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();
    }

}