View source  
  <?php
namespace Drupal\webform\Element;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Element\FormElement;
use Drupal\webform\Plugin\WebformElement\TableSelect;
abstract class WebformExcludedBase extends FormElement {
  
  public function getInfo() {
    $class = get_class($this);
    return [
      '#input' => TRUE,
      '#process' => [
        [
          $class,
          'processWebformExcluded',
        ],
      ],
      '#webform_id' => NULL,
      '#theme_wrappers' => [
        'form_element',
      ],
      '#default_value' => [],
    ];
  }
  
  public static function processWebformExcluded(&$element, FormStateInterface $form_state, &$complete_form) {
    $options = static::getWebformExcludedOptions($element);
    $default_value = array_diff(array_keys($options), array_keys($element['#default_value'] ?: []));
    $element['#tree'] = TRUE;
    
    $element += [
      '#element_validate' => [],
    ];
    array_unshift($element['#element_validate'], [
      get_called_class(),
      'validateWebformExcluded',
    ]);
    $element['tableselect'] = [
      '#type' => 'tableselect',
      '#header' => static::getWebformExcludedHeader(),
      '#options' => $options,
      '#js_select' => TRUE,
      '#empty' => t('No elements are available.'),
      '#default_value' => array_combine($default_value, $default_value),
    ];
    TableSelect::setProcessTableSelectCallback($element['tableselect']);
    if (isset($element['#parents'])) {
      $element['tableselect']['#parents'] = array_merge($element['#parents'], [
        'tableselect',
      ]);
    }
    
    $properties = [
      '#title',
      '#title_display',
      '#description',
      '#description_display',
      '#ajax',
      '#states',
    ];
    $element['tableselect'] += array_intersect_key($element, array_combine($properties, $properties));
    return $element;
  }
  
  public static function validateWebformExcluded(array &$element, FormStateInterface $form_state, &$complete_form) {
    $value = array_filter($element['tableselect']['#value']);
    
    $options = array_keys($element['tableselect']['#options']);
    $excluded = array_diff($options, $value);
    
    $form_state
      ->setValueForElement($element['tableselect'], NULL);
    $value = array_combine($excluded, $excluded);
    $element['#value'] = $value;
    $form_state
      ->setValueForElement($element, $value);
  }
  
  public static function getWebformExcludedHeader() {
    return [];
  }
  
  public static function getWebformExcludedOptions(array $element) {
    return [];
  }
}