */
function book_library_info() {
  $libraries['drupal.book'] = array(
    'title' => 'Book',
    'version' => VERSION,
    'js' => array(
      drupal_get_path('module', 'book') . '/book.js' => array(),
    ),
    'dependencies' => array(
      array('system', 'jquery'),
      array('system', 'drupal'),
      array('system', 'drupal.form'),
    ),
  );

  return $libraries;
}