qwt: howto use QVariant(QwtPlotItem*) for QwtPlotCurve->setVisible?

[Cedric-air]

I'm trying to make a graph where the user can choose what traces to have visible.
I do get a signal when the QwtLegend is clicked, but I can't figure out howto choose the correct trace to setVisible().

In the slot, I have these variables. On toggles as expected dependent if the label is pressed, but index stays 0. I don't know how to handle iteminfo to extract what tracelabel has just been changed.

const QVariant &iteminfo, bool on, int index

Here's the screenshot after the curve_A,B,C buttons have been clicked a couple of times. Note the visibility of the traces is out of sync with the buttons:

Here's the qdebug output of the program: when first curve_A is clicked two times, then curve_B is clicked two times, then curve_C is clicked two times.

dpi is invalid got from xsettings(Qt/DPI/ and Xft/DPI), fallback to get dpi from QXcbScreen::logicalDpi()
dpi is invalid got from xsettings(Qt/DPI/ and Xft/DPI), fallback to get dpi from QXcbScreen::logicalDpi()
dpi is invalid got from xsettings(Qt/DPI/ and Xft/DPI), fallback to get dpi from QXcbScreen::logicalDpi()
dpi is invalid got from xsettings(Qt/DPI/ and Xft/DPI), fallback to get dpi from QXcbScreen::logicalDpi()
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  true
index:  0
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  false
index:  0
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  true
index:  0
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  false
index:  0
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  true
index:  0
Function Name:  void MainWindow::slot_qwtlegendChecked(const QVariant&, bool, int)
Item:  QVariant(QwtPlotItem*, )
on:  false
index:  0

Here's the program.
Mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QwtPlot>
#include <QwtPlotCurve>
#include <QwtPlotGrid>
#include <QwtSymbol>
#include <QwtLegend>
#include <QDebug>
#include <QSettings>

#include <qwt_magnifier.h>
#include <qwt_plot_canvas.h>
#include <qwt_plot_magnifier.h>
#include <qwt_plot_zoomer.h>
#include <qwt_date_scale_draw.h>

#include <qdebug.h>
#include <qdatetime.h>

typedef enum
{
    CURVE_TF_A = 0,
    CURVE_TF_B,
    CURVE_TF_C,
    NUM_TF_CURVES,
}TF_CURVES_ENUM;

const QString TF_STRINGS [NUM_TF_CURVES] =
{
    "curve_A",
    "curve_B",
    "curve_C",
};

QwtPlot* plotTF0 = nullptr;
QwtPlotGrid* gridTF0 = nullptr;
QwtPlotCurve* myCurvesTF[NUM_TF_CURVES];
QPolygonF myPontsTF[NUM_TF_CURVES];
QwtLegend* myLegend0 = nullptr;

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    plotTF0 = new QwtPlot();
    gridTF0 = new QwtPlotGrid();
    myLegend0 = new QwtLegend();
    for (int curve = 0; curve < NUM_TF_CURVES; ++curve)
    {
        myCurvesTF[curve] = new QwtPlotCurve();
    }

    myLegend0 -> setDefaultItemMode(QwtLegendData::Checkable);
    plotTF0->insertLegend( myLegend0 );

    connect(myLegend0,&QwtLegend::checked,this,&MainWindow::slot_qwtlegendChecked);

    myPontsTF[CURVE_TF_A] << QPointF((qreal)0,(qreal)1);
    myPontsTF[CURVE_TF_A] << QPointF((qreal)1,(qreal)2);
    myPontsTF[CURVE_TF_A] << QPointF((qreal)2,(qreal)3);
    myPontsTF[CURVE_TF_B] << QPointF((qreal)0,(qreal)0.5);
    myPontsTF[CURVE_TF_B] << QPointF((qreal)1,(qreal)1.5);
    myPontsTF[CURVE_TF_B] << QPointF((qreal)2,(qreal)2.5);
    myPontsTF[CURVE_TF_C] << QPointF((qreal)0,(qreal)0.7);
    myPontsTF[CURVE_TF_C] << QPointF((qreal)1,(qreal)1.7);
    myPontsTF[CURVE_TF_C] << QPointF((qreal)2,(qreal)2.7);

    for (int i = 0; i < NUM_TF_CURVES; ++i)
    {
        myCurvesTF[i]->setSamples(myPontsTF[i]);
        myCurvesTF[i]->setTitle(TF_STRINGS[i]);
        //myCurvesTF[i]->setRenderHint( QwtPlotItem::RenderAntialiased, true );
        myCurvesTF[i]->attach(plotTF0);
        qint16 hue = (360/NUM_TF_CURVES) * i; //so each color is as far away as possible from other colors
        QColor color;
        color.setHsv(hue,255,255,128); //last number is alpha
        myCurvesTF[i]->setPen(color,2);
    }
    plotTF0->resize( 600, 400 );
    plotTF0->show();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::slot_qwtlegendChecked(const QVariant &iteminfo, bool on, int index)
{
    qDebug() << "Function Name: " << Q_FUNC_INFO;
    qDebug() << "Item: " << iteminfo;
    qDebug() << "on: " << on;
    qDebug() << "index: " << index;

    //(myCurvesTF)iteminfo->setVisible(on); ///home/werk/qwt/mainwindow.cpp:97: error: Member reference type 'const QVariant' is not a pointer; did you mean to use '.'? (fix available)
    myCurvesTF[index]->setVisible(on); //this works, but index is always 0
    plotTF0->replot();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

