How to extract values of multiple topics in onMessageArrived(message) function of Paho MQTT JavaScript API?

Question

I have created a Publisher Script using Paho MQTT JavaScript API which publishes values to two topics MyHome/Temp and MyHome/Hum. The script is running successfully and publishing data to CloudMQTT broker. In my Subscriber script I have subscribed to these two topics and printing them in Console as following:

function onConnect() {
  console.log("onConnect");
  client.subscribe("MyHome/Temp");
  client.subscribe("MyHome/Hum");
}

function onMessageArrived(message) { 
  console.log(message.destinationName +" : "+ message.payloadString);
}

It is printing both the topic names and corresponding values. Now I want to extract the values of both topics using message.payloadString and store in variable as following:

function onMessageArrived(message) { 
  var temp = message.payloadString;
  var hum = message.payloadString;
  ...
}

But I am getting only on value in both variable i.e. the value of last topic 'hum'. Can anyone please help me solving this.

Answer 1

onMessageArrived will be called separately for each message that arrives so if the payload only contains one value then you will only be able to get one value at a time.

You need to include a if statement in the onMessageArrived callback to determine which topic the message arrived on and then set the respective value.

Answer 2

i found different solution to this in python

client.py

 class Messages(threading.Thread):
    def __init__(
        self,
        clientname,
        broker="127.0.0.1",
        pub_topic="msg",
        sub_topic=[("msg", 0)],
    ):
        super().__init__()
        self.broker = broker
        self.sub_topic = sub_topic
        self.pub_topic = pub_topic
        self.clientname = clientname
        self.client = mqtt.Client(self.clientname)
        self.client.on_connect = self.on_connect
        self.client.on_message = self.on_message
        self.client.on_subscibe = self.on_subscribe
        self.received = {}
        self.topicTemp = "" <==== define temporary topic name

    def on_connect(self, client, userdata, flags, rc):
        if rc == 0:
            print("Server Connection Established")
        else:
            print("bad connection Returned code=", rc)
        self.client.subscribe(self.sub_topic)

    def on_subscribe(self, client, userdata, mid, granted_qos):
        print("Subscription complete")

    def on_message(self, client, userdata, msg):
        if self.topicTemp != msg.topic:  <=== compare incoming topic name
            self.received[msg.topic] = {  <=== create new element of received dict
                "topic": msg.topic,
                "payload": str(msg.payload.decode()),
            }
        self.topicTemp = msg.topic <== store handlede topic to compare next one

    def begin(self):
        print("Setting up connection")
        self.client.connect(self.broker)
        self.client.loop_start()
        # self.client.loop_forever()

    def end(self):
        time.sleep(1)
        print("Ending Connection")
        self.client.loop_stop()
        self.client.disconnect()

    def send(self, msg, topic=None):
        if topic is None:
            topic = self.pub_topic
        self.client.publish(topic, msg)

    def get(self, msg, topic=None):
        if topic is None:
            topic = self.sub_topic[0]
        self.client.subscribe(topic, msg)
        # self.send(msg, topic)


def main():
    remote = Messages(clientname="PC", broker="127.0.0.1")
    remote.begin()


if __name__ == "__main__":
    main()

function.py

    from mqtt_client_test import Messages
import time

remote = Messages(
    clientname="Camera",
    broker="127.0.0.1",
    pub_topic="pc/camera",
    sub_topic=[("pc/camera", 0), ("commands/detect", 1)],
)
remote.begin()
while True:
    msg = remote.received
       # print(msg)
    if "commands/detect" in msg:
        print("command", msg["commands/detect"]["payload"])
    if "pc/camera" in msg:
        print("camera", msg["pc/camera"]["payload"])
    time.sleep(1)
    

Now you can get payloads from different topics in one on_message function and can extract how you want to use somewhere else...