class DraggableListBuilder

abstract class DraggableListBuilder extends ConfigEntityListBuilder implements FormInterface {
  
  /**
   * The key to use for the form element containing the entities.
   *
   * @var string
   */
  protected $entitiesKey = 'entities';
  
  /**
   * The entities being listed.
   *
   * @var \Drupal\Core\Entity\EntityInterface[]
   */
  protected $entities = [];
  
  /**
   * Name of the entity's weight field or FALSE if no field is provided.
   *
   * @var string|bool
   */
  protected $weightKey = FALSE;
  
  /**
   * The form builder.
   *
   * @var \Drupal\Core\Form\FormBuilderInterface
   */
  protected $formBuilder;
  
  /**
   * {@inheritdoc}
   */
  public function __construct(EntityTypeInterface $entity_type, EntityStorageInterface $storage) {
    parent::__construct($entity_type, $storage);
    // Check if the entity type supports weighting.
    if ($this->entityType
      ->hasKey('weight')) {
      $this->weightKey = $this->entityType
        ->getKey('weight');
    }
    $this->limit = FALSE;
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildHeader() {
    $header = [];
    if (!empty($this->weightKey)) {
      $header['weight'] = t('Weight');
    }
    return $header + parent::buildHeader();
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildRow(EntityInterface $entity) {
    $row = [];
    if (!empty($this->weightKey)) {
      // Override default values to markup elements.
      $row['#attributes']['class'][] = 'draggable';
      $row['#weight'] = $entity->get($this->weightKey);
      // Add weight column.
      $row['weight'] = [
        '#type' => 'weight',
        '#title' => t('Weight for @title', [
          '@title' => $entity->label(),
        ]),
        '#title_display' => 'invisible',
        '#default_value' => $entity->get($this->weightKey),
        '#attributes' => [
          'class' => [
            'weight',
          ],
        ],
      ];
    }
    return $row + parent::buildRow($entity);
  }
  
  /**
   * {@inheritdoc}
   */
  public function render() {
    if (!empty($this->weightKey)) {
      return $this->formBuilder()
        ->getForm($this);
    }
    return parent::render();
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form[$this->entitiesKey] = [
      '#type' => 'table',
      '#header' => $this->buildHeader(),
      '#empty' => t('There are no @label yet.', [
        '@label' => $this->entityType
          ->getPluralLabel(),
      ]),
      '#tabledrag' => [
        [
          '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'] = [
          '#plain_text' => $row['label'],
        ];
      }
      if (isset($row['weight'])) {
        $row['weight']['#delta'] = $delta;
      }
      $form[$this->entitiesKey][$entity->id()] = $row;
    }
    $form['actions']['#type'] = 'actions';
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => t('Save'),
      '#button_type' => 'primary',
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    // No validation.
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    foreach ($form_state->getValue($this->entitiesKey) as $id => $value) {
      if (isset($this->entities[$id]) && $this->entities[$id]
        ->get($this->weightKey) != $value['weight']) {
        // Save entity only when its weight was changed.
        $this->entities[$id]
          ->set($this->weightKey, $value['weight']);
        $this->entities[$id]
          ->save();
      }
    }
  }
  
  /**
   * Returns the form builder.
   *
   * @return \Drupal\Core\Form\FormBuilderInterface
   *   The form builder.
   */
  protected function formBuilder() {
    if (!$this->formBuilder) {
      $this->formBuilder = \Drupal::formBuilder();
    }
    return $this->formBuilder;
  }

}