Radio.php

<?php

namespace Drupal\Core\Render\Element;

use Drupal\Core\Render\Element;

/**
 * Provides a form element for a single radio button.
 *
 * This is an internal element that is primarily used to render the radios form
 * element. Refer to \Drupal\Core\Render\Element\Radios for more documentation.
 *
 * @see \Drupal\Core\Render\Element\Radios
 * @see \Drupal\Core\Render\Element\Checkbox
 *
 * @FormElement("radio")
 */
class Radio extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#default_value' => NULL,
            '#process' => [
                [
                    $class,
                    'processAjaxForm',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderRadio',
                ],
            ],
            '#theme' => 'input__radio',
            '#theme_wrappers' => [
                'form_element',
            ],
            '#title_display' => 'after',
        ];
    }
    
    /**
     * Prepares a #type 'radio' render element for input.html.twig.
     *
     * @param array $element
     *   An associative array containing the properties of the element.
     *   Properties used: #required, #return_value, #value, #attributes, #title,
     *   #description. The #name property will be sanitized before output. This is
     *   currently done by initializing Drupal\Core\Template\Attribute with all
     *   the attributes.
     *
     * @return array
     *   The $element with prepared variables ready for input.html.twig.
     */
    public static function preRenderRadio($element) {
        $element['#attributes']['type'] = 'radio';
        Element::setAttributes($element, [
            'id',
            'name',
            '#return_value' => 'value',
        ]);
        // To avoid auto-casting during '==' we convert $element['#value'] and
        // $element['#return_value'] to strings. It will prevent wrong true-checking
        // for both cases: 0 == 'string' and 'string' == 0, this will occur because
        // all numeric array values will be integers and all submitted values will
        // be strings. Array values are never valid for radios and are skipped. To
        // account for FALSE and empty string values in the #return_value, we will
        // consider any #value that evaluates to empty to be the same as any
        // #return_value that evaluates to empty.
        if (isset($element['#return_value']) && $element['#value'] !== FALSE && !is_array($element['#value']) && (empty($element['#value']) && empty($element['#return_value']) || (string) $element['#value'] === (string) $element['#return_value'])) {
            $element['#attributes']['checked'] = 'checked';
        }
        static::setAttributes($element, [
            'form-radio',
        ]);
        return $element;
    }

}