class RssFeedsForm

class RssFeedsForm extends ConfigFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'system_rss_feeds_settings';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'system.rss',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $rss_config = $this->config('system.rss');
    $form['feed_description'] = [
      '#type' => 'textarea',
      '#title' => t('Feed description'),
      '#default_value' => $rss_config->get('channel.description'),
      '#description' => t('Description of your site, included in each feed.'),
    ];
    $options = [
      1,
      2,
      3,
      4,
      5,
      6,
      7,
      8,
      9,
      10,
      15,
      20,
      25,
      30,
    ];
    $form['feed_default_items'] = [
      '#type' => 'select',
      '#title' => t('Number of items in each feed'),
      '#default_value' => $rss_config->get('items.limit'),
      '#options' => array_combine($options, $options),
      '#description' => t('Default number of items to include in each feed.'),
    ];
    $form['feed_view_mode'] = [
      '#type' => 'select',
      '#title' => t('Feed content'),
      '#default_value' => $rss_config->get('items.view_mode'),
      '#options' => [
        'title' => t('Titles only'),
        'teaser' => t('Titles plus teaser'),
        'fulltext' => t('Full text'),
      ],
      '#description' => t('Global setting for the default display of content items in each feed.'),
    ];
    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('system.rss')
      ->set('channel.description', $form_state->getValue('feed_description'))
      ->set('items.limit', $form_state->getValue('feed_default_items'))
      ->set('items.view_mode', $form_state->getValue('feed_view_mode'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}