Qt Creator: query on GraphsView and C++ model interaction:

avio

Hello! I am new to Qt. I "simply" want to plot points in a x-y axis coordinate system by using the model-view concept.
Having attended some relevant Qt Academy on line courses, the idea is as follows:

C++ code to generate the points and stores them in a model that inherits from QAbstractListModel
the GraphsView is in charge of plotting the x-axis and y-axis, retrieving the points from the C++ model and plotting them.
I have created my graphsview and my C++ model. The latter has a QScatterSeries and is correctly populated with points by the C++ code. (I have used the beging/endinsertRows, etc.). A nice x-y axis system is rendered on the screen, if I run my application, but no points from the model are rendered. (If I draw a point defined in the QML graphsview itself, that point is rendered as expected.)
I "believe" I have correctly exposed to QML). Debug logging shows that QML calls "int rowCount(.......)" but it does not call QVariant data(.......) to retrieve the points to plot: hence, no points are rendered.
I have found no example for this in the qt examples and qt academy courses (they deal with ListViews). Have I missed something? Has anyone got a code example I could have a look at to understand what I am doing wrong? Thanks.

JKSH

Hi, and welcome!

@avio said in Qt Creator: query on GraphsView and C++ model interaction::

C++ code to generate the points and stores them in a model that inherits from QAbstractListModel

I would make your custom class inherit QXYSeries or QScatterSeries instead of QAbstractListModel: https://doc.qt.io/qt-6/qxyseries.html

That way, you don't need to transfer data between your model and your graph. Simply add your custom series to the GraphsView, and your graph will automatically update whenever you call append()/replace()/clear() on your series.

If you need further assistance, please share your code.

avio

@JKSH Many thanks, your help is very much appreciated. I have followed your advice: excellent result! I guess this approach should also work well for real-time data streaming from the C++ backend.
Before I close this conversation, I have a question, though, if I may, about the view-model approach I originally attempted. Surely, that must work too. Further debugging has shown that QML does call "QVariant data(const QModelIndex &index, int role) with a valid "index". Yet, "role" has got "random" values nowhere near the ones (257, 258) in the enum role below

class List2dPlotModel : public QAbstractListModel
{ Q_OBJECT
  QML_ELEMENT
public:
  enum Roles {XRole = Qt::UserRole + 1, YRole};
  ......
private:
  QScatterSeries allPoints;
};

QHash<int, QByteArray> roleNames() {
  QHash<int, QByteArray> roles;
  roles[XRole] = "x";
  roles[YRole] = "y";
  return roles;
}

I have seen examples of how this mapping (i.e., string-datum) works for "simple" class members (i.e., not array-like), but not one, so far, that deals with vectors or series. My "roles" also does not seem to make much sense to me, really, because, in principle, I guess the QML GraphsView should 1) be told it is dealing with "a" series (i.e., without knowing it's precisely my allPoints) of QPointfs, and 2) ask C++ for a point and 3) expect "QVariant Data(......)" to return a point. Series can indeed be added to the GraphsView. Parameter index"(.row() could do the job without using any "roles"? I am clearly missing something. Could you please give me at least a hint as to how to solve this QML-C++ mapping problem? Thanks.

JKSH

@avio said in Qt Creator: query on GraphsView and C++ model interaction::

@JKSH Many thanks, your help is very much appreciated. I have followed your advice: excellent result! I guess this approach should also work well for real-time data streaming from the C++ backend.

Great to hear! You're welcome.

Before I close this conversation, I have a question, though, if I may, about the view-model approach I originally attempted. Surely, that must work too. Further debugging has shown that QML does call "QVariant data(const QModelIndex &index, int role) with a valid "index". Yet, "role" has got "random" values nowhere near the ones (257, 258) in the enum role below

We'll need to see your QML code (how you integrate your model with the GraphsView) to be able to say what went wrong. Seeing your implementation of List2dPlotModel::data() could also be helpful.

avio

Hello @JKSH ! Here is the code...that shows my Qt inexperience. It displays the x-y system and the test point: nothing else. I have tried a few things, played with the Mapper and its values (the value of "series" below is definitely wrong, since I get an "unable to assign [undefined] to QXYSeries*) [some model-view "links" are clearly not in place], but all to no avail. Any suggestion will be appreciated. All I want to do is to use the view-model approach with QML asking the C++ backend for data. [I might have to divide this reply in segments in order to prevent "Akismet.com" to block the whole message as spam....]
List2dPlotModel. h below

class List2dPlotModel : public QAbstractListModel
{
    Q_OBJECT
public:
    enum Roles {XRole = Qt::UserRole + 1, YRole};
    explicit List2dPlotModel(QObject* parent = nullptr);

    int rowCount(const QModelIndex& parent = QModelIndex()) const override {
        qDebug() << "rowCount() called, count =" << allPoints.count();
        return allPoints.count();
    }

    QVariant data(const QModelIndex &index, int role) const override {
        qDebug() << "entered data():" << "role=" << role  << "index=" << index;
        if (!index.isValid()) {
            qDebug() << "modelindex NOT valid:";
            return {};
        }
        int idx = index.row();
        qDebug() << "   index.row()=" << idx << "  col=" << index.column() << "   parent=" << index.parent();
        if (idx < 0 || idx>= allPoints.count()) {
            qDebug() << "   Bad idx! ";
            return {};
        }
        QPointF thePoint = allPoints.at(idx);
        if (role == XRole) return thePoint.x();
        if (role == YRole) return thePoint.y();
        qDebug() << "   Bad role!";
        return{};
    }

