class ForumBlockBase

abstract class ForumBlockBase extends BlockBase {
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    $result = $this->buildForumQuery()
      ->execute();
    $elements = [];
    if ($node_title_list = node_title_list($result)) {
      $elements['forum_list'] = $node_title_list;
      $elements['forum_more'] = [
        '#type' => 'more_link',
        '#url' => Url::fromRoute('forum.index'),
        '#attributes' => [
          '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 [
      'properties' => [
        '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'] = [
      '#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 Cache::mergeContexts(parent::getCacheContexts(), [
      'user.node_grants:view',
    ]);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    return Cache::mergeTags(parent::getCacheTags(), [
      'node_list',
    ]);
  }

}