class PrerenderList

abstract class PrerenderList extends FieldPluginBase implements MultiItemsFieldHandlerInterface {
    
    /**
     * Stores all items which are used to render the items.
     *
     * It should be keyed first by the id of the base table, for example nid.
     * The second key is the id of the thing which is displayed multiple times
     * per row, for example the tid.
     *
     * @var array
     */
    public $items = [];
    
    /**
     * {@inheritdoc}
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['type'] = [
            'default' => 'separator',
        ];
        $options['separator'] = [
            'default' => ', ',
        ];
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        $form['type'] = [
            '#type' => 'radios',
            '#title' => $this->t('Display type'),
            '#options' => [
                'ul' => $this->t('Unordered list'),
                'ol' => $this->t('Ordered list'),
                'separator' => $this->t('Simple separator'),
            ],
            '#default_value' => $this->options['type'],
        ];
        $form['separator'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Separator'),
            '#default_value' => $this->options['separator'],
            '#states' => [
                'visible' => [
                    ':input[name="options[type]"]' => [
                        'value' => 'separator',
                    ],
                ],
            ],
        ];
        parent::buildOptionsForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function renderItems($items) {
        if (!empty($items)) {
            if ($this->options['type'] == 'separator') {
                $render = [
                    '#type' => 'inline_template',
                    '#template' => '{{ items|safe_join(separator) }}',
                    '#context' => [
                        'items' => $items,
                        'separator' => $this->sanitizeValue($this->options['separator'], 'xss_admin'),
                    ],
                ];
            }
            else {
                $render = [
                    '#theme' => 'item_list',
                    '#items' => $items,
                    '#title' => NULL,
                    '#list_type' => $this->options['type'],
                ];
            }
            return \Drupal::service('renderer')->render($render);
        }
    }
    
    /**
     * {@inheritdoc}
     *
     * Items should be stored in the result array, if possible, as an array
     * with 'value' as the actual displayable value of the item, plus
     * any items that might be found in the 'alter' options array for
     * creating links, such as 'path', 'fragment', 'query' etc, such a thing
     * is to be made. Additionally, items that might be turned into tokens
     * should also be in this array.
     */
    public function getItems(ResultRow $values) {
        $field = $this->getValue($values);
        if (!empty($this->items[$field])) {
            return $this->items[$field];
        }
        return [];
    }

}