#include <qwt_plot.h> // for QwtPlotItem

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

private slots:
    void slot_qwtlegendChecked(const QVariant &itemInfo, bool on, int index);
};
#endif // MAINWINDOW_H

main.cpp

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

qwt.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17
CONFIG += qwt

win32: {
    INCLUDEPATH += C:\Qwt-6.2.0\include
    CONFIG(debug, debug|release) {
        message("Windows debug build")
        LIBS += -LC:\Qwt-6.2.0\lib\ -lqwtd
    }

    CONFIG(release, debug|release) {
        message("Windows release build")
        LIBS += -LC:\Qwt-6.2.0\lib\ -lqwt
    }
}

unix: {
    message("Linux build")
    LIBS += -lqwt
}

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

FORMS += \
    mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

[Cedric-air]

I forgot to post my versions:

$ uname -a
Linux cedric 6.6.1-arch1-1 #1 SMP PREEMPT_DYNAMIC Wed, 08 Nov 2023 16:05:38 +0000 x86_64 GNU/Linux
Qt 5.15.11 in PATH(system)
$ pacman -Q qt5-base
qt5-base 5.15.11+kde+r146-1
$ pacman -Q qwt
qwt 6.2.0-1

[Cedric-air]

@Cedric-air The QWT CpuPlot example* works, but I can't integrate it into my program.
https://github.com/opencor/qwt/tree/master/examples/cpuplot

I get the error ‘infoToItem’ was not declared in this scope in my slot definition:

void MainWindow::slot_qwtlegendChecked(const QVariant &itemInfo, bool onn, int index)
{
    qDebug() << "Function Name: " << Q_FUNC_INFO;
    qDebug() << "Iteminfo: " << itemInfo;
    qDebug() << "on: " << onn;
    qDebug() << "index: " << index;

    //QwtPlotItem* plotitem = QwtPlot::infoToItem(iteminfo); //error: cannot call member function ‘virtual QwtPlotItem* QwtPlot::infoToItem(const QVariant&) const’ without object
    //QwtPlotItem* plotItem = infoToItem( itemInfo ); //from cpuplot, error: ‘infoToItem’ was not declared in this scope

    qDebug() << "plotItem: " << plotItem;

    if (plotItem)
    {
        plotItem->setVisible(on);
    }
}

These are the includes in my program:

#include "mainwindow-cont.h"
#include "ui_mainwindow-cont.h"

#include <QwtPlot>
#include <QwtPlotCurve>
#include <QwtPlotGrid>
#include <QwtSymbol>
#include <QwtLegend>
#include <QDebug>
#include <QSettings>

#include <qwt_magnifier.h>
#include <qwt_plot_canvas.h>
#include <qwt_plot_magnifier.h>
#include <qwt_plot_zoomer.h>
#include <qwt_date_scale_draw.h>
#include <qwt_plot.h> //for infoToItem

#include <qdebug.h>
#include <qdatetime.h>

this is the connect line in CpuPlot.cpp

 connect( legend, SIGNAL(checked(const QVariant&,bool,int)),
        SLOT(legendChecked(const QVariant&,bool)) );

this is the slot definition in CpuPlot.cpp

void CpuPlot::legendChecked( const QVariant& itemInfo, bool on )
{
    QwtPlotItem* plotItem = infoToItem( itemInfo );
    if ( plotItem )
        showCurve( plotItem, on );
}

void CpuPlot::showCurve( QwtPlotItem* item, bool on )
{
    item->setVisible( on );

    QwtLegend* lgd = qobject_cast< QwtLegend* >( legend() );

    QList< QWidget* > legendWidgets =
        lgd->legendWidgets( itemToInfo( item ) );

    if ( legendWidgets.size() == 1 )
    {
        QwtLegendLabel* legendLabel =
            qobject_cast< QwtLegendLabel* >( legendWidgets[0] );

        if ( legendLabel )
            legendLabel->setChecked( on );
    }

    replot();
}

this is in CpuPlot.h

private Q_SLOTS:
    void legendChecked( const QVariant&, bool on );

[JonB]

@Cedric-air said in qwt: howto use QVariant(QwtPlotItem*) for QwtPlotCurve->setVisible?:

//(myCurvesTF)iteminfo->setVisible(on); ///home/werk/qwt/mainwindow.cpp:97: error: Member reference type 'const QVariant' is not a pointer; did you mean to use '.'? (fix available)

I don't know about the rest of what you have written, but if you were intending to use the const QVariant &iteminfo parameter did you try correcting your C++ just as it says?

Having said that, you are not going to be able to call a method like setVisible() on a const variable. Where did you get told that the signal/slot signature should be const QVariant&, bool, int? What is this parameter supposed to represent?

[Cedric-air]

@JonB said in qwt: howto use QVariant(QwtPlotItem*) for QwtPlotCurve->setVisible?:

Having said that, you are not going to be able to call a method like setVisible() on a const variable. Where did you get told that the signal/slot signature should be const QVariant&, bool, int? What is this parameter supposed to represent?

https://qwt.sourceforge.io/class_qwt_legend.html

I didn't know const was optional there. I'll try to see if that makes a difference.

[JonB]

@Cedric-air
No, that shows it is not optional, it shows that you should not be trying to alter itemInfo in the signal/slot. But your commented out/erroneous code was trying to set that visible.