class LayoutBuilderWidget

class LayoutBuilderWidget extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element += [
      '#type' => 'layout_builder',
      '#section_storage' => $this->getSectionStorage($form_state),
    ];
    $element['#process'][] = [
      static::class,
      'layoutBuilderElementGetKeys',
    ];
    return $element;
  }
  
  /**
   * Form element #process callback.
   *
   * Save the layout builder element array parents as a property on the top form
   * element so that they can be used to access the element within the whole
   * render array later.
   *
   * @see \Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController
   */
  public static function layoutBuilderElementGetKeys(array $element, FormStateInterface $form_state, &$form) {
    $form['#layout_builder_element_keys'] = $element['#array_parents'];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function extractFormValues(FieldItemListInterface $items, array $form, FormStateInterface $form_state) {
    // @todo This isn't resilient to being set twice, during validation and
    //   save https://www.drupal.org/project/drupal/issues/2833682.
    if (!$form_state->isValidationComplete()) {
      return;
    }
    $items->setValue($this->getSectionStorage($form_state)
      ->getSections());
  }
  
  /**
   * Gets the section storage.
   *
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   *
   * @return \Drupal\layout_builder\SectionStorageInterface
   *   The section storage loaded from the tempstore.
   */
  private function getSectionStorage(FormStateInterface $form_state) {
    return $form_state->getFormObject()
      ->getSectionStorage();
  }

}