Details.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Render\Element;

/**
 * Provides a render element for a details element, similar to a fieldset.
 *
 * Fieldsets can only be used in forms, while details elements can be used
 * outside of forms. Users click on the title to open or close the details
 * element, showing or hiding the contained elements.
 *
 * Properties:
 * - #title: The title of the details container. Defaults to "Details".
 * - #open: Indicates whether the container should be open by default.
 *   Defaults to FALSE.
 * - #summary_attributes: An array of attributes to apply to the <summary>
 *   element.
 *
 * Usage example:
 * @code
 * $form['author'] = array(
 *   '#type' => 'details',
 *   '#title' => $this->t('Author'),
 * );
 *
 * $form['author']['name'] = array(
 *   '#type' => 'textfield',
 *   '#title' => $this->t('Name'),
 * );
 * @endcode
 *
 * @see \Drupal\Core\Render\Element\Fieldset
 * @see \Drupal]Core\Render\Element\VerticalTabs
 *
 * @RenderElement("details")
 */
class Details extends RenderElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#open' => FALSE,
            '#summary_attributes' => [],
            '#value' => NULL,
            '#process' => [
                [
                    $class,
                    'processGroup',
                ],
                [
                    $class,
                    'processAjaxForm',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderDetails',
                ],
                [
                    $class,
                    'preRenderGroup',
                ],
            ],
            '#theme_wrappers' => [
                'details',
            ],
        ];
    }
    
    /**
     * Adds form element theming to details.
     *
     * @param $element
     *   An associative array containing the properties and children of the
     *   details.
     *
     * @return
     *   The modified element.
     */
    public static function preRenderDetails($element) {
        Element::setAttributes($element, [
            'id',
        ]);
        // The .js-form-wrapper class is required for #states to treat details like
        // containers.
        static::setAttributes($element, [
            'js-form-wrapper',
            'form-wrapper',
        ]);
        // Collapsible details.
        $element['#attached']['library'][] = 'core/drupal.collapse';
        // Open the detail if specified or if a child has an error.
        if (!empty($element['#open']) || !empty($element['#children_errors'])) {
            $element['#attributes']['open'] = 'open';
        }
        // Do not render optional details elements if there are no children.
        if (isset($element['#parents'])) {
            $group = implode('][', $element['#parents']);
            if (!empty($element['#optional']) && !Element::getVisibleChildren($element['#groups'][$group])) {
                $element['#printed'] = TRUE;
            }
        }
        return $element;
    }

}