class Range

class Range extends Number {
  
  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $info = parent::getInfo();
    $class = get_class($this);
    return [
      '#min' => 0,
      '#max' => 100,
      '#pre_render' => [
        [
          $class,
          'preRenderRange',
        ],
      ],
      '#theme' => 'input__range',
    ] + $info;
  }
  
  /**
   * Prepares a #type 'range' render element for input.html.twig.
   *
   * @param array $element
   *   An associative array containing the properties of the element.
   *   Properties used: #title, #value, #description, #min, #max, #attributes,
   *   #step.
   *
   * @return array
   *   The $element with prepared variables ready for input.html.twig.
   */
  public static function preRenderRange($element) {
    $element['#attributes']['type'] = 'range';
    Element::setAttributes($element, [
      'id',
      'name',
      'value',
      'step',
      'min',
      'max',
    ]);
    static::setAttributes($element, [
      'form-range',
    ]);
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
    if ($input === '') {
      $offset = ($element['#max'] - $element['#min']) / 2;
      // Round to the step.
      if (strtolower($element['#step']) != 'any') {
        $steps = round($offset / $element['#step']);
        $offset = $element['#step'] * $steps;
      }
      return $element['#min'] + $offset;
    }
  }

}