class Checkbox

class Checkbox extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = static::class;
        return [
            '#input' => TRUE,
            '#return_value' => 1,
            '#process' => [
                [
                    $class,
                    'processCheckbox',
                ],
                [
                    $class,
                    'processAjaxForm',
                ],
                [
                    $class,
                    'processGroup',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderCheckbox',
                ],
                [
                    $class,
                    'preRenderGroup',
                ],
            ],
            '#theme' => 'input__checkbox',
            '#theme_wrappers' => [
                'form_element',
            ],
            '#title_display' => 'after',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        if ($input === FALSE) {
            // Use #default_value as the default value of a checkbox, except change
            // NULL to 0, because FormBuilder::handleInputElement() would otherwise
            // replace NULL with empty string, but an empty string is a potentially
            // valid value for a checked checkbox.
            return $element['#default_value'] ?? 0;
        }
        else {
            // Checked checkboxes are submitted with a value (possibly '0' or ''):
            // http://www.w3.org/TR/html401/interact/forms.html#successful-controls.
            // For checked checkboxes, browsers submit the string version of
            // #return_value, but we return the original #return_value. For unchecked
            // checkboxes, browsers submit nothing at all, but
            // FormBuilder::handleInputElement() detects this, and calls this
            // function with $input=NULL. Returning NULL from a value callback means
            // to use the default value, which is not what is wanted when an unchecked
            // checkbox is submitted, so we use integer 0 as the value indicating an
            // unchecked checkbox. Therefore, modules must not use integer 0 as a
            // #return_value, as doing so results in the checkbox always being treated
            // as unchecked. The string '0' is allowed for #return_value. The most
            // common use-case for setting #return_value to either 0 or '0' is for the
            // first option within a 0-indexed array of checkboxes, and for this,
            // \Drupal\Core\Render\Element\Checkboxes::processCheckboxes() uses the
            // string rather than the integer.
            return isset($input) ? $element['#return_value'] : 0;
        }
    }
    
    /**
     * Prepares a #type 'checkbox' render element for input.html.twig.
     *
     * @param array $element
     *   An associative array containing the properties of the element.
     *   Properties used: #title, #value, #return_value, #description, #required,
     *   #attributes, #checked.
     *
     * @return array
     *   The $element with prepared variables ready for input.html.twig.
     */
    public static function preRenderCheckbox($element) {
        $element['#attributes']['type'] = 'checkbox';
        Element::setAttributes($element, [
            'id',
            'name',
            '#return_value' => 'value',
        ]);
        // Unchecked checkbox has #value of integer 0.
        if (!empty($element['#checked'])) {
            $element['#attributes']['checked'] = 'checked';
        }
        static::setAttributes($element, [
            'form-checkbox',
        ]);
        return $element;
    }
    
    /**
     * Sets the #checked property of a checkbox element.
     */
    public static function processCheckbox(&$element, FormStateInterface $form_state, &$complete_form) {
        $value = $element['#value'];
        $return_value = $element['#return_value'];
        // On form submission, the #value of an available and enabled checked
        // checkbox is #return_value, and the #value of an available and enabled
        // unchecked checkbox is integer 0. On not submitted forms, and for
        // checkboxes with #access=FALSE or #disabled=TRUE, the #value is
        // #default_value (integer 0 if #default_value is NULL). Most of the time,
        // a string comparison of #value and #return_value is sufficient for
        // determining the "checked" state, but a value of TRUE always means checked
        // (even if #return_value is 'foo'), and a value of FALSE or integer 0
        // always means unchecked (even if #return_value is '' or '0').
        if ($value === TRUE || $value === FALSE || $value === 0) {
            $element['#checked'] = (bool) $value;
        }
        else {
            // Compare as strings, so that 15 is not considered equal to '15foo', but
            // 1 is considered equal to '1'. This cast does not imply that either
            // #value or #return_value is expected to be a string.
            $element['#checked'] = (string) $value === (string) $return_value;
        }
        return $element;
    }

}