public function signedRoute($name, $parameters = [], $expiration = null, $absolute = true)
    {
        $parameters = $this->formatParameters($parameters);

        if ($expiration) {
            $parameters = $parameters + ['expires' => $this->availableAt($expiration)];
        }

        ksort($parameters);

        $key = call_user_func($this->keyResolver);

        return $this->route($name, $parameters + [
            'signature' => hash_hmac('sha256', $this->route($name, $parameters, $absolute), $key),
        ], $absolute);
    }