modules/forum/forum.module

<?php

/**
 * @file
 * Provides discussion forums.
 */

use Drupal\comment\CommentInterface;
use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Link;
use Drupal\Core\Url;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\taxonomy\VocabularyInterface;
use Drupal\user\Entity\User;

/**
 * Implements hook_help().
 */
function forum_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.forum':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Forum module lets you create threaded discussion forums with functionality similar to other message board systems. In a forum, users post topics and threads in nested hierarchies, allowing discussions to be categorized and grouped.') . '</p>';
      $output .= '<p>' . t('The Forum module adds and uses a content type called <em>Forum topic</em>. For background information on content types, see the <a href=":node_help">Node module help page</a>.', [':node_help' => Url::fromRoute('help.page', ['name' => 'node'])->toString()]) . '</p>';
      $output .= '<p>' . t('A forum is represented by a hierarchical structure, consisting of:');
      $output .= '<ul>';
      $output .= '<li>' . t('<em>Forums</em> (for example, <em>Recipes for cooking vegetables</em>)') . '</li>';
      $output .= '<li>' . t('<em>Forum topics</em> submitted by users (for example, <em>How to cook potatoes</em>), which start discussions.') . '</li>';
      $output .= '<li>' . t('Threaded <em>comments</em> submitted by users (for example, <em>You wash the potatoes first and then...</em>).') . '</li>';
      $output .= '<li>' . t('Optional <em>containers</em>, used to group similar forums. Forums can be placed inside containers, and vice versa.') . '</li>';
      $output .= '</ul>';
      $output .= '</p>';
      $output .= '<p>' . t('For more information, see the <a href=":forum">online documentation for the Forum module</a>.', [':forum' => 'https://www.drupal.org/documentation/modules/forum']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Setting up the forum structure') . '</dt>';
      $output .= '<dd>' . t('Visit the <a href=":forums">Forums page</a> to set up containers and forums to hold your discussion topics.', [':forums' => Url::fromRoute('forum.overview')->toString()]) . '</dd>';
      $output .= '<dt>' . t('Starting a discussion') . '</dt>';
      $output .= '<dd>' . t('The <a href=":create-topic">Forum topic</a> link on the <a href=":content-add">Add content</a> page creates the first post of a new threaded discussion, or thread.', [':create-topic' => Url::fromRoute('node.add', ['node_type' => 'forum'])->toString(), ':content-add' => Url::fromRoute('node.add_page')->toString()]) . '</dd>';
      $output .= '<dt>' . t('Navigating in the forum') . '</dt>';
      $output .= '<dd>' . t('Enabling the Forum module provides a default <em>Forums</em> menu item in the Tools menu that links to the <a href=":forums">Forums page</a>.', [':forums' => Url::fromRoute('forum.index')->toString()]) . '</dd>';
      $output .= '<dt>' . t('Moving forum topics') . '</dt>';
      $output .= '<dd>' . t('A forum topic (and all of its comments) may be moved between forums by selecting a different forum while editing a forum topic. When moving a forum topic between forums, the <em>Leave shadow copy</em> option creates a link in the original forum pointing to the new location.') . '</dd>';
      $output .= '<dt>' . t('Locking and disabling comments') . '</dt>';
      $output .= '<dd>' . t('Selecting <em>Closed</em> under <em>Comment settings</em> while editing a forum topic will lock (prevent new comments on) the thread. Selecting <em>Hidden</em> under <em>Comment settings</em> while editing a forum topic will hide all existing comments on the thread, and prevent new ones.') . '</dd>';
      $output .= '</dl>';
      return $output;

    case 'forum.overview':
      $output = '<p>' . t('Forums contain forum topics. Use containers to group related forums.') . '</p>';
      $more_help_link = [
        '#type' => 'link',
        '#url' => Url::fromRoute('help.page', ['name' => 'forum']),
        '#title' => t('More help'),
        '#attributes' => [
          'class' => ['icon-help'],
        ],
      ];
      $container = [
        '#theme' => 'container',
        '#children' => $more_help_link,
        '#attributes' => [
          'class' => ['more-link'],
        ],
      ];
      $output .= \Drupal::service('renderer')->renderPlain($container);
      return $output;

    case 'forum.add_container':
      return '<p>' . t('Use containers to group related forums.') . '</p>';

    case 'forum.add_forum':
      return '<p>' . t('A forum holds related forum topics.') . '</p>';

    case 'forum.settings':
      return '<p>' . t('Adjust the display of your forum topics. Organize the forums on the <a href=":forum-structure">forum structure page</a>.', [':forum-structure' => Url::fromRoute('forum.overview')->toString()]) . '</p>';
  }
}

