QML undefined variable from Qt

Question

I try to implement cpp variables to qml using Q_PROPERTY. First, I have created Q_PROPERTY of float numbers xValue, yValue, and zValue in the header file.

fileio.h
...
    Q_PROPERTY(int xValue
               MEMBER xValue
               NOTIFY xValueChanged)
    Q_PROPERTY(int yValue
               MEMBER yValue
               NOTIFY yValueChanged)
    Q_PROPERTY(int zValue
               MEMBER zValue
               NOTIFY zValueChanged)
...

Then I also make them private and created their signal functions.

signals:
    void xValueChanged();
    void yValueChanged();
    void zValueChanged();

private:
    float xValue,yValue,zValue;

After that, I have assigned a changeable variable to these float numbers in fileio.cpp file

...
       xValue = (line.split(',')[0]).toFloat();
       yValue = (line.split(',')[4]).toFloat();
       zValue = (line.split(',')[5]).toFloat();
...

Lastly, I tried to use them in QML

    FileIO{
        id: dataCSV
        source: "qrc:/data.csv"
    }

    Component.onCompleted: {
        console.log(FileIO.xValue)

    }

but qml returnsundefined.

Now I have 2 main questions. The first is should I define a function that updates values of variables, because (as I know) in cpp it is not needed to create an update function. And the second one is, if the return doesn't cause from the lack of update function, what should I do to use and read these variables' values in qml?

selbie · Accepted Answer · 2021-02-18 18:50:59Z

Two things:

You need to emit the change such that the QML can become aware of the property change after you set the local variable

xValue = (line.split(',')[0]).toFloat();
emit xValueChanged();

Further, your C++ class, which derives from QObject needs to be exposed up to the QML as a named object and connected to it.

Typically, you have a code path that looks like this when you create the QQuickView:

TheModel model;  // a C++ Class that derives from QObject

QQuickView view(QUrl(QStringLiteral("qrc:/main.qml")));
view.create();
view.rootContext()->setContextProperty("model", &model);  // projects the C++ class instance of "TheModel" to QML and Javascript as a an object named "model"

Then in QML you can say this:

Component.onCompleted: console.log(model.xValue)

I have some sample code on my github as a minimal example of how to setup a connection between the QML and C++ Class.

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