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 -');
            }
        }
    }

}