/**
 * Implements hook_theme().
 */
function forum_theme() {
  return [
    'forums' => [
      'variables' => ['forums' => [], 'topics' => [], 'topics_pager' => [], 'parents' => NULL, 'term' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL, 'header' => []],
    ],
    'forum_list' => [
      'variables' => ['forums' => NULL, 'parents' => NULL, 'tid' => NULL],
    ],
    'forum_icon' => [
      'variables' => ['new_posts' => NULL, 'num_posts' => 0, 'comment_mode' => 0, 'sticky' => 0, 'first_new' => FALSE],
    ],
    'forum_submitted' => [
      'variables' => ['topic' => NULL],
    ],
  ];
}

/**
 * Implements hook_entity_type_build().
 */
function forum_entity_type_build(array &$entity_types) {
  /** @var $entity_types \Drupal\Core\Entity\EntityTypeInterface[] */
  // Register forum specific forms.
  $entity_types['taxonomy_term']
    ->setFormClass('forum', 'Drupal\forum\Form\ForumForm')
    ->setFormClass('container', 'Drupal\forum\Form\ContainerForm')
    ->setLinkTemplate('forum-edit-container-form', '/admin/structure/forum/edit/container/{taxonomy_term}')
    ->setLinkTemplate('forum-delete-form', '/admin/structure/forum/delete/forum/{taxonomy_term}')
    ->setLinkTemplate('forum-edit-form', '/admin/structure/forum/edit/forum/{taxonomy_term}');
}

/**
 * Implements hook_entity_bundle_info_alter().
 */
function forum_entity_bundle_info_alter(&$bundles) {
  // Take over URI construction for taxonomy terms that are forums.
  if ($vid = \Drupal::config('forum.settings')->get('vocabulary')) {
    if (isset($bundles['taxonomy_term'][$vid])) {
      $bundles['taxonomy_term'][$vid]['uri_callback'] = 'forum_uri';
    }
  }
}

/**
 * Entity URI callback used in forum_entity_bundle_info_alter().
 */
function forum_uri($forum) {
  return Url::fromRoute('forum.page', ['taxonomy_term' => $forum->id()]);
}

/**
 * Implements hook_entity_bundle_field_info_alter().
 */
function forum_entity_bundle_field_info_alter(&$fields, EntityTypeInterface $entity_type, $bundle) {
  if ($entity_type->id() == 'node'  && !empty($fields['taxonomy_forums'])) {
    $fields['taxonomy_forums']->addConstraint('ForumLeaf', []);
  }
}

/**
 * Implements hook_ENTITY_TYPE_presave() for node entities.
 *
 * Assigns the forum taxonomy when adding a topic from within a forum.
 */
function forum_node_presave(EntityInterface $node) {
  if (\Drupal::service('forum_manager')->checkNodeType($node)) {
    // Make sure all fields are set properly:
    $node->icon = !empty($node->icon) ? $node->icon : '';
    if (!$node->taxonomy_forums->isEmpty()) {
      $node->forum_tid = $node->taxonomy_forums->target_id;
      // Only do a shadow copy check if this is not a new node.
      if (!$node->isNew()) {
        $old_tid = \Drupal::service('forum.index_storage')->getOriginalTermId($node);
        if ($old_tid && isset($node->forum_tid) && ($node->forum_tid != $old_tid) && !empty($node->shadow)) {
          // A shadow copy needs to be created. Retain new term and add old term.
          $node->taxonomy_forums[count($node->taxonomy_forums)] = ['target_id' => $old_tid];
        }
      }
    }
  }
}

/**
 * Implements hook_ENTITY_TYPE_update() for node entities.
 */
function forum_node_update(EntityInterface $node) {
  if (\Drupal::service('forum_manager')->checkNodeType($node)) {
    // If this is not a new revision and does exist, update the forum record,
    // otherwise insert a new one.
    /** @var \Drupal\forum\ForumIndexStorageInterface $forum_index_storage */
    $forum_index_storage = \Drupal::service('forum.index_storage');
    if ($node->getRevisionId() == $node->original->getRevisionId() && $forum_index_storage->getOriginalTermId($node)) {
      if (!empty($node->forum_tid)) {
        $forum_index_storage->update($node);
      }
      // The node is removed from the forum.
      else {
        $forum_index_storage->delete($node);
      }
    }
    else {
      if (!empty($node->forum_tid)) {
        $forum_index_storage->create($node);
      }
    }
    // If the node has a shadow forum topic, update the record for this
    // revision.
    if (!empty($node->shadow)) {
      $forum_index_storage->deleteRevision($node);
      $forum_index_storage->create($node);
    }

