Method test

/**
 * Method to test if the folder path is valid and points to an existing folder below the Joomla root
 *
 * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
 * @param   mixed              $value    The form field value to validate.
 * @param   string             $group    The field name group control value. This acts as an array container for the field.
 *                                       For example if the field has name="foo" and the group value is set to "bar" then the
 *                                       full field name would end up being "bar[foo]".
 * @param   Registry           $input    An optional Registry object with the entire data set to validate against the entire form.
 * @param   Form               $form     The form object for which the field is being tested.
 *
 * @return  boolean  True if the value is valid and points to an existing folder below the Joomla root, false otherwise.
 *
 * @since   4.0.0
 */
public function test(\SimpleXMLElement $element, $value, $group = null, Registry $input = null, Form $form = null)
{
    if (!parent::test($element, $value, $group, $input, $form)) {
        return false;
    }
    // If the field is empty and not required so the previous test hasn't failed, the field is valid.
    if ($value === '' || $value === null) {
        return true;
    }
    // Spaces only would result in Joomla root which is not allowed
    if (!trim($value)) {
        return false;
    }
    $pathCleaned = rtrim(Path::clean(JPATH_ROOT . '/' . $value), \DIRECTORY_SEPARATOR);
    $rootCleaned = rtrim(Path::clean(JPATH_ROOT), \DIRECTORY_SEPARATOR);
    // JPATH_ROOT is not allowed
    if ($pathCleaned === $rootCleaned) {
        return false;
    }
    return Folder::exists($pathCleaned);
}