Qt: QPainter + GDI in the same widget?

问题

I'm trying to use the method described here to use a QPainter and GDI calls on the same widget.
Unfortunately this tutorial seem to have been written on an earlier version of Qt and now it does not work.

I set the WA_PaintOnScreen flag and reimplement paintEngine() to return NULL.

Then on the paintEvent() I create a QPainter, use it and then use some GDI calls to paint a bitmap. The GDI calls work fine but the QPainter does nothing. I get the following error on the console:

QPainter::begin: Paint device returned engine == 0, type: 1

Is this simply not supported anymore? how can I do it?

I've also tried creating an additional widget on top of the GDI-painting widget but that didn't go well as well since the top widget appears black and blocks the GDI widget.

回答1:

I got this working in QT 4.7-beta 2 as follows

1. In the constructor call setAttribute(Qt::WA_PaintOnScreen,true);
2. Do NOT reimplement paintEngine() to return NULL;

3. Use the following code in the paintEvent();

   QPainter painter(this);
   HDC hdc = painter.paintEngine()->getDC();   // THIS IS THE CRITICAL STEP! 
   HWND hwnd = winID();
   
      // From this point on it is all regular GDI 
   QString text("Test GDI Paint");
   RECT rect;
   GetClientRect(hwnd, &rect);
   
   HBRUSH hbrRed = CreateSolidBrush(RGB(255,0,0));
   FillRect(hdc, &rect, hbrRed);
   HBRUSH hbrBlue = CreateSolidBrush(RGB(40,40,255));
   HPEN bpenGreen = CreatePen(PS_SOLID, 4, RGB(0,255,0));
   SelectObject(hdc,bpenGreen);
   SelectObject(hdc,hbrBlue);
   
   Ellipse(hdc,10,10,rect.right-20,rect.bottom-20);
   SetTextAlign(hdc, TA_CENTER | TA_BASELINE);
   TextOutW(hdc, width() / 2, height() / 2, text.utf16(), text.size());
   ReleaseDC(hwnd, hdc);
   

回答2:

This worked with Qt 4.0 and 4.1, but stopped working in either 4.2 or 4.3, when Trolltech reimplemented the Windows paint engine. In the second edition of the Qt 4 book, we added the sentence:

"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."

I haven't tested it using later versions of Qt (I'm no longer at Trolltech/Nokia and have no Windows machine) but I hope it will still work.

来源：https://stackoverflow.com/questions/2616349/qt-qpainter-gdi-in-the-same-widget