QwtPlot MVC

Hello,

I have a QwtPlotCurve attached to QwtPlot. The curve is made up of a large number of QPointF's. I want to keep only one copy of these points in memory(so I need to avoid deep copies). Hence I chose QwtPointSeriesData(Vector<QPointF>&). I use QwtPlotCurve::setSamples() to make sure my curve reflects any changes that happen to the vector of points. I provide appropriate boundingRect() and everything works fine.

Code snippet:

Code:

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m_mainPlot = new QwtPlot(this);
curve1        = new QwtPlotCurve("Curve 1");
QVector<QPointF>    m_2dPointsVector;
curve1->setSamples(QwtPointSeriesData(m_2dPointsVector));

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I want to use Qt's Model-View pattern and create a Graph which acts like my View. The graph which is a QWidget has a child QwtPlot. Create a QAbstractListModel to hold the data: QVector<QPointF> m_2dPointsVector;. Everytime this data changes the QwtPlotCurve updates itself as I described before. But to reflect this change in my Graph I need to listen to the dataChanged() signal from the QAbstractListModel. Inside the slot I call the QwtPlot::replot(). This works fine!

My Question is:
I think I am not doing it right since the graph is a normal widget which has a pointer to the QAbstractListModel and listen's to its dataChanged() signal. Is there a cleaner way of doing this so that I can make the best use of Qt's Model-View pattern?

Quote:

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Originally Posted by Vikram.Saralaya

Is there a cleaner way of doing this so that I can make the best use of Qt's Model-View pattern?

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The model API is just too slow/fat for iterating over many points and the concept of working around type checks of the compiler by hiding the data behind a generic API does not make much sense, when every class knows that it is a QVector<QPointF>.
So IMHO the best way to use Qt's Model-View pattern for this particular use case is - not to use it. If you want to have a MVC design for your plot better implement your own without using QAbstractItemModel.

Just my 2 cents,
Uwe

Hello Uwe,

That's indeed very helpful. After playing with it for a while I did strip off all the model view constructs of Qt.

I treat QwtPointSeriesData as my model now. QwtPlotCurve is my view. QVector<QPointF> is my data. All the changes to my data is obtained by the curve since it references to it. But to update the curve I need to call QwtPlot::replot(). Do you think I need to emit a signal when data changes and call replot() manually? or is there a way the model-view structure used internally in Qwt can provide a refresh/replot ?!

Regards
Vikram

The auto-replot feature ( QwtPlot::autoReplot() ) is triggered whenever QwtPlotItem::itemChanged() is called. This happens f.e when assigning a new data set.
When you plan to change the data behind the back of the QwtPointSeriesData you would have to add some sort of notification, so that itemChanged() gets called.

But I would be careful with any auto-update patterns - look always nice in books, but in reality you soon lose control over the program flow and when expensive operations are involved ( like replot of heavy plots ) it might end in low performance.

Uwe