QwtLegend: different behaviour when inserting a legend after/before adding curves

[agarny]

Ok, I have found out why I am having the behaviour I described above. It has to do with my calling QWidget::setUpdatesEnabled() on my QwtPlot object. Indeed, I customise quite a few things in my QwtPlot object, so I thought I would temporarily prevent it from updating itself. In other words, I have something like this:

myPlot->setUpdatesEnabled(false);
...
myPlot->insertLegend(addLegend?new QwtLegend(myPlot):0);
...
myPlot->setUpdatesEnabled(true);

myPlot->setUpdatesEnabled(false);
...
myPlot->insertLegend(addLegend?new QwtLegend(myPlot):0);
...
myPlot->setUpdatesEnabled(true);

To copy to clipboard, switch view to plain text mode 

This can easily be reproduced in the simpleplot example:

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_symbol.h>
#include <qwt_legend.h>
 
int main( int argc, char **argv )
{
    QApplication a( argc, argv );
 
    QwtPlot plot;
plot.setUpdatesEnabled(false);
    plot.setTitle( "Plot Demo" );
    plot.setCanvasBackground( Qt::white );
    plot.setAxisScale( QwtPlot::yLeft, 0.0, 10.0 );
    plot.insertLegend( new QwtLegend() );
 
    QwtPlotGrid *grid = new QwtPlotGrid();
    grid->attach( &plot );
 
    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setTitle( "Some Points" );
    curve->setPen( Qt::blue, 4 ),
    curve->setRenderHint( QwtPlotItem::RenderAntialiased, true );
 
    QwtSymbol *symbol = new QwtSymbol( QwtSymbol::Ellipse,
        QBrush( Qt::yellow ), QPen( Qt::red, 2 ), QSize( 8, 8 ) );
    curve->setSymbol( symbol );
 
    QPolygonF points;
    points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
        << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
        << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
    curve->setSamples( points );
 
    curve->attach( &plot );
plot.setUpdatesEnabled(true);
 
    plot.resize( 600, 400 );
    plot.show();
 
    return a.exec();
}

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_symbol.h>
#include <qwt_legend.h>

int main( int argc, char **argv )
{
    QApplication a( argc, argv );

    QwtPlot plot;
plot.setUpdatesEnabled(false);
    plot.setTitle( "Plot Demo" );
    plot.setCanvasBackground( Qt::white );
    plot.setAxisScale( QwtPlot::yLeft, 0.0, 10.0 );
    plot.insertLegend( new QwtLegend() );

    QwtPlotGrid *grid = new QwtPlotGrid();
    grid->attach( &plot );

    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setTitle( "Some Points" );
    curve->setPen( Qt::blue, 4 ),
    curve->setRenderHint( QwtPlotItem::RenderAntialiased, true );

    QwtSymbol *symbol = new QwtSymbol( QwtSymbol::Ellipse,
        QBrush( Qt::yellow ), QPen( Qt::red, 2 ), QSize( 8, 8 ) );
    curve->setSymbol( symbol );

    QPolygonF points;
    points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
        << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
        << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
    curve->setSamples( points );

    curve->attach( &plot );
plot.setUpdatesEnabled(true);

    plot.resize( 600, 400 );
    plot.show();

    return a.exec();
}

To copy to clipboard, switch view to plain text mode 

So, is that behaviour 'normal'? I mean, I would really like to be able to make calls to QWidget::setUpdatesEnabled() whenever I see fit, so how can I go about it without messing up my legend?

[agarny]

Ok, I have 'fixed' it. In the end, the 'problem' is with QwtLegendLabel::setData(). Indeed, it disables updates, but then only reenables them if it was originally enabling them. Now, because my application temporarily disable them at some point, QwtLegendLabel doesn't reenable updates, hence the 'blank' legend I mentioned in my original message.

