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.
     */
    protected abstract 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',
        ]);
    }

}