problem with QwtPlotCanvas::paintEvent

[MickyJhon]

hello all,
i want to paint some thing (in this example , i do nothing) to a QwtPlotCanvas, so i subclass MyCanvas, in class MyCanvas constructor function , i configure object and implement paintEvent, However, the displaying QwtPlotCanvas is not correct as my except . Wrong result.JPG

my code is listed as following.

in mycanvas.h

#ifndef MYCANVAS_H
#define MYCANVAS_H
#include <qwt_plot_canvas.h>
#include<QPainter>
 
class MyCanvas:public QwtPlotCanvas
{
public:
    MyCanvas();
    void paintEvent(QPaintEvent *event);
};
#endif // MYCANVAS_H

#ifndef MYCANVAS_H
#define MYCANVAS_H
#include <qwt_plot_canvas.h>
#include<QPainter>

class MyCanvas:public QwtPlotCanvas
{
public:
    MyCanvas();
    void paintEvent(QPaintEvent *event);
};
#endif // MYCANVAS_H

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in mycanvas.cpp

#include "mycanvas.h"
 
MyCanvas::MyCanvas()
{
    this->setBorderRadius( 15 );
}
void MyCanvas::paintEvent(QPaintEvent *event)
{
}

#include "mycanvas.h"

MyCanvas::MyCanvas()
{
    this->setBorderRadius( 15 );
}
void MyCanvas::paintEvent(QPaintEvent *event)
{
}

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in mainwindow.h

#define MAINWINDOW_H
#include <QMainWindow>
#include"mycanvas.h"
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();    
private:
    Ui::MainWindow *ui;
    MyCanvas * canvas;
};
#endif // MAINWINDOW_H

#define MAINWINDOW_H
#include <QMainWindow>
#include"mycanvas.h"
namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();    
private:
    Ui::MainWindow *ui;
    MyCanvas * canvas;
};
#endif // MAINWINDOW_H

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in mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <qwt_plot_canvas.h>
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    canvas(new MyCanvas)
{
    ui->setupUi(this);
    ui->qwtPlot->setCanvas(canvas);
}
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <qwt_plot_canvas.h>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    canvas(new MyCanvas)
{
    ui->setupUi(this);
    ui->qwtPlot->setCanvas(canvas);
}
MainWindow::~MainWindow()
{
    delete ui;
}

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any suggestions？

[Uwe]

Your code has to be:

void MyCanvas::paintEvent(QPaintEvent *event)
{
    QwtPlotCanvas::paintEvent( event );
 
     // now do your stuff 
}

void MyCanvas::paintEvent(QPaintEvent *event)
{
    QwtPlotCanvas::paintEvent( event );

     // now do your stuff 
}

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But I'm very sure that you are trying to do something the wrong way. In almost 100% of the use cases it is better to derive a new type of QwtPlotItem that can be attached to the plot scene.

Uwe

[MickyJhon]

Thank you very much, Uwe, Your advice is very useful .

Your code has to be:

void MyCanvas::paintEvent(QPaintEvent *event)
{
    QwtPlotCanvas::paintEvent( event );
 
     // now do your stuff 
}

void MyCanvas::paintEvent(QPaintEvent *event)
{
    QwtPlotCanvas::paintEvent( event );

     // now do your stuff 
}

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But I'm very sure that you are trying to do something the wrong way. In almost 100% of the use cases it is better to derive a new type of QwtPlotItem that can be attached to the plot scene.

Uwe