paintEvent is not painting complete objects

NIXIN

I am trying this piece of code below:

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QLabel>
#include <QPainter>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
Q_OBJECT

public:
explicit Widget(QWidget *parent = 0);
~Widget();

void paintEvent(QPaintEvent *e);
void operation();
void drawLabel();

int flag = 0;
int startX, startY;
QString labelText;

private:
Ui::Widget *ui;
};

#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
QWidget(parent),
ui(new Ui::Widget)
{
ui->setupUi(this);
}

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

void Widget::paintEvent(QPaintEvent *e)
{
QPainter *painter = new QPainter(this);
if(flag == 1)
{
painter->setPen(QPen(Qt::black, 1));
painter->drawLine(startX, startY, startX+30, startY);
painter->drawLine(startX+50, startY, startX+80, startY);
drawLabel();
}
}

void Widget::drawLabel()
{
QLabel *label = new QLabel(this);
label->setStyleSheet("border: 1px solid black");
label->setGeometry(startX+30, startY-10, 20, 20);
label->setText(labelText);
label->show();
}

void Widget::operation()
{
int found = 1;

if(found)
{
    labelText = "2";
    startX = 20;
    startY = 50;
    flag = 1;
    update();

    labelText = "3";
    startX = 100;
    startY = 50;
    flag = 1;
    update();
}

}

main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.operation();
w.show();

return a.exec();

}

I want to draw a label between two lines side by side, but when I run the above code label with labelText 3, between two lines is visible. labelText 2, between two lines is not painted. I don,t understand why? Can anybody help me to solve this?

raven-worx

@NIXIN
you are doing multiple mistakes in your code.

you are creating widgets on every paintEvent call! This is very bad design.
in your Widget::operation() you update your data and call update() to immediately set the data again and call update() again. You should know that update() schedules a paint-event, where multiple calls to update() still result in a single paint-event (in the next event loop iteration). So it doesn't repaint immediately. There is a repaint() method for this, but as i said you already have a bad design by creating widgets on every paintEvent call.

You should draw the text using QPainter instead creating a QLabel every time. But still the label text "2" would never be rendered, since you immediately overwrite it afterwards.

NIXIN

can you provide a small piece of code, how to do it?

raven-worx

@NIXIN

void Widget::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);  // do create the painter on the stack
    ....
    drawLabel( p );
}

void Widget::drawLabel(QPainter & p)
{
QRect rect(startX+30, startY-10, 20, 20);
p.setPen( QPen::QPen(Qt::black, 1.0, Qt::SolidLine) );
p.setBrush( Qtt::NoBrush );
p.drawRect( rect );
p.drawText( rect, Qt::AlignCenter, labelText );
}

But as i said, this only draws the last label text set. You should store your data (position, label text,...) in a list (and call update() once). And draw each using drawLabel() for example.

NIXIN

That's fine, what I want to know is how to not delete the drawing from previous paintevent,
because as per your statement:
" But still the label text "2" would never be rendered, since you immediately overwrite it afterwards."
I want to keep the drawing for both:

if(found)
{
labelText = "2";
startX = 20;
startY = 50;
flag = 1;
update();

labelText = "3";
startX = 100;
startY = 50;
flag = 1;
update();

}

raven-worx

@NIXIN
as i said store your data (e.g. struct) in a list and draw each item separately while iterating over the list in the paintEvent()

NIXIN

Can you please provide a small code

raven-worx

@NIXIN
come on thats not that hard...

struct Data {
    Data(QString t, int x, int y, int f)
         : labelText(t), startX(x), startY(y), flag(f)
    {
    }

    QString labelText;;
    int startX;;
    int startY;
    int flag;
}
QList<Data> dataList;
...
void Widget::operation()
{
int found = 1;

if(found)
{
    dataList.clear();

    dataList << Data("2", 20, 50, 1);
    dataList << Data("3", 100, 50, 1);
    update();
}

}

void Widget::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);  // do create the painter on the stack
    ....
    foreach( Data d, dataList )
         drawLabel( p, data );
}

void Widget::drawLabel(QPainter & p, const Data & d)
{
QRect rect(d.startX+30, d.startY-10, 20, 20);
p.setPen( QPen::QPen(Qt::black, 1.0, Qt::SolidLine) );
p.setBrush( Qt::NoBrush );
p.drawRect( rect );
p.drawText( rect, Qt::AlignCenter, d.labelText );
}

Jan-Willem

Perhaps this might be also useful?

http://www.cplusplus.com/doc/tutorial/structures/

http://doc.qt.io/qt-5/qlist.html

VRonin

How about composing the widget instead of painting it.
Put two QFrame with a HLine style and a label in a QVBoxLayout. So much easier!

Jan-Willem

@VRonin Depends on what he wants to achieve.
Is it a widget, than I agree with you.
Is it some CAD-drawing (looks like a diagram symbol to me), then I would prefer drawing.
For the latter the Graphics View Framework would be more practical though.