class Weight

class Weight extends FormElementBase {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#delta' => 10,
      '#default_value' => 0,
      '#process' => [
        [
          $class,
          'processWeight',
        ],
        [
          $class,
          'processAjaxForm',
        ],
      ],
    ];
  }

  /**
   * Expands a weight element into a select/number element.
   */
  public static function processWeight(&$element, FormStateInterface $form_state, &$complete_form) {

    // If the number of options is small enough, use a select field. Otherwise,
    // use a number field.
    $type = $element['#delta'] <= \Drupal::config('system.site')
      ->get('weight_select_max') ? 'select' : 'number';
    $element = array_merge($element, \Drupal::service('element_info')
      ->getInfo($type));
    $element['#is_weight'] = TRUE;
    if ($type === 'select') {
      $weights = [];
      for ($n = -1 * $element['#delta']; $n <= $element['#delta']; $n++) {
        $weights[$n] = $n;
      }
      $default_value = (int) $element['#default_value'];
      if (!isset($weights[$default_value])) {
        $weights[$default_value] = $default_value;
        ksort($weights);
      }
      $element['#options'] = $weights;
    }
    else {

      // Use a field big enough to fit most weights.
      $element['#size'] = 10;
    }
    return $element;
  }

}