Changing the content of a QVector in a C++ model doesn’t trigger the QML bindings between the model and the custom C++ component used as a delegate

JL SABATIER

Good Morning,

Here is my problem: Changing the content of a QVector in a C++ model doesn’t trigger the QML bindings between the model and the custom C++ component used as a delegate

I’ve got a QVector in my C++ model. I’ve got a C++ custom component based on a QQuickPaintedItem.
Adding a value in the vector of the model doesn’t trigger the bindings between the model and the component.

Context :

I’m trying to draw the successive results from a measurement instrument.
This makes a curve displayed with a QQuickPaintedItem.

I’ve got a « VolmeterChannel » class, in which I’m defining a « results » property that is accessible to the QML.
(NB : my Voltmeter class has two channels, managed by two member classes VoltmeterChannel).

Class VoltmeterChannel : public QObject {
    Q_OBJECT
    …
    Q_PROPERTY (QVector<QPointF> results READ get_results WRITE set_results NOTIFY resultsChanged)
    …
    QVector<QPoint> m_results ;
    …
}

Every time I get a result from the Voltmeter, I’m adding the last measurement to my result vector :

void VoltmeterChannel::setResult(int channel, qreal result) {
    static int x=0;        
    if (m_results.count() >= NB_SAMPLES)
        m_results.removeLast();
    m_results(QPointF(x, result));
    set_results(m_results);
    ++x;
    emit resultsChanged(m_results);
}

The QML bindings are as follow :

                    Repeater {
                        model: [
                            Global.voltmeter.channel1,
                            Global.voltmeter.channel2,
                        ];
                        delegate: PaintedGraph {
                            id: _graph;
                            xMin: _graph.object.xMin;
                            xMax: _graph.object.xMax;
                            yMin: _graph.object.yMin;
                            yMax: _graph.object.yMax;
                            points: _graph.object.results;
                            visible: _graph.object.selected;
                            color: {
                                switch (_graph.object.number) {
                                case VoltmeterSelection.CHANNEL_1: return "#FEDB00";
                                case VoltmeterSelection.CHANNEL_2: return "#001489";
                                }
                                return "gray";
                            }
                            anchors {
                                fill: parent;
                                topMargin: Global.theme.spaceM;
                                leftMargin: 2;
                                rightMargin: 2;
                                bottomMargin: Global.theme.spaceM;
                            }

                            readonly property VoltmeterChannel object: modelData;
                        }
                    }

I found a « quick and dirty » fix to that problem, by reconstructing a whole QVector every time and using setResults(results).

Then, the QML bindings are triggered and the curve is drawn.
This is working but I don’t want to copy twice every value from the QVector every time I get a new result, as this is very bad coding…

So, how could I program this so that adding a new value in the QVector will be signaled to the QQuickPaintedItem so it will continue drawing the curve?

JKSH

@JL-SABATIER said in Changing the content of a QVector in a C++ model doesn’t trigger the QML bindings between the model and the custom C++ component used as a delegate:

Adding a value in the vector of the model doesn’t trigger the bindings between the model and the component.

That is expected.

The QML bindings are not triggered by modifying a variable. The QML bindings are triggered by the NOTIFY signal.

how could I program this so that adding a new value in the QVector will be signaled to the QQuickPaintedItem so it will continue drawing the curve?

Emit your resultsChanged() signal every time a new value is added.

JL SABATIER

Thanks for your reply and for pointing me out where to look for the bug.
I was trying to emit the signal, but there was a bug in the code. Which is now fixed. Thanks again.

JKSH

@JL-SABATIER You're most welcome. Happy coding!