Can't get values from QList in QML

[Atr0p0s]

Hello!
I have my simple class that should give access to QList:

struct ChartPoint {
    int x;
    int y;
};

class ChartPointList : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QList<ChartPoint> list READ chartPointList /*WRITE setChartPointList*/ NOTIFY chartPointListChanged)

public:
    ChartPointList(QObject* parent) : QObject(parent)
    {
        /* for test */
        m_chartPointList.append(ChartPoint{1, 1});
    } 
QList<ChartPoint> chartPointList() const;
signals:
    void chartPointListChanged();
private:
    QList<ChartPoint> m_chartPointList;
};

I registered type in main.cpp:

qmlRegisterType<ChartPointList>("com.myinc.ChartPointList", 1, 0, "ChartPointList");

but I can't get values from the list.

console.log(first.list);
console.log(first[0].x);

show

qml: QVariant(QList<ChartPoint>, )
qrc:/QML-graph/Main.qml:20: TypeError: Cannot read property 'x' of undefined

Perhaps I should use QVariant in some way?
Or maybe the whole approach is wrong...

My goal is to read a file with c++ and make a list of points (x, y) from it. And then via ChartView draw these points on the chart.

Thanks for any tips!

[J.Hilk]

@Atr0p0syou in your case, you need to should work with QProperties, if you want to access the data from QML:

class ChartPoint {
    Q_GADGET
    Q_PROPERTY(int x MEMBER m_x)
    Q_PROPERTY(int y MEMBER m_y)
    
public:
    int m_x;
    int m_y;
};

[Atr0p0s]

@J-Hilk Thanks, the list is now recognizable as I can see, but the point isn't:

console.log(first.list); --- qml: [ChartPoint(1, 1)]
console.log(first[0]); --- qml: undefined
console.log(first[0].x); --- Cannot read property 'x' of undefined

Header:

class ChartPoint
{
    Q_GADGET
    Q_PROPERTY(int x MEMBER m_x)
    Q_PROPERTY(int y MEMBER m_y)

public:
    int m_x;
    int m_y;
};

class ChartPointList : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QList<ChartPoint> list READ chartPointList NOTIFY chartPointListChanged)
    QML_ELEMENT

public:
    ChartPointList(QObject* parent = nullptr);
    QList<ChartPoint> chartPointList() const;

signals:
    void chartPointListChanged();

private:
    QList<ChartPoint> m_chartPointList;
};

I registered both classes:

qmlRegisterType<ChartPoint>("com.myinc.chartPoint", 1, 0, "ChartPoint");
qmlRegisterType<ChartPointList>("com.myinc.chartPointList", 1, 0, "ChartPointList");

Also, I get the warning in the output: qt.qml.typeregistration: Invalid QML element name "ChartPoint"; value type names should begin with a lowercase letter, but if I write chartPoint instead of ChartPoint then instead of this warning I get a new one in IDE: QML types must begin with uppercase.

Of course, I could use special functions to get x and y values such as

int ChartPointList::valueX(int index) const
{
    return m_chartPointList.value(index).m_x;
}

but I wish it was a little prettier.

[J.Hilk]

@Atr0p0s It's possible that QGadget is not enough, I thought it is but I may be wrong. Make it a proper QObject and use the Q_OBJECT macro instead

[Atr0p0s]

@J-Hilk Still doesn't work. Perhaps I should use a model or functions for direct access to the data?

What I tried:

//thing-object.h
class Thing : public QObject
{
    Q_OBJECT
    Q_PROPERTY (int     size READ getSize CONSTANT)
    Q_PROPERTY (QString name READ getName CONSTANT)

public:
    Thing(QObject * parent = NULL) : QObject(parent) {}

    int     getSize () const { return m_size; }
    void    setSize (int s) { m_size = s; }
    QString getName () const { return m_name; }
    void    setName (QString n) { m_name = n; }

private:
    int     m_size;
    QString m_name;
};

class ThingManager : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QList<Thing*> things READ getThings NOTIFY thingsChanged)

public:
    QList<Thing*> getThings () const { return m_things; }
    void append (Thing* thg) { m_things.append(thg); }
signals:
    void thingsChanged ();
private:
    QList<Thing*> m_things;
};

//main.cpp
#include "thing-object.h"
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<Thing>("com.myinc.thing", 1, 0, "Thing");
    qmlRegisterType<ThingManager>("com.myinc.thingManager", 1, 0, "ThingManager");

    Thing* thg = new Thing;
    thg->setSize(5); thg->setName("Cow");
    ThingManager man;
    man.append(thg);

    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty(QStringLiteral("first"), &man);
    const QUrl url(u"qrc:/testAgain/Main.qml"_qs);
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
        &app, []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

//main.qml
import QtQuick
import QtQuick.Window
import com.myinc.thing
import com.myinc.thingManager
import QtQuick.Controls

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Button {
        text: "Press"
        onClicked: {
            console.log(first.things);
            console.log(first[0].size);
        }
    }
}

[J.Hilk]

@Atr0p0s said in Can't get values from QList in QML:

console.log(first[0].size);

shouldn't this be first.things[0].size ?

[Atr0p0s]

@J-Hilk I am so sorry, keep forgetting that there is no compile time error checking :(
You are right - it works also with Q_GADGET.
Thanks a lot!

Could you also advise please, if qml copies the entire list to access one element?

[J.Hilk]

@Atr0p0s said in Can't get values from QList in QML:

I am so sorry, keep forgetting that there is no compile time error checking :(
You are right - it works also with Q_GADGET.

Don't worry a pit fall I myself fall into regularly :D

Could you also advise please, if qml copies the entire list to access one element?

That I'm unsure about, sorry. I know that QObjects can't be copied, and its also kind of irrelevant, as its a list of pointers :D

Just make sure to handle ownership of the objects correctly or the QML will take ownership and may GC them at any moment it sees fit.

[Atr0p0s]

@J-Hilk Got it. Thanks again and have a great day :)