    // If the node is published, update the forum index.
    if ($node->isPublished()) {
      $forum_index_storage->deleteIndex($node);
      $forum_index_storage->createIndex($node);
    }
    // When a forum node is unpublished, remove it from the forum_index table.
    else {
      $forum_index_storage->deleteIndex($node);
    }
  }
}

/**
 * Implements hook_ENTITY_TYPE_insert() for node entities.
 */
function forum_node_insert(EntityInterface $node) {
  if (\Drupal::service('forum_manager')->checkNodeType($node)) {
    /** @var \Drupal\forum\ForumIndexStorageInterface $forum_index_storage */
    $forum_index_storage = \Drupal::service('forum.index_storage');
    if (!empty($node->forum_tid)) {
      $forum_index_storage->create($node);
    }

    // If the node is published, update the forum index.
    if ($node->isPublished()) {
      $forum_index_storage->createIndex($node);
    }
  }
}

/**
 * Implements hook_ENTITY_TYPE_predelete() for node entities.
 */
function forum_node_predelete(EntityInterface $node) {
  if (\Drupal::service('forum_manager')->checkNodeType($node)) {
    /** @var \Drupal\forum\ForumIndexStorageInterface $forum_index_storage */
    $forum_index_storage = \Drupal::service('forum.index_storage');
    $forum_index_storage->delete($node);
    $forum_index_storage->deleteIndex($node);
  }
}

/**
 * Implements hook_ENTITY_TYPE_storage_load() for node entities.
 */
function forum_node_storage_load($nodes) {
  $node_vids = [];
  foreach ($nodes as $node) {
    if (\Drupal::service('forum_manager')->checkNodeType($node)) {
      $node_vids[] = $node->getRevisionId();
    }
  }
  if (!empty($node_vids)) {
    $result = \Drupal::service('forum.index_storage')->read($node_vids);
    foreach ($result as $record) {
      $nodes[$record->nid]->forum_tid = $record->tid;
    }
  }
}

/**
 * Implements hook_ENTITY_TYPE_update() for comment entities.
 */
function forum_comment_update(CommentInterface $comment) {
  if ($comment->getCommentedEntityTypeId() == 'node') {
    \Drupal::service('forum.index_storage')->updateIndex($comment->getCommentedEntity());
  }
}

/**
 * Implements hook_ENTITY_TYPE_insert() for comment entities.
 */
function forum_comment_insert(CommentInterface $comment) {
  if ($comment->getCommentedEntityTypeId() == 'node') {
    \Drupal::service('forum.index_storage')->updateIndex($comment->getCommentedEntity());
  }
}

/**
 * Implements hook_ENTITY_TYPE_delete() for comment entities.
 */
function forum_comment_delete(CommentInterface $comment) {
  if ($comment->getCommentedEntityTypeId() == 'node') {
    \Drupal::service('forum.index_storage')->updateIndex($comment->getCommentedEntity());
  }
}

/**
 * Implements hook_form_BASE_FORM_ID_alter() for \Drupal\taxonomy\VocabularyForm.
 */
function forum_form_taxonomy_vocabulary_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $vid = \Drupal::config('forum.settings')->get('vocabulary');
  $vocabulary = $form_state->getFormObject()->getEntity();
  if ($vid == $vocabulary->id()) {
    $form['help_forum_vocab'] = [
      '#markup' => t('This is the designated forum vocabulary. Some of the normal vocabulary options have been removed.'),
      '#weight' => -1,
    ];
    // Forum's vocabulary always has single hierarchy. Forums and containers
    // have only one parent or no parent for root items. By default this value
    // is 0.
    $form['hierarchy']['#value'] = VocabularyInterface::HIERARCHY_SINGLE;
    // Do not allow to delete forum's vocabulary.
    $form['actions']['delete']['#access'] = FALSE;
    // Do not allow to change a vid of forum's vocabulary.
    $form['vid']['#disabled'] = TRUE;
  }
}

