paintEvent help

[seltra]

I want to draw an ellipse when i click to pushbutton.

i write this signal-slots:
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(paintEvent()));


then as paintevent:
void MainWindow:aintEvent(QPaintEvent *pe)
{
QPainter painter(this);
painter.drawEllipse(20,20,100,100);
}

when i run program. it builds successfully.
but it draws ellipse automatically. program doesn't need to click pushbutton.
and gives that message "QPainter::begin: Paint device returned engine == 0, type: 1"


what should i do to perform painting when i click push button.
thank you very much

[SixDegrees]

paintEvent() gets called automatically by the underlying Qt framework for all sorts of reasons. If you only want to paint something when a specific action takes place, move it to another routine, do your painting just as you've done, and call update() when you're finished. That way, the routine will only be invoked when whatever actions you connect to it are triggered.

[seltra]

ok i made code like this.
i can draw an ellipse by clicking push button.
but when i click button it draws the last ellipse.
i want to draw them one on the top of the other.
how can i do this?
thank you very much.

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(Drawit()));

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(Drawit()));

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void MainWindow::Drawit()
{
    drawEllipse=1;
    positionx=positionx+1;
    positiony=positiony+1;
    update();
}

void MainWindow::Drawit()
{
    drawEllipse=1;
    positionx=positionx+1;
    positiony=positiony+1;
    update();
}

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void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse )
    {
        QPainter painter( this );
        painter.drawEllipse(positionx,positiony,100,100);
    }
}

void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse )
    {
        QPainter painter( this );
        painter.drawEllipse(positionx,positiony,100,100);
    }
}

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[Lykurg]

You have to append the positions to a member variable. Then in the event use the informations of that member variable and draw your ellipses. Or you can use Qt::WA_OpaquePaintEvent to "save" the previous drawn ellipses.

[seltra]

You have to append the positions to a member variable. Then in the event use the informations of that member variable and draw your ellipses. Or you can use Qt::WA_OpaquePaintEvent to "save" the previous drawn ellipses.

dear lykurg, thank you for your help.
"positionx" and "positiony" are my member variables, i declare them as:

class MainWindow : public QMainWindow
{
...
        int positionx;
        int positiony;
...
}

class MainWindow : public QMainWindow
{
...
        int positionx;
        int positiony;
...
}

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how can i append the member variables?
should i define them as "vector"s and "push back"?
thank you

[seltra]

i don't know if it is an efficient solution, i have found this solution.
what do you suggest?
thank you.

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(Drawit()));

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(Drawit()));

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void MainWindow::Drawit()
{
    drawEllipse=1;
}

void MainWindow::Drawit()
{
    drawEllipse=1;
}

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void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse  )
    {
        QPainter painter( this );
        for (int i=1; i<50; i=i+6)
        {
            positionx=i;
            positiony=i;
            update();
            painter.drawEllipse(positionx,positiony,80,80);
        }
 
    }
}

void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse  )
    {
        QPainter painter( this );
        for (int i=1; i<50; i=i+6)
        {
            positionx=i;
            positiony=i;
            update();
            painter.drawEllipse(positionx,positiony,80,80);
        }

    }
}

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[Lykurg]

You can use whatever you like. I'd go probably for a QList:

class MainWindow : public QMainWindow
{
...
        QList<int> positionx;
        QList<int> positiony;
...
}
 
void MainWindow::Drawit()
{
    drawEllipse=1;
  // check if you list is not empty
    positionx.append(positionx.last()+1);
    positiony.append(positiony.last()+1);
    update();
}

class MainWindow : public QMainWindow
{
...
        QList<int> positionx;
        QList<int> positiony;
...
}

void MainWindow::Drawit()
{
    drawEllipse=1;
  // check if you list is not empty
    positionx.append(positionx.last()+1);
    positiony.append(positiony.last()+1);
    update();
}

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And then loop the list in you paint event.

[Lykurg]

i don't know if it is an efficient solution, i have found this solution.

No, do not call update() in a paint event!

[seltra]

No, do not call update() in a paint event!

When i remove update() from the code below, it doesn't draw multiple ellipses.
makes a weird behaviour.

void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse  )
    {
        QPainter painter( this );
        for (int i=1; i<50; i=i+6)
        {
            positionx=i;
            positiony=i;
            update();
            painter.drawEllipse(positionx,positiony,80,80);
        }
 
    }
}

void MainWindow::paintEvent(QPaintEvent *pe)
{
    if( drawEllipse  )
    {
        QPainter painter( this );
        for (int i=1; i<50; i=i+6)
        {
            positionx=i;
            positiony=i;
            update();
            painter.drawEllipse(positionx,positiony,80,80);
        }
 
    }
}

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[Lykurg]

makes a weird behaviour.

Please post a screen or describe it.

[seltra]

Please post a screen or describe it.

when i start program and i don't press to "draw it" push button.


when i use update() in paintEvent method, and click to pushbutton:


when i don't use update() in paintEvent method, and click to pushbutton:

[tbscope]

I suggest you create a new widget to do they painting on.
Place everything nicely in layouts.

This will prevent the weird behavior you see now.