Draggable Views custom field to save weight (index)

Here is the solution with the custom code. In the example the draggable view name is the 'packages_order' and the display is the 'page_1'.

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // The name and display of the draggable view.
  $view_name = 'packages_order';
  $display_name = 'page_1';
  // The id of the draggableviews form (that contains the 'Save order' button).
  $view_form_id = 'views_form_' . $view_name . '_' . $display_name;

  if ($view_form_id != $form_id) {
    return;
  }
  // Add the custom submit hanler (fired when a user click on the 'Save order').
  $form['actions']['save_order']['#submit'][] = 'my_module_draggableviews_views_submit';
}

/**
 * Custom form submit handler.
 */
function my_module_draggableviews_views_submit(&$form, FormStateInterface $form_state) {
  $input = $form_state->getUserInput();

  foreach ($input['draggableviews'] as $item) {

    $nid = $item['id'];
    $weight = $item['weight'];
    $node = \Drupal::entityTypeManager()->getStorage('node')->load($nid);

    if ($node) {
      $node->set('field_index', $weight);
      $node->save();
    }
  }
}