class Select

class Select extends FormElementBase {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#multiple' => FALSE,
      '#sort_options' => FALSE,
      '#sort_start' => NULL,
      '#process' => [
        [
          $class,
          'processSelect',
        ],
        [
          $class,
          'processAjaxForm',
        ],
      ],
      '#pre_render' => [
        [
          $class,
          'preRenderSelect',
        ],
      ],
      '#theme' => 'select',
      '#theme_wrappers' => [
        'form_element',
      ],
      '#options' => [],
    ];
  }

  /**
   * Processes a select list form element.
   *
   * This process callback is mandatory for select fields, since all user agents
   * automatically preselect the first available option of single (non-multiple)
   * select lists.
   *
   * @param array $element
   *   The form element to process.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param array $complete_form
   *   The complete form structure.
   *
   * @return array
   *   The processed element.
   *
   * @see _form_validate()
   */
  public static function processSelect(&$element, FormStateInterface $form_state, &$complete_form) {

    // #multiple select fields need a special #name.
    if ($element['#multiple']) {
      $element['#attributes']['multiple'] = 'multiple';
      $element['#attributes']['name'] = $element['#name'] . '[]';
    }
    else {

      // If the element is set to #required through #states, override the
      // element's #required setting.
      $required = isset($element['#states']['required']) ? TRUE : $element['#required'];

      // If the element is required and there is no #default_value, then add an
      // empty option that will fail validation, so that the user is required to
      // make a choice. Also, if there's a value for #empty_value or
      // #empty_option, then add an option that represents emptiness.
      if ($required && !isset($element['#default_value']) || isset($element['#empty_value']) || isset($element['#empty_option'])) {
        $element += [
          '#empty_value' => '',
          '#empty_option' => $required ? t('- Select -') : t('- None -'),
        ];

        // The empty option is prepended to #options and purposively not merged
        // to prevent another option in #options mistakenly using the same value
        // as #empty_value.
        $empty_option = [
          $element['#empty_value'] => $element['#empty_option'],
        ];
        $element['#options'] = $empty_option + $element['#options'];
      }
    }

    // Provide the correct default value for #sort_start.
    $element['#sort_start'] = $element['#sort_start'] ?? (isset($element['#empty_value']) ? 1 : 0);
    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
    if ($input !== FALSE) {
      if (isset($element['#multiple']) && $element['#multiple']) {

        // If an enabled multi-select submits NULL, it means all items are
        // unselected. A disabled multi-select always submits NULL, and the
        // default value should be used.
        if (empty($element['#disabled'])) {
          return is_array($input) ? array_combine($input, $input) : [];
        }
        else {
          return isset($element['#default_value']) && is_array($element['#default_value']) ? $element['#default_value'] : [];
        }
      }
      elseif (isset($element['#empty_value']) && $input === (string) $element['#empty_value']) {
        return $element['#empty_value'];
      }
      else {
        return $input;
      }
    }
  }

  /**
   * Prepares a select render element.
   */
  public static function preRenderSelect($element) {
    Element::setAttributes($element, [
      'id',
      'name',
      'size',
    ]);
    static::setAttributes($element, [
      'form-select',
    ]);
    return $element;
  }

}