/**
 * Implements hook_form_FORM_ID_alter() for \Drupal\taxonomy\TermForm.
 */
function forum_form_taxonomy_term_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $vid = \Drupal::config('forum.settings')->get('vocabulary');
  if (isset($form['vid']['#value']) && $form['vid']['#value'] == $vid) {
    // Hide multiple parents select from forum terms.
    $form['relations']['parent']['#access'] = FALSE;
  }
}

/**
 * Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
 */
function forum_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $node = $form_state->getFormObject()->getEntity();
  if (isset($node->taxonomy_forums) && !$node->isNew()) {
    $forum_terms = $node->taxonomy_forums;
    // If editing, give option to leave shadows.
    $shadow = (count($forum_terms) > 1);
    $form['shadow'] = [
      '#type' => 'checkbox',
      '#title' => t('Leave shadow copy'),
      '#default_value' => $shadow,
      '#description' => t('If you move this topic, you can leave a link in the old forum to the new forum.'),
    ];
    $form['forum_tid'] = ['#type' => 'value', '#value' => $node->forum_tid];
  }

  if (isset($form['taxonomy_forums'])) {
    $widget =& $form['taxonomy_forums']['widget'];
    $widget['#multiple'] = FALSE;
    if (empty($widget['#default_value'])) {
      // If there is no default forum already selected, try to get the forum
      // ID from the URL (e.g., if we are on a page like node/add/forum/2, we
      // expect "2" to be the ID of the forum that was requested).
      $requested_forum_id = \Drupal::request()->query->get('forum_id');
      $widget['#default_value'] = is_numeric($requested_forum_id) ? $requested_forum_id : '';
    }
  }
}

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

/**
 * Implements hook_theme_suggestions_HOOK().
 */
function forum_theme_suggestions_forums(array $variables) {
  $suggestions = [];
  $tid = $variables['term']->id();

  // Provide separate template suggestions based on what's being output. Topic
  // ID is also accounted for. Check both variables to be safe then the inverse.
  // Forums with topic IDs take precedence.
  if ($variables['forums'] && !$variables['topics']) {
    $suggestions[] = 'forums__containers';
    $suggestions[] = 'forums__' . $tid;
    $suggestions[] = 'forums__containers__' . $tid;
  }
  elseif (!$variables['forums'] && $variables['topics']) {
    $suggestions[] = 'forums__topics';
    $suggestions[] = 'forums__' . $tid;
    $suggestions[] = 'forums__topics__' . $tid;
  }
  else {
    $suggestions[] = 'forums__' . $tid;
  }

  return $suggestions;
}

/**
 * Prepares variables for forums templates.
 *
 * Default template: forums.html.twig.
 *
 * @param array $variables
 *   An array containing the following elements:
 *   - forums: An array of all forum objects to display for the given taxonomy
 *     term ID. If tid = 0 then all the top-level forums are displayed.
 *   - topics: An array of all the topics in the current forum.
 *   - parents: An array of taxonomy term objects that are ancestors of the
 *     current term ID.
 *   - term: Taxonomy term of the current forum.
 *   - sortby: One of the following integers indicating the sort criteria:
 *     - 1: Date - newest first.
 *     - 2: Date - oldest first.
 *     - 3: Posts with the most comments first.
 *     - 4: Posts with the least comments first.
 *   - forum_per_page: The maximum number of topics to display per page.
 */
function template_preprocess_forums(&$variables) {
  $variables['tid'] = $variables['term']->id();
  if ($variables['forums_defined'] = count($variables['forums']) || count($variables['parents'])) {
    if (!empty($variables['forums'])) {
      $variables['forums'] = [
        '#theme' => 'forum_list',
        '#forums' => $variables['forums'],
        '#parents' => $variables['parents'],
        '#tid' => $variables['tid'],
      ];
    }

    if ($variables['term'] && empty($variables['term']->forum_container->value) && !empty($variables['topics'])) {
      $forum_topic_list_header = $variables['header'];

      $table = [
        '#theme' => 'table__forum_topic_list',
        '#responsive' => FALSE,
        '#attributes' => ['id' => 'forum-topic-' . $variables['tid']],
        '#header' => [],
        '#rows' => [],
      ];

      if (!empty($forum_topic_list_header)) {
        $table['#header'] = $forum_topic_list_header;
      }

      /** @var \Drupal\node\NodeInterface $topic */
      foreach ($variables['topics'] as $id => $topic) {
        $variables['topics'][$id]->icon = [
          '#theme' => 'forum_icon',
          '#new_posts' => $topic->new,
          '#num_posts' => $topic->comment_count,
          '#comment_mode' => $topic->comment_mode,
          '#sticky' => $topic->isSticky(),
          '#first_new' => $topic->first_new,
        ];

