Weight.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Attribute\FormElement;

/**
 * Provides a form element for input of a weight.
 *
 * Weights are integers used to indicate ordering, with larger numbers later in
 * the order.
 *
 * Properties:
 * - #delta: The range of possible weight values used. A delta of 10 would
 *   indicate possible weight values between -10 and 10.
 *
 * Usage example:
 * @code
 * $form['weight'] = [
 *   '#type' => 'weight',
 *   '#title' => $this->t('Weight'),
 *   '#default_value' => $edit['weight'],
 *   '#delta' => 10,
 * ];
 * @endcode
 */
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;
    }

}