public function notification(string $method, array $parameters)
    {
        if ($this->isSingleRequest) {
            $this->units = [];
        }

        $this->units[] = new Notification($method, $parameters);

        if ($this->isSingleRequest) {
            $this->execute(new InvokeSpec($this->units, true));
            return null;
        }

        return $this;
    }