class HtmlList

class HtmlList extends StylePluginBase {
    
    /**
     * {@inheritdoc}
     */
    protected $usesRowPlugin = TRUE;
    
    /**
     * Does the style plugin support custom css class for the rows.
     *
     * @var bool
     */
    protected $usesRowClass = TRUE;
    
    /**
     * Set default options.
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['type'] = [
            'default' => 'ul',
        ];
        $options['class'] = [
            'default' => '',
        ];
        $options['wrapper_class'] = [
            'default' => 'item-list',
        ];
        return $options;
    }
    
    /**
     * Render the given style.
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        parent::buildOptionsForm($form, $form_state);
        $form['type'] = [
            '#type' => 'radios',
            '#title' => $this->t('List type'),
            '#options' => [
                'ul' => $this->t('Unordered list'),
                'ol' => $this->t('Ordered list'),
            ],
            '#default_value' => $this->options['type'],
        ];
        $form['wrapper_class'] = [
            '#title' => $this->t('Wrapper class'),
            '#description' => $this->t('The class to provide on the wrapper, outside the list.'),
            '#type' => 'textfield',
            '#size' => '30',
            '#default_value' => $this->options['wrapper_class'],
        ];
        $form['class'] = [
            '#title' => $this->t('List class'),
            '#description' => $this->t('The class to provide on the list element itself.'),
            '#type' => 'textfield',
            '#size' => '30',
            '#default_value' => $this->options['class'],
        ];
    }

}