Drawing an overlay on top of an application's window

Question

I want to be able to paint on top of my application\'s window so that I can annotate all the widgets with some extra diagnostic information, similar to the CSS developer too

Answer 1

You can create own style class based on QMotifStyle or other ... and paint on any widget/control related to him information.

void MyStyle::drawPrimitive(PrimitiveElement element, const QStyleOption *option,QPainter *painter, const QWidget *widget) const
{
     QStyle::State flags = option->state;
     QRect      rect     = option->rect;
     QPalette   pal      = option->palette;
     QBrush brush;

    switch (element)
    {
        case PE_FrameTabWidget:
        {
             painter->save();

                 // for example: draw anything on TabWidget
                painter->drawPixmap(rect,centerPm,centerPm.rect());
             painter->restore();
        }
        break;
        default:
         QMotifStyle::drawPrimitive(element, option, painter, widget);
         break;

    }
}

Answer 2

Somewhere in Qt5 the styles (GTK, Windows, etc) were made internal. Now you need to use QCommonStyle.

If anyone's wondering how to do this with Qt5+. Here's a self-contained version of @the_mandrill's code above.

class DiagnosticStyle : public QCommonStyle
{
Q_OBJECT

public: 
    typedef QStyle BaseStyle;
    void drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const
    {
        QCommonStyle::drawControl(element, option, painter, widget);
        if (widget && painter) {
            // draw a border around the widget
            painter->setPen(QColor("red"));
            painter->drawRect(widget->rect());

            // show the classname of the widget
            QBrush translucentBrush(QColor(255,246,240, 100));
            painter->fillRect(widget->rect(), translucentBrush);
            painter->setPen(QColor("darkblue"));
            painter->drawText(widget->rect(), Qt::AlignLeft | Qt::AlignVCenter, widget->metaObject()->className()); 
        }
    };
};

Then, in your main window constructor call

qApp->setStyle(new DiagnosticStyle());