Checkbox::valueCallback

public static Checkbox::valueCallback(&$element, $input, FormStateInterface $form_state)

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 isset($element['#default_value']) ? $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;
  }
}