Checkboxes::processCheckboxes

public static Checkboxes::processCheckboxes(&$element, FormStateInterface $form_state, &$complete_form)

public static function processCheckboxes(&$element, FormStateInterface $form_state, &$complete_form) {
  $value = is_array($element['#value']) ? $element['#value'] : array();
  $element['#tree'] = TRUE;
  if (count($element['#options']) > 0) {
    if (!isset($element['#default_value']) || $element['#default_value'] == 0) {
      $element['#default_value'] = array();
    }
    $weight = 0;
    foreach ($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;

      $element += array($key => array());
      $element[$key] += array(
        '#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;
}