To address this issue, I had to something like:

class MyLegend : public QwtLegend
{
    Q_OBJECT
 
public:
    explicit MyLegend(QWidget *pParent) :
        QwtLegend(pParent)
    {
    }
 
protected:
    virtual void updateWidget(QWidget *pWidget,
                              const QwtLegendData &pLegendData)
    {
        QwtLegend::updateWidget(pWidget, pLegendData);
 
        pWidget->setUpdatesEnabled(true);
    }
};

class MyLegend : public QwtLegend
{
    Q_OBJECT

public:
    explicit MyLegend(QWidget *pParent) :
        QwtLegend(pParent)
    {
    }

protected:
    virtual void updateWidget(QWidget *pWidget,
                              const QwtLegendData &pLegendData)
    {
        QwtLegend::updateWidget(pWidget, pLegendData);

        pWidget->setUpdatesEnabled(true);
    }
};

To copy to clipboard, switch view to plain text mode 

I wish I didn't have to do this, but at least my application is now working as expected.

[Uwe]

On my box ( Linux/X11 Qt-5.9.1 ) disabling the updates of the plot leads to not painting anything at all - f.e the simpleplot example comes with an empty window frame.
But this is the expected behavior: if you explicitly disable automatic updates you have to call update manually and of course you need to know what to update.

But does disabling updates totally make sense in your case ? Usually only repainting of the canvas should be expensive and if you really have unwanted replots - triggered by update - it might be better to disable updates for the canvas only. Or maybe introducing your own flag, that could be handled in YourPlot::replot().

Uwe

[agarny]

On my box ( Linux/X11 Qt-5.9.1 ) disabling the updates of the plot leads to not painting anything at all - f.e the simpleplot example comes with an empty window frame.
But this is the expected behavior: if you explicitly disable automatic updates you have to call update manually and of course you need to know what to update.

Note that in 'my' version of the simpleplot example, I reenable updates by calling setUpdatesEnabled(true) on the QwtPlot object, which results in update() being automatically called on the QwtPlot object. Thus, on Ubuntu 16.04 and with Qt 5.9.3, I am getting the expected result when it comes to the plot, just not the legend (which is 'blank').

But does disabling updates totally make sense in your case ? Usually only repainting of the canvas should be expensive and if you really have unwanted replots - triggered by update - it might be better to disable updates for the canvas only. Or maybe introducing your own flag, that could be handled in YourPlot::replot().

I appreciate what you are saying, but there are two cases where I am currently calling setUpdatesEnabled():

1. I need to customise a plot by modifying several of its properties (line, symbols, colours, etc.). Here, I agree that your approach would work.
2. I need to customise a 'big' widget that can contain one or several plots (among other things). Because that widget is relatively 'heavy', I call setUpdatesEnabled() on it to disable updates, so that I can do my customisation. Once I am done with my customisation, I reenable updates on my 'big' widget. If I wasn't to do that, I could risk some parts of my widget flashing, which is really ugly in some cases.

So, although my first case could easily be addressed (using, for example, the approach you described), my second case is much more complex and not worth spending time on it since calling setUpdatesEnabled() to disable and then reenable updates works fine, just not when a QwtPlot object's legend... but I have now got it to work by 'manually' reenabling updates on my QwtPlot object's legend items.

[Uwe]

I need to customise a plot by modifying several of its properties (line, symbols, colours, etc.).

Well, update() only schedules a paint event ( in opposite to repaint what has an immediate effect ). So as long as you don't return to event loop you usually don't win anything by disabling updates.
Also the plot canvas usually has a backing store and as long as the size of the canvas is not changing and you don't invalidate the backing store ( by replot ) manually nothing expensive will happen.

I need to customise a 'big' widget that can contain one or several plots (among other things). Because that widget is relatively 'heavy', I call setUpdatesEnabled() on it to disable updates, so that I can do my customisation. Once I am done with my customisation, I reenable updates on my 'big' widget.

Instead of calling the final update you could call replot on your plots. If you don't want to do this you could try to send QEvent::LayoutRequest to the plots instead. Those events will let the plots recalculate their layout - what should be the missing part to give your legend item a proper geometry. But as soon as you have a different layout, the size of the plot canvas changes and ...

Uwe