Changing the properties of the chart from Qt Widget Application

pirates21

Hello

In my "Qt Widget Application I load a QML object, which is a graph:

ChartView {
title: "Line"
anchors.fill: parent
antialiasing: true

LineSeries {
    name: "LineSeries"
    XYPoint { x: 0; y: 0 }
    XYPoint { x: 1.1; y: 2.1 }
    XYPoint { x: 1.9; y: 3.3 }
    XYPoint { x: 2.1; y: 2.1 }
    XYPoint { x: 2.9; y: 4.9 }
    XYPoint { x: 3.4; y: 3.0 }
    XYPoint { x: 4.1; y: 3.3 }
}

}

I would like to change the values of the "LineSeries" chart from my application.

In other QML objects, I do it in this way:
QQmlProperty(ui->quickWidget->rootObject(),"currentValue").write(variable);

How do you change the "LineSeries" values in the same way?

Best Regards

oria66

Hi @pirates21. In order to send data to qml components in a Qt Widget Application, you must obtain the QQuickItem* pointer, then you can use two ways (it's possible that exists others but I used these two):

You can use the SIGNAL/SLOT mechanism.
You can use the QMetaObject::invokeMethod(...).

Please refer to http://doc.qt.io/qt-5/qtqml-cppintegration-interactqmlfromcpp.html.

Good luck.

pirates21

@oria66 : thank you for your answer. Could you give an example that would refer to lineseries in QML?

raven-worx

@pirates21
as the docs state LineSeries instantiates QLineSeries.
So you need a reference to your LineSeries object.

QML:

ChartView {
    title: "Line"
    anchors.fill: parent
    antialiasing: true

   property alias  lineSeries: series // <<<<<<<<<<

    LineSeries {
        id: series
        ....
    }
}

Then in C++:

QObject* lineSeriesObj = QQmlProperty(ui->quickWidget->rootObject(),"lineSeries").read().value<QObject*>();
QLineSeries* lineSeries = qobject_cast<QLineSeries*>(lineSeriesObj);
    lineSeries->clear();
    lineSeries->append( ... );

Note that i've haven't tested this code. I don't know if thats enough to also let the chart view also update itself.

pirates21

@raven-worx: You are Genius! Thank You very much!

GrecKo

What @raven-worx showed is not recommended (cf. http://doc.qt.io/qt-5/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c & https://youtu.be/vzs5VPTf4QQ?t=23m20s & https://doc.qt.io/qt-5/qtquick-bestpractices.html#interacting-with-qml-from-c)

Accessing a QML object from C++ add unnecessary coupling. Your c++ business layer should merely expose data that is then used by the QML layer.

Here what you could do instead is expose a c++ QAbstractListModel, and use that in QML with a VXYModelMapper.

Alternatively, you could pass your line series to your c++ layer via a slot (or invokable function) and it will then populate it. Your c++ code will still modify a qml object, but at least it didn't have to retrieve it by itself. It's less than ideal but still better.