    QHash<int, QByteArray> roleNames() const override {
        qDebug() << "entered roleNames(): XRole=" << XRole << "YRole=" << YRole;
        QHash<int, QByteArray> roles;
        roles[XRole] = "x";
        roles[YRole] = "y";
        return roles;
    }

    void appendPoint(qreal x, qreal y);

private:
    //Series that contains the points to be rendered by a GraphsView.
    QScatterSeries allPoints;
};

list2dplotmodel.cpp below:


List2dPlotModel::List2dPlotModel(QObject *parent)
  : QAbstractListModel(parent)
{
    qDebug() << "ctor executed";
}

void List2dPlotModel::appendPoint(qreal x, qreal y)
{
    int rowIdx = allPoints.count();
    beginInsertRows(QModelIndex(), rowIdx, rowIdx);
    allPoints.append({x, y});
    endInsertRows();
    qDebug() << "Appended point:" << allPoints.at(rowIdx);
}

main.cpp below:

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    List2dPlotModel twoDPlotModel{};

    twoDPlotModel.appendPoint(0.5, 0.8);
    twoDPlotModel.appendPoint(0.2, -0.8);
    twoDPlotModel.appendPoint(0.8, 1);

    QAbstractItemModelTester(&twoDPlotModel,
        QAbstractItemModelTester::FailureReportingMode::Fatal,
        nullptr);

    QQmlApplicationEngine engine;
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreationFailed,
        &app,
        []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.rootContext()->setContextProperty("plotModel", &twoDPlotModel);
    engine.loadFromModule("plot2dWithCppModel", "Main");

    return app.exec();
}

Main.qml below:

import QtQuick
import QtGraphs
import QtQml
import QtQuick.Controls
import plot2dWithCppModel

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Sine function")

    GraphsView {
        id: myView
        anchors.fill: parent

        axisX: ValueAxis {
            min: 0
            max: 3
            titleText: "x"
            gridVisible: true
        }

        axisY: ValueAxis {
            min: -1.5
            max: 1.5
            titleText: "sin(x)"
            gridVisible: true
        }

        Component {
            id: xyBufferMarker
            Rectangle {
                width: 6
                height: 6
                color: "red"
                border.color: "black"
                border.width: 1
                radius: 3
            }
        }

        ScatterSeries {
            id: mySeries
            visible: true
            color: "#00ff00"
            pointDelegate: xyBufferMarker
            XYPoint {x:0.5; y:0.8}  // Rendering test.
            function logData() {
                console.log("data:" + myView.seriesList)  //debug purposes.
            }
        }

        XYModelMapper {
            count: 0
            model: plotModel
            series: plotModel.allPoints //Where is the model data abstraction, then??
            xSection: 0  // Not too sure where I can find the definition of section.......
            ySection: 0
        }
    }

    Button {
        text: "Click to see debug data"
        onClicked: {
            mySeries.logData();
            }
    }
}

avio

@JKSH Here's the second part of the message. [uploaded file, again to prevent "Akismet.com" to regard my post as spam and block it. Slightly annoying....] Many thanks.
Debug output:

jeremy_k

@avio said in Qt Creator: query on GraphsView and C++ model interaction::

Further debugging has shown that QML does call "QVariant data(const QModelIndex &index, int role) with a valid "index". Yet, "role" has got "random" values nowhere near the ones (257, 258) in the enum role below

Those roles are defined by the model-view framework, rather than random:
https://doc.qt.io/qt-6/qt.html#ItemDataRole-enum

JKSH

@avio said in Qt Creator: query on GraphsView and C++ model interaction::

series: plotModel.allPoints //Where is the model data abstraction, then??

XYModelMapper reads data from a source table model and writes it to a destination/target XYSeries. XYModelMapper.model is the source, and XYModelMapper.series is the destination.

So, what you want is series: mySeries

xSection: 0  // Not too sure where I can find the definition of section.......

It's a bit obscure, but when talking about Qt models, "section" refers to "row" or "column" depending on the orientation. See https://doc.qt.io/qt-6/qabstractitemmodel.html#headerData for example.

For a quick sanity check: Ensure that your model's data can be displayed by a TableView (see https://doc.qt.io/qt-6/qml-qtquick-tableview.html#example-usage ). If TableView can't see your data, then XYModelMapper can't either.

if (role == XRole) return thePoint.x();
if (role == YRole) return thePoint.y();

XYModelMapper operates on columns (sections), so you don't need to define any custom roles. Just use the default Qt::DisplayRole. What you want is a setup like this:

Column 0 gives x value
Column 1 gives y value
Row N corresponds to allPoints.at(N)

But at the end of the day, your data flow is now Data -> inner QScatterSeries -> List2dPlotModel -> XYModelMapper -> outer QScatterSeries -> GraphsView which forms a Rube Goldberg machine.

What you want is Data -> QScatterSeries -> GraphsView

avio

@jeremy_k , @JKSH , thanks for your comments and observations. some good food for thought. I think this query can now be regarded as solved, since the originally proposed solution works.