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) {
        $form['feed_view_mode'] = [
            '#type' => 'select',
            '#title' => $this->t('Feed content'),
            '#default_value' => $this->config('system.rss')
                ->get('items.view_mode'),
            '#options' => [
                'title' => $this->t('Titles only'),
                'teaser' => $this->t('Titles plus teaser'),
                'fulltext' => $this->t('Full text'),
            ],
            '#description' => $this->t('Global setting for the default display of content items in each feed.'),
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('system.rss')
            ->set('items.view_mode', $form_state->getValue('feed_view_mode'))
            ->save();
        parent::submitForm($form, $form_state);
    }

}