DraggableListBuilder::buildForm

public DraggableListBuilder::buildForm(array $form, FormStateInterface $form_state)

public function buildForm(array $form, FormStateInterface $form_state) {
  $form[$this->entitiesKey] = array(
    '#type' => 'table',
    '#header' => $this->buildHeader(),
    '#empty' => t('There is no @label yet.', array('@label' => $this->entityType->getLabel())),
    '#tabledrag' => array(
      array(
        'action' => 'order',
        'relationship' => 'sibling',
        'group' => 'weight',
      ),
    ),
  );

  $this->entities = $this->load();
  $delta = 10;
  // Change the delta of the weight field if have more than 20 entities.
  if (!empty($this->weightKey)) {
    $count = count($this->entities);
    if ($count > 20) {
      $delta = ceil($count / 2);
    }
  }
  foreach ($this->entities as $entity) {
    $row = $this->buildRow($entity);
    if (isset($row['label'])) {
      $row['label'] = array('#markup' => $row['label']);
    }
    if (isset($row['weight'])) {
      $row['weight']['#delta'] = $delta;
    }
    $form[$this->entitiesKey][$entity->id()] = $row;
  }

  $form['actions']['#type'] = 'actions';
  $form['actions']['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
    '#button_type' => 'primary',
  );

  return $form;
}