Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/forum/src/Plugin/Block/ActiveTopicsBlock.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\forum\Plugin\Block\ActiveTopicsBlock.
+ */
+
+namespace Drupal\forum\Plugin\Block;
+
+/**
+ * Provides an 'Active forum topics' block.
+ *
+ * @Block(
+ *   id = "forum_active_block",
+ *   admin_label = @Translation("Active forum topics"),
+ *   category = @Translation("Lists (Views)")
+ * )
+ */
+class ActiveTopicsBlock extends ForumBlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildForumQuery() {
+    return db_select('forum_index', 'f')
+      ->fields('f')
+      ->addTag('node_access')
+      ->addMetaData('base_table', 'forum_index')
+      ->orderBy('f.last_comment_timestamp', 'DESC')
+      ->range(0, $this->configuration['block_count']);
+  }
+
+}

core/modules/forum/src/Plugin/Block/ForumBlockBase.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\forum\Plugin\Block\ForumBlockBase.
+ */
+
+namespace Drupal\forum\Plugin\Block;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Url;
+
+/**
+ * Provides a base class for Forum blocks.
+ */
+abstract class ForumBlockBase extends BlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $result = $this->buildForumQuery()->execute();
+    $elements = array();
+    if ($node_title_list = node_title_list($result)) {
+      $elements['forum_list'] = $node_title_list;
+      $elements['forum_more'] = array(
+        '#type' => 'more_link',
+        '#url' => Url::fromRoute('forum.index'),
+        '#attributes' => array('title' => $this->t('Read the latest forum topics.')),
+      );
+    }
+    return $elements;
+  }
+
+  /**
+   * Builds the select query to use for this forum block.
+   *
+   * @return \Drupal\Core\Database\Query\Select
+   *   A Select object.
+   */
+  abstract protected function buildForumQuery();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return array(
+      'properties' => array(
+        'administrative' => TRUE,
+      ),
+      'block_count' => 5,
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    return AccessResult::allowedIfHasPermission($account, 'access content');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockForm($form, FormStateInterface $form_state) {
+    $range = range(2, 20);
+    $form['block_count'] = array(
+      '#type' => 'select',
+      '#title' => $this->t('Number of topics'),
+      '#default_value' => $this->configuration['block_count'],
+      '#options' => array_combine($range, $range),
+    );
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockSubmit($form, FormStateInterface $form_state) {
+    $this->configuration['block_count'] = $form_state->getValue('block_count');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheContexts() {
+    return ['user.node_grants:view'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheTags() {
+    return ['node_list'];
+  }
+
+}

core/modules/forum/src/Plugin/Block/NewTopicsBlock.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\forum\Plugin\Block\NewTopicsBlock.
+ */
+
+namespace Drupal\forum\Plugin\Block;
+
+/**
+ * Provides a 'New forum topics' block.
+ *
+ * @Block(
+ *   id = "forum_new_block",
+ *   admin_label = @Translation("New forum topics"),
+ *   category = @Translation("Lists (Views)")
+ * )
+ */
+class NewTopicsBlock extends ForumBlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildForumQuery() {
+    return db_select('forum_index', 'f')
+      ->fields('f')
+      ->addTag('node_access')
+      ->addMetaData('base_table', 'forum_index')
+      ->orderBy('f.created', 'DESC')
+      ->range(0, $this->configuration['block_count']);
+  }
+}

core/modules/forum/src/Plugin/Validation/Constraint/ForumLeafConstraint.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\forum\Plugin\Validation\Constraint\ForumLeafConstraint.
+ */
+
+namespace Drupal\forum\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Checks that the node is assigned only a "leaf" term in the forum taxonomy.
+ *
+ * @Constraint(
+ *   id = "ForumLeaf",
+ *   label = @Translation("Forum leaf", context = "Validation"),
+ * )
+ */
+class ForumLeafConstraint extends Constraint {
+
+  public $selectForum = 'Select a forum.';
+  public $noLeafMessage = 'The item %forum is a forum container, not a forum. Select one of the forums below instead.';
+}

core/modules/forum/src/Plugin/Validation/Constraint/ForumLeafConstraintValidator.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\forum\Plugin\Validation\Constraint\ForumLeafConstraintValidator.
+ */
+
+namespace Drupal\forum\Plugin\Validation\Constraint;
+
+use Drupal\Component\Utility\Unicode;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the ForumLeaf constraint.
+ */
+class ForumLeafConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($items, Constraint $constraint) {
+    $item = $items->first();
+    if (!isset($item)) {
+      return NULL;
+    }
+
+    // Verify that a term has been selected.
+    if (!$item->entity) {
+      $this->context->addViolation($constraint->selectForum);
+    }
+
+    // The forum_container flag must not be set.
+    if (!empty($item->entity->forum_container->value)) {
+      $this->context->addViolation($constraint->noLeafMessage, array('%forum' => $item->entity->getName()));
+    }
+  }
+
+}