QT: Scroll widget that renders directly to the DC

I'm trying to create a widget which开发者_运维百科 paints directly to the windows Device Context by calling getDC() and painting an HBITMAP to it.

The widget I'm painting resides inside a scroll widget.

I've implemented the paintEvent() and it does seem to paint but immediatly after painting the widget gets painted over again with a blank gray color.

I've tried setting WA_PaintOnScreen and Qt::WA_NoSystemBackground but none of those help.

In theory this should be possible since this is basically how the GLWidget works.

What am I missing?

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Found the answer here:

http://www.qtchina.net/qt4c++guiprogramming/ch20lev1sec1.html/

void GdiControl::paintEvent(QPaintEvent * /* event */)
{
    RECT rect;
    GetClientRect(winId(), &rect);
    HDC hdc = GetDC(winId());
    FillRect(hdc, &rect, HBRUSH(COLOR_WINDOW + 1));
    SetTextAlign(hdc, TA_CENTER | TA_BASELINE);
    TextOutW(hdc, width() / 2, height() / 2, text.utf16(), text.size());
    ReleaseDC(winId(), hdc);
}

For this to work, we must also reimplement QPaintDevice::paintEngine() to return a null pointer and set the Qt::WA_PaintOnScreen attribute in the widget's constructor.