class Checkboxes

class Checkboxes extends FormElement {
    use CompositeFormElementTrait;
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#process' => [
                [
                    $class,
                    'processCheckboxes',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderCompositeFormElement',
                ],
            ],
            '#theme_wrappers' => [
                'checkboxes',
            ],
        ];
    }
    
    /**
     * Processes a checkboxes form element.
     */
    public static function processCheckboxes(&$element, FormStateInterface $form_state, &$complete_form) {
        $value = is_array($element['#value']) ? $element['#value'] : [];
        $element['#tree'] = TRUE;
        if (count($element['#options']) > 0) {
            if (!isset($element['#default_value']) || $element['#default_value'] == 0) {
                $element['#default_value'] = [];
            }
            $weight = 0;
            foreach ($element['#options'] as $key => $choice) {
                // Integer 0 is not a valid #return_value, so use '0' instead.
                // @see \Drupal\Core\Render\Element\Checkbox::valueCallback().
                // @todo For Drupal 8, cast all integer keys to strings for consistency
                //   with \Drupal\Core\Render\Element\Radios::processRadios().
                if ($key === 0) {
                    $key = '0';
                }
                // Maintain order of options as defined in #options, in case the element
                // defines custom option sub-elements, but does not define all option
                // sub-elements.
                $weight += 0.001;
                $element += [
                    $key => [],
                ];
                $element[$key] += [
                    '#type' => 'checkbox',
                    '#title' => $choice,
                    '#return_value' => $key,
                    '#default_value' => isset($value[$key]) ? $key : NULL,
                    '#attributes' => $element['#attributes'],
                    '#ajax' => isset($element['#ajax']) ? $element['#ajax'] : NULL,
                    // Errors should only be shown on the parent checkboxes element.
'#error_no_message' => TRUE,
                    '#weight' => $weight,
                ];
            }
        }
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        if ($input === FALSE) {
            $value = [];
            $element += [
                '#default_value' => [],
            ];
            foreach ($element['#default_value'] as $key) {
                $value[$key] = $key;
            }
            return $value;
        }
        elseif (is_array($input)) {
            // Programmatic form submissions use NULL to indicate that a checkbox
            // should be unchecked. We therefore remove all NULL elements from the
            // array before constructing the return value, to simulate the behavior
            // of web browsers (which do not send unchecked checkboxes to the server
            // at all). This will not affect non-programmatic form submissions, since
            // all values in \Drupal::request()->request are strings.
            // @see \Drupal\Core\Form\FormBuilderInterface::submitForm()
            foreach ($input as $key => $value) {
                if (!isset($value)) {
                    unset($input[$key]);
                }
            }
            return array_combine($input, $input);
        }
        else {
            return [];
        }
    }
    
    /**
     * Determines which checkboxes were checked when a form is submitted.
     *
     * @param array $input
     *   An array returned by the FormAPI for a set of checkboxes.
     *
     * @return array
     *   An array of keys that were checked.
     */
    public static function getCheckedCheckboxes(array $input) {
        // Browsers do not include unchecked options in a form submission. The
        // FormAPI tries to normalize this to keep checkboxes consistent with other
        // form elements. Checkboxes show up as an array in the form of option_id =>
        // option_id|0, where integer 0 is an unchecked option.
        //
        // @see \Drupal\Core\Render\Element\Checkboxes::valueCallback()
        // @see https://www.w3.org/TR/html401/interact/forms.html#checkbox
        $checked = array_filter($input, function ($value) {
            return $value !== 0;
        });
        return array_keys($checked);
    }
    
    /**
     * Determines if all checkboxes in a set are unchecked.
     *
     * @param array $input
     *   An array returned by the FormAPI for a set of checkboxes.
     *
     * @return bool
     *   TRUE if all options are unchecked. FALSE otherwise.
     */
    public static function detectEmptyCheckboxes(array $input) {
        return empty(static::getCheckedCheckboxes($input));
    }

}