class OptionsSelectWidget

class OptionsSelectWidget extends OptionsWidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);
    $element += [
      '#type' => 'select',
      '#options' => $this->getOptions($items->getEntity()),
      '#default_value' => $this->getSelectedOptions($items),
      // Do not display a 'multiple' select box if there is only one option.
'#multiple' => $this->multiple && count($this->options) > 1,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function sanitizeLabel(&$label) {
    // Select form inputs allow unencoded HTML entities, but no HTML tags.
    $label = Html::decodeEntities(strip_tags($label));
  }
  
  /**
   * {@inheritdoc}
   */
  protected function supportsGroups() {
    return TRUE;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEmptyLabel() {
    if ($this->multiple) {
      // Multiple select: add a 'none' option for non-required fields.
      if (!$this->required) {
        return $this->t('- None -');
      }
    }
    else {
      // Single select: add a 'none' option for non-required fields,
      // and a 'select a value' option for required fields that do not come
      // with a value selected.
      if (!$this->required) {
        return $this->t('- None -');
      }
      if (!$this->has_value) {
        return $this->t('- Select a value -');
      }
    }
  }

}