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);
    }

}