        // We keep the actual tid in forum table, if it's different from the
        // current tid then it means the topic appears in two forums, one of
        // them is a shadow copy.
        if ($variables['tid'] != $topic->forum_tid) {
          $variables['topics'][$id]->moved = TRUE;
          $variables['topics'][$id]->title = $topic->getTitle();
          $variables['topics'][$id]->message = Link::fromTextAndUrl(t('This topic has been moved'), Url::fromRoute('forum.page', ['taxonomy_term' => $topic->forum_tid]))->toString();
        }
        else {
          $variables['topics'][$id]->moved = FALSE;
          $variables['topics'][$id]->title_link = Link::fromTextAndUrl($topic->getTitle(), $topic->toUrl())->toString();
          $variables['topics'][$id]->message = '';
        }
        $forum_submitted = [
          '#theme' => 'forum_submitted',
          '#topic' => (object) [
            'uid' => $topic->getOwnerId(),
            'name' => $topic->getOwner()->getDisplayName(),
            'created' => $topic->getCreatedTime(),
          ],
        ];
        $variables['topics'][$id]->submitted = \Drupal::service('renderer')->render($forum_submitted);
        $forum_submitted = [
          '#theme' => 'forum_submitted',
          '#topic' => isset($topic->last_reply) ? $topic->last_reply : NULL,
        ];
        $variables['topics'][$id]->last_reply = \Drupal::service('renderer')->render($forum_submitted);

        $variables['topics'][$id]->new_text = '';
        $variables['topics'][$id]->new_url = '';

        if ($topic->new_replies) {
          $page_number = \Drupal::entityTypeManager()->getStorage('comment')
            ->getNewCommentPageNumber($topic->comment_count, $topic->new_replies, $topic, 'comment_forum');
          $query = $page_number ? ['page' => $page_number] : NULL;
          $variables['topics'][$id]->new_text = \Drupal::translation()->formatPlural($topic->new_replies, '1 new post<span class="visually-hidden"> in topic %title</span>', '@count new posts<span class="visually-hidden"> in topic %title</span>', ['%title' => $variables['topics'][$id]->label()]);
          $variables['topics'][$id]->new_url = Url::fromRoute('entity.node.canonical', ['node' => $topic->id()], ['query' => $query, 'fragment' => 'new'])->toString();
        }

        // Build table rows from topics.
        $row = [];
        $row[] = [
          'data' => [
            $topic->icon,
            [
              '#markup' => '<div class="forum__title"><div>' . $topic->title_link . '</div><div>' . $topic->submitted . '</div></div>',
            ],
          ],
          'class' => ['forum__topic'],
        ];

        if ($topic->moved) {
          $row[] = [
            'data' => $topic->message,
            'colspan' => '2',
          ];
        }
        else {
          $new_replies = '';
          if ($topic->new_replies) {
            $new_replies = '<br /><a href="' . $topic->new_url . '">' . $topic->new_text . '</a>';
          }

          $row[] = [
            'data' => [
              [
                '#prefix' => $topic->comment_count,
                '#markup' => $new_replies,
              ],
            ],
            'class' => ['forum__replies'],
          ];
          $row[] = [
            'data' => $topic->last_reply,
            'class' => ['forum__last-reply'],
          ];
        }
        $table['#rows'][] = $row;
      }

      $variables['topics_original'] = $variables['topics'];
      $variables['topics'] = $table;
      $variables['topics_pager'] = [
        '#type' => 'pager',
      ];
    }
  }
}

/**
 * Prepares variables for forum list templates.
 *
 * Default template: forum-list.html.twig.
 *
 * @param array $variables
 *   An array containing the following elements:
 *   - forums: An array of all forum objects to display for the given taxonomy
 *     term ID. If tid = 0 then all the top-level forums are displayed.
 *   - parents: An array of taxonomy term objects that are ancestors of the
 *     current term ID.
 *   - tid: Taxonomy term ID of the current forum.
 */
function template_preprocess_forum_list(&$variables) {
  $user = \Drupal::currentUser();
  $row = 0;
  // Sanitize each forum so that the template can safely print the data.
  foreach ($variables['forums'] as $id => $forum) {
    $variables['forums'][$id]->description = ['#markup' => $forum->description->value];
    $variables['forums'][$id]->link = forum_uri($forum);
    $variables['forums'][$id]->name = $forum->label();
    $variables['forums'][$id]->is_container = !empty($forum->forum_container->value);
    $variables['forums'][$id]->zebra = $row % 2 == 0 ? 'odd' : 'even';
    $row++;

