Method test

/**
 * Method to test the username for uniqueness.
 *
 * @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, false otherwise.
 *
 * @since   1.7.0
 */
public function test(\SimpleXMLElement $element, $value, $group = null, Registry $input = null, Form $form = null)
{
    // Get the database object and a new query object.
    $db = Factory::getDbo();
    $query = $db->getQuery(true);
    // Get the extra field check attribute.
    $userId = $form instanceof Form ? (int) $form->getValue('id') : 0;
    // Build the query.
    $query->select('COUNT(*)')->from($db->quoteName('#__users'))->where([$db->quoteName('username') . ' = :username', $db->quoteName('id') . ' <> :userId'])->bind(':username', $value)->bind(':userId', $userId, ParameterType::INTEGER);
    // Set and query the database.
    $db->setQuery($query);
    $duplicate = (bool) $db->loadResult();
    if ($duplicate) {
        return false;
    }
    return true;
}