Subscriptions in MQTT

Pamast

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.

SGaist

Hi and welcome to devnet,

What broker are you using ?
How is it setup ?

Pamast

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.

SGaist

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

Pamast

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".

SGaist

Glad you found out and thanks for sharing !