在Qt内部通过Windows GDI绘图

>重新实现paintEngine返回nullptr.
>设置WA_PaintOnScreen属性.
>(可选)设置WA_NativeWindow属性.这只会加快小部件的第一次重绘速度.
>重新实现QObject :: event并捕获Paint和UpdateRequest事件.这些事件应该导致调用一个执行GDI绘制的方法.不得将事件转发到基类.

此外,在通过paintEvent / QPainter绘制的内容之上通过GDI绘制的QWidget还必须：

>在构造函数中调用基类的paintEngine()方法一次.这将实例化窗口小部件的本机绘制引擎.
>在QObject :: event实现中,必须在执行GDI绘制之前调用基类的事件.这将使用光栅绘制引擎绘制内容,并将控制权返回给您以覆盖其上的一些其他内容.

下面的示例显示了如何在Qt的绘图系统完成的绘图之上进行重绘.当然,由于这幅画是在Qt已经绘制的顶级内容上完成的,因此有闪烁.

在Qt的后备存储上进行GDI绘制,避免闪烁也是可能的,但需要采用稍微不同的方法.

#include <QApplication>
#include <QGraphicsObject>
#include <QPropertyAnimation>
#include <QGraphicsView>
#include <QPainter>
#include <QPaintEvent>
#include <windows.h>

class View : public QGraphicsView {
public:
   View(QWidget *parent = 0) : QGraphicsView(parent)
   {
      // This brings the original paint engine alive.
      QGraphicsView::paintEngine();
      //setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
      setAttribute(Qt::WA_NativeWindow);
      setAttribute(Qt::WA_PaintOnScreen);
      setRenderHint(QPainter::Antialiasing);
   }
   QPaintEngine * paintEngine() const Q_DECL_OVERRIDE { return 0; }
   bool event(QEvent * event) Q_DECL_OVERRIDE {
      if (event->type() == QEvent::Paint) {
         bool result = QGraphicsView::event(event);
         paintOverlay();
         return result;
      }
      if (event->type() == QEvent::UpdateRequest) {
         bool result = QGraphicsView::event(event);
         paintOverlay();
         return result;
      }
      return QGraphicsView::event(event);
   }
   void resizeEvent(QResizeEvent *) {
       fitInView(-2,-2,4,Qt::KeepAspectRatio);
   }
   virtual void paintOverlay();
};

void View::paintOverlay()
{
   // We're called after the native painter has done its thing
   HWND hwnd = (HWND)viewport()->winId();
   HDC hdc = GetDC(hwnd);
   HBRUSH hbrGreen = CreateHatchBrush(HS_BDIAGONAL,RGB(0,0));

   RECT rect;
   GetClientRect(hwnd,&rect);

   SetBkMode(hdc,TRANSPARENT);
   SelectObject(hdc,hbrGreen);
   Rectangle(hdc,rect.right,rect.bottom);

   SelectObject(hdc,GetStockObject(NULL_BRUSH));
   Ellipse(hdc,rect.bottom - 100);

   QString text("Test GDI Paint");
   SetTextAlign(hdc,TA_CENTER | TA_BASELINE);
   TextOutW(hdc,text.size());

   DeleteObject(hbrGreen);
   ReleaseDC(hwnd,hdc);
}

class EmptyGraphicsObject : public QGraphicsObject
{
public:
    EmptyGraphicsObject() {}
    QRectF boundingRect() const { return QRectF(0,0); }
    void paint(QPainter *,const QStyleOptionGraphicsItem *,QWidget *) {}
};

void setupScene(QGraphicsScene &s)
{
    QGraphicsObject * obj = new EmptyGraphicsObject;
    QGraphicsRectItem * rect = new QGraphicsRectItem(-1,0.3,2,obj);
    QPropertyAnimation * anim = new QPropertyAnimation(obj,"rotation",&s);
    s.addItem(obj);
    rect->setPen(QPen(Qt::darkBlue,0.1));
    anim->setDuration(2000);
    anim->setStartValue(0);
    anim->setEndValue(360);
    anim->setEasingCurve(QEasingCurve::InBounce);
    anim->setLoopCount(-1);
    anim->start();
}

int main(int argc,char *argv[])
{
   QApplication a(argc,argv);
   QGraphicsScene s;
   setupScene(s);
   View view;
   view.setScene(&s);
   view.show();
   return a.exec();
}