qt trouble overriding paintEvent

Question

I'm subclassing QProgressBar in a custom widget, and I overwrote the paintEvent method with the following code :

void myProg::paintEvent(QPaintEvent *pe)
{
    QProgressBar::paintEvent(pe);
    QRect region = pe->rect();
    QPainter *painter = new QPainter(this);
    QPen *pen = new QPen;
    painter->begin(this);
    painter->setBrush(Qt::red);
    int x = this->x();
    int y = this->y();
    pen->setWidth(10);
    painter->setPen(*pen);
    painter->drawLine(x,y,x+100,y);
    painter->end();

}

I'm trying to display a red line, as a starting point, to see that I can add my own modifications to the widget. However, this isn't working. I only see the widget as a regular QProgressBar. Any ideas on what could be wrong ?

Answer 1

The coordinate system you need to use is relative to the top-left of the widget, but you're apparently using one relative to the widget's parent. (Widget's x and y coords are relative to their parent). So your line will be getting clipped.

Also, it's unnecessary to call QPainter::begin and QPainter::end when you construct it using a QWidget * parameter. And the painter in your code doesn't get deleted, either. It's not necessary to create a painter on the heap with new: I'd just create it on the stack.

Try:

void myProg::paintEvent(QPaintEvent *pe)
{
    QProgressBar::paintEvent(pe);
    QRect region = pe->rect();
    QPainter painter(this);
    QPen pen(Qt::red); //Note: set line colour like this

    //(Brush line removed; not necessary when drawing a line)
    int x = 0; //Note changed
    int y = height() / 2; //Note changed
    pen.setWidth(10);
    painter.setPen(pen);
    painter.drawLine(x,y,x+100,y);

}

This should draw a red horizontal line 100 pixels long starting from the middle-left of the widget.