Hidden.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Render\Element;

/**
 * Provides a form element for an HTML 'hidden' input element.
 *
 * Specify either #default_value or #value but not both.
 *
 * Properties:
 * - #default_value: The initial value of the form element. JavaScript may
 *   alter the value prior to submission.
 * - #value: The value of the form element. The Form API ensures that this
 *   value remains unchanged by the browser.
 *
 * Usage example:
 * @code
 * $form['entity_id'] = array('#type' => 'hidden', '#value' => $entity_id);
 * @endcode
 *
 * @see \Drupal\Core\Render\Element\Value
 *
 * @FormElement("hidden")
 */
class Hidden extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#process' => [
                [
                    $class,
                    'processAjaxForm',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderHidden',
                ],
            ],
            '#theme' => 'input__hidden',
        ];
    }
    
    /**
     * Prepares a #type 'hidden' render element for input.html.twig.
     *
     * @param array $element
     *   An associative array containing the properties of the element.
     *   Properties used: #name, #value, #attributes.
     *
     * @return array
     *   The $element with prepared variables ready for input.html.twig.
     */
    public static function preRenderHidden($element) {
        $element['#attributes']['type'] = 'hidden';
        Element::setAttributes($element, [
            'name',
            'value',
        ]);
        return $element;
    }

}