    $variables['forums'][$id]->new_text = '';
    $variables['forums'][$id]->new_url = '';
    $variables['forums'][$id]->new_topics = 0;
    $variables['forums'][$id]->old_topics = $forum->num_topics;
    $variables['forums'][$id]->icon_class = 'default';
    $variables['forums'][$id]->icon_title = t('No new posts');
    if ($user->isAuthenticated()) {
      $variables['forums'][$id]->new_topics = \Drupal::service('forum_manager')->unreadTopics($forum->id(), $user->id());
      if ($variables['forums'][$id]->new_topics) {
        $variables['forums'][$id]->new_text = \Drupal::translation()->formatPlural($variables['forums'][$id]->new_topics, '1 new post<span class="visually-hidden"> in forum %title</span>', '@count new posts<span class="visually-hidden"> in forum %title</span>', ['%title' => $variables['forums'][$id]->label()]);
        $variables['forums'][$id]->new_url = Url::fromRoute('forum.page', ['taxonomy_term' => $forum->id()], ['fragment' => 'new'])->toString();
        $variables['forums'][$id]->icon_class = 'new';
        $variables['forums'][$id]->icon_title = t('New posts');
      }
      $variables['forums'][$id]->old_topics = $forum->num_topics - $variables['forums'][$id]->new_topics;
    }
    $forum_submitted = ['#theme' => 'forum_submitted', '#topic' => $forum->last_post];
    $variables['forums'][$id]->last_reply = \Drupal::service('renderer')->render($forum_submitted);
  }

  $variables['pager'] = [
   '#type' => 'pager',
  ];

  // Give meaning to $tid for themers. $tid actually stands for term ID.
  $variables['forum_id'] = $variables['tid'];
  unset($variables['tid']);
}

/**
 * Prepares variables for forum icon templates.
 *
 * Default template: forum-icon.html.twig.
 *
 * @param array $variables
 *   An array containing the following elements:
 *   - new_posts: Indicates whether or not the topic contains new posts.
 *   - num_posts: The total number of posts in all topics.
 *   - comment_mode: An integer indicating whether comments are open, closed,
 *     or hidden.
 *   - sticky: Indicates whether the topic is sticky.
 *   - first_new: Indicates whether this is the first topic with new posts.
 */
function template_preprocess_forum_icon(&$variables) {
  $variables['hot_threshold'] = \Drupal::config('forum.settings')->get('topics.hot_threshold');

  if ($variables['num_posts'] > $variables['hot_threshold']) {
    $variables['icon_status'] = $variables['new_posts'] ? 'hot-new' : 'hot';
    $variables['icon_title'] = $variables['new_posts'] ? t('Hot topic, new comments') : t('Hot topic');
  }
  else {
    $variables['icon_status'] = $variables['new_posts'] ? 'new' : 'default';
    $variables['icon_title'] = $variables['new_posts'] ? t('New comments') : t('Normal topic');
  }

  if ($variables['comment_mode'] == CommentItemInterface::CLOSED || $variables['comment_mode'] == CommentItemInterface::HIDDEN) {
    $variables['icon_status'] = 'closed';
    $variables['icon_title'] = t('Closed topic');
  }

  if ($variables['sticky'] == 1) {
    $variables['icon_status'] = 'sticky';
    $variables['icon_title'] = t('Sticky topic');
  }

  $variables['attributes']['title'] = $variables['icon_title'];
}

/**
 * Prepares variables for forum submission information templates.
 *
 * The submission information will be displayed in the forum list and topic
 * list.
 *
 * Default template: forum-submitted.html.twig.
 *
 * @param array $variables
 *   An array containing the following elements:
 *   - topic: The topic object.
 */
function template_preprocess_forum_submitted(&$variables) {
  $variables['author'] = '';
  if (isset($variables['topic']->uid)) {
    $username = ['#theme' => 'username', '#account' => User::load($variables['topic']->uid)];
    $variables['author'] = \Drupal::service('renderer')->render($username);
  }
  $variables['time'] = isset($variables['topic']->created) ? \Drupal::service('date.formatter')->formatTimeDiffSince($variables['topic']->created) : '';
}