为 QML-Usage 返回 QString

Question

idk why, but I can't read a return value of a QString method from.cpp-code in.qml-code, although I did everything logically correct (I'll put only relatable parts of code):

.h File:

public:
    Q_INVOKABLE QString displayOnQml();
    QString message;
public slots:
   void updateMessage(const QMqttMessage &msg);

.cpp FIle:

connect(m_sub, &QMqttSubscription::messageReceived, this, &mqtt::updateMessage);
connect(m_sub, &QMqttSubscription::messageReceived, this, &mqtt::displayOnQml);
void mqtt::updateMessage(const QMqttMessage &msg)
{
    message=msg.payload();
    qDebug()<<message;
}
QString mqtt::displayOnQml()
{
    return message;
}

.qml File

  Text {
     id: text1
     x: 63
     y: 68
     text: mqttClient.displayOnQml()
     font.pixelSize: 12
  }

(I embedded.cpp-Methodes into.qml as a engine.rootContext()->setContextProperty("mqttClient",mqttClient); , so that I can use other functions as well.)

Finally the question: why by debugging I catch the breakpoint in.cpp-File on displayOnQml() ( when I send some data to the topic ), but afterall the object message has "no value", though it was as a public declared? And still, the simple returns from.cpp-Methodes (such as 1 or 0) are working..

Answer 1

The Q_INVOKABLE do not create a binding since you cannot know when the string changed. In this case it is better to create a Q_PROPERTY:

* .cpp

class mqtt: ...{
    Q_OBJECT
    // ...
    Q_PROPERTY(QString message READ message NOTIFY messageChanged)
public:
    // ...
    QString message() const;
    // ...
Q_SIGNALS:
    void messageChanged();
    // ...
private:
    QString m_message;
    // ...
};

* .h

void mqtt::updateMessage(const QMqttMessage &msg)
{
    m_message = msg.payload();
    Q_EMIT messageChanged();
}
QString mqtt::message() const{
    return m_message;
}

Text {
    id: text1
    x: 63
    y: 68
    text: mqttClient.message
    font.pixelSize: 12
}