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;
    }

}