Qt Designer – How to get a widget to span columns

You need to override paintEvent() and do your painting there. You don't really need the begin() and end(). Declare the painter with

QPainter painter(this);

The constructor will handle begin(), and end() will be called when the painter object goes out of scope and is destroyed.

You also won't need the click event to trigger the painting. paintEvent() will be called whenever the widget needs to draw itself. You could use the the button click to toggle a boolean that the paintEvent() checks to determine whether or not it should display the rectangle and arc. Just make sure you call update() after you toggle the variable.

void Widget::on_pushButton_clicked()
{
    drawShapes = !drawShapes;
    update();
}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);

    if(drawShapes)
    {
        QRectF rectangle(10.0, 20.0, 80.0, 60.0);
        int startAngle = 30 * 16;
        int spanAngle = 120 * 16;
        painter.drawArc(rectangle, startAngle, spanAngle);
    }
}

UPDATE:

To avoid having to override the paintEvent() of a widget, you could use a QLabel and assign a pixmap to it and paint to that. Note: As far as I can tell, you will need to set the pixmap each time you modify it.

void MainForm::slot_buttonClick()
{
    QPixmap pixmap(100,100);
    pixmap.fill(QColor("transparent"));

    QPainter painter(&pixmap);
    painter.setBrush(QBrush(Qt::black));
    painter.drawRect(10, 10, 100, 100);

    label.setPixmap(pixmap);
}

Use QObject::sender() in the slot, like in the following Example:

void MainWindow::someSetupFunction( void )
{
   ...
   connect( _foobarButton, SIGNAL(clicked()), this, SLOT(buttonPressedSlot()) );
}

void MainWindow::buttonPressedSlot()
{
   // e.g. check with member variable _foobarButton
   QObject* obj = sender();
   if( obj == _foobarButton )
   { 
      ...
   }

   // e.g. casting to the class you know its connected with
   QPushButton* button = qobject_cast<QPushButton*>(sender());
   if( button != NULL ) 
   { 
      ...
   }

}