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 [];
  }

}