@IBAction func subscribeTopic(_ sender: Any) {
        let topic = mqttTopic.text

        if !subscribeBool{
            subscribeBool = true
            mqtt?.subscribe(topic!)
        }
        else{
            subscribeBool = false
            mqtt?.unsubscribe(topic!)
        }

        mqtt?.didSubscribeTopic = {  mqtt , topic in
          //訂閱後要執行的程式碼
        }

        mqtt?.didUnsubscribeTopic = { mqtt , topic in
          //解除訂閱後要執行的程式碼
        }

    }