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Subscriptions in MQTT

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  • P Offline
    P Offline
    Pamast
    wrote on last edited by
    #1

    Hi,

    I built QMQTT module for the community version of the ide. I did a simple program to connect to a broker and publish some frames. It worked.

    I tried to add a subscription to receive frames from another client and I came across a problem and I am not able to figure it out.

    In all examples I see on the Internet, I see things like:

    auto subs = m_client->subscribe("topic"); //Being "topic" a QString.
    if(!subs){
       qDebug() << "can't subscribe!";
    }
    

    But, once I try that, I get an error. subscribe mandatory param is a QMqttTopicFilter.

    I tried something like:

    QMqttTopicFilter topic;
    topic.setFilter("topic");
    auto subs = m_client->subscribe(topic, 1);
    if(!subs){
        qDebug() << "can't subs";
    }
    

    But I get always the message "can't subs". Neither do I receibe frames with this:

    connect(m_client, &QMqttClient::messageReceived, this, [this](const QByteArray &message, const QMqttTopicName &topic) {
            const QString content = QDateTime::currentDateTime().toString()
                        + QLatin1String(" Received Topic: ")
                        + topic.name()
                        + QLatin1String(" Message: ")
                        + message
                        + QLatin1Char('\n');
            qDebug() << (content);
        });
    

    I guess that subscription is not made at all. Debugging I get that subs value is 0x0. It does not create QMqttSubscription object as it should.

    Can you give me an idea about what I can try? This is my first time with QT and I am still reading tutorials.

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    • SGaistS Offline
      SGaistS Offline
      SGaist
      Lifetime Qt Champion
      wrote on last edited by
      #2

      Hi and welcome to devnet,

      What broker are you using ?
      How is it setup ?

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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      0
      • P Offline
        P Offline
        Pamast
        wrote on last edited by
        #3

        I tested with 2 instances of Mosquitto Broker with MQTT 3.1

        Configuration is the standard one. I have not modified anything but user, password and port in one of them. The other remain as default.

        I can publish with this program and subscribe&publish with other clients.

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        • SGaistS Offline
          SGaistS Offline
          SGaist
          Lifetime Qt Champion
          wrote on last edited by
          #4

          Ok, can you provide a minimal compilable sample that shows the situation you have ?

          Interested in AI ? www.idiap.ch
          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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          0
          • P Offline
            P Offline
            Pamast
            wrote on last edited by
            #5

            Well I found the solution.

            It seems that in some version of the MQTT module something changed:

            QMqttTopicFilter topic;
            
            topic.setFilter("hello");
            auto subs = m_client->subscribe(topic, 1);
            qDebug() << "Subs state: " << subs->Subscribed;
            

            Now you need to check with the "Subscribed" property and not with the response in "subs".

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            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #6

              Glad you found out and thanks for sharing !

              Interested in AI ? www.idiap.ch
              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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