class ForumSettingsForm

class ForumSettingsForm extends ConfigFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'forum_admin_settings';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'forum.settings',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('forum.settings');
    $options = [
      5,
      10,
      15,
      20,
      25,
      30,
      35,
      40,
      50,
      60,
      80,
      100,
      150,
      200,
      250,
      300,
      350,
      400,
      500,
    ];
    $form['forum_hot_topic'] = [
      '#type' => 'select',
      '#title' => $this->t('Hot topic threshold'),
      '#default_value' => $config->get('topics.hot_threshold'),
      '#options' => array_combine($options, $options),
      '#description' => $this->t('The number of replies a topic must have to be considered "hot".'),
    ];
    $options = [
      10,
      25,
      50,
      75,
      100,
    ];
    $form['forum_per_page'] = [
      '#type' => 'select',
      '#title' => $this->t('Topics per page'),
      '#default_value' => $config->get('topics.page_limit'),
      '#options' => array_combine($options, $options),
      '#description' => $this->t('Default number of forum topics displayed per page.'),
    ];
    $order = [
      1 => $this->t('Date - newest first'),
      2 => $this->t('Date - oldest first'),
      3 => $this->t('Posts - most active first'),
      4 => $this->t('Posts - least active first'),
    ];
    $form['forum_order'] = [
      '#type' => 'radios',
      '#title' => $this->t('Default order'),
      '#default_value' => $config->get('topics.order'),
      '#options' => $order,
      '#description' => $this->t('Default display order for topics.'),
    ];
    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('forum.settings')
      ->set('topics.hot_threshold', $form_state->getValue('forum_hot_topic'))
      ->set('topics.page_limit', $form_state->getValue('forum_per_page'))
      ->set('topics.order', $form_state->getValue('forum_order'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}