private function validateUsername(Request $request)
    {
        $username = self::$validator->isString('username');

        $result = $username
            ->isRequired()
            ->isAlpha()
            ->isBetween(4, 12, true)
            ->validate($request->username);

        $this->isValid = $this->isValid && $result;
        $this->errors  = array_merge($this->errors, $username->getErrors());
    }