Range.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Element;

/**
 * Provides a slider for input of a number within a specific range.
 *
 * Provides an HTML5 input element with type of "range".
 *
 * Properties:
 * - #min: Minimum value (defaults to 0).
 * - #max: Maximum value (defaults to 100).
 * Refer to \Drupal\Core\Render\Element\Number for additional properties.
 *
 * Usage example:
 * @code
 * $form['quantity'] = array(
 *   '#type' => 'range',
 *   '#title' => $this->t('Quantity'),
 * );
 * @endcode
 *
 * @see \Drupal\Core\Render\Element\Number
 *
 * @FormElement("range")
 */
class Range extends Number {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $info = parent::getInfo();
        $class = static::class;
        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;
        }
    }

}