QML and QtMQTT publish example

[Pablo J. Rogina]

@Egn1n3 spoiler alert: not that I've tried it myself...
from this blog (see code snippet there for publishing data) and the example in your post, I imagine that you need to use the publish() method instead of or in addition to subscribe() method.

Pseudo-code for your QmlMqttClient class I assume you already have:

...
qint32 QmlMqttClient::publish(const QString &topic, const QByteArray &message, quint8 qos, bool retain)
{
    auto id = QMqttClient::publish(topic, message, qos, retain);
    return id;
}
...

[Egn1n3]

Thanks.

I extended the example with the publish call but nothing is published.
Not sure if the arguments I provide are correct?

MqttClient {
    id: client
    hostname: hostnameField.text
    port: portField.text
}

client.publish("mytopic", "testmessage",0,false)

[Pablo J. Rogina]

@Egn1n3 said in QML and QtMQTT publish example:

but nothing is published.

How do you know?
What broker are you using to connect to?
Have you tried using a retained message?

[Egn1n3]

@Pablo-J.-Rogina I subscribed to "mytopic" on my mosquitto broker and I enabled debugging on the broker too. Nothing....
The publish function returns a -1 which indicates an error.

I am wondering if I need to register something like you need to do with subscription:

qmlRegisterUncreatableType<QmlMqttSubscription>("MqttClient", 1, 0, "MqttSubscription", QLatin1String("Subscriptions are read-only"));

[SGaist]

Hi,

Did you test your installation with the Simple Client Exemple ?

[Egn1n3]

@SGaist I did and Simple Client Example works perfect.
I'll try to do some more tests....

[Pablo J. Rogina]

@Egn1n3 I did some test on using QmlMqttClient example and I made it work only when I provided a publish() method with slightly changed parameters, so to avoid QML -> C++ interaction with a QByteArray (as expected for param message). So in C++ I made a Q_INVOKABLE method like this (pseudo-code):

qint32 QmlMqttClient::publish(const QString &topic, const QString &message, int qos, bool retain)
{
    auto id = publish(topic, message.toUtf8(), (quint8)qos, retain);
    return id;
}

which works fine with QML and then inside it calls the original publish() method with the proper type of parameters

[Egn1n3]

@Pablo-J.-Rogina said in QML and QtMQTT publish example:

qint32 QmlMqttClient::publish(const QString &topic, const QString &message, int qos, bool retain)
{
auto id = publish(topic, message.toUtf8(), (quint8)qos, retain);
return id;
}

Thanks Pablo, this is finally working. Thanks!

I had to change the method name from publish to publishme. If I didn't the program crashed.

This is what I did:
Added to qmlmqttclient.h:

Q_INVOKABLE qint32 publishme(const QString &topic, const QString &message, int qos, bool retain);

qmlmqttclient.h:
class QmlMqttClient : public QMqttClient
{
Q_OBJECT
public:
QmlMqttClient(QObject *parent = nullptr);

Q_INVOKABLE QmlMqttSubscription *subscribe(const QString &topic);
Q_INVOKABLE qint32 publishme(const QString &topic, const QString &message, int qos, bool retain);

private:
Q_DISABLE_COPY(QmlMqttClient)
};

And to qmlmqttclient.cpp I added my new publishme method:

qint32 QmlMqttClient::publishme(const QString &topic, const QString &message, int qos, bool retain)
{
auto id = publish(topic, message.toUtf8(), (quint8)qos, retain);
return id;
}

From main.qml:
Button {
id: publish

    text: qsTr("Publish")
    onClicked: {
        
        if (client.state==MqttClient.Connected){
            result=client.publishme("mytopic","mymessage",0,false)
            console.log(result)
         } else {
             console.log("Client is not connected")
        }
    }
}

Not 100% sure if I did this all correctly, but it seems to work.

Thanks again!

[Egn1n3]

Some last remarks: it seems this is a bug as indicated by Maurice Kalinowski:
https://bugreports.qt.io/browse/QTBUG-69066

[mkalinow]

Hi,

this has also been discussed on the qt-interest mailing list. Here is the link, also with some solution and patches how to get it working.http://lists.qt-project.org/pipermail/interest/2018-June/030309.html