如果您希望它可以在所有平台上运行,则需要一些小心.您无法直接将效果应用于顶级窗口.层次结构需要如下所示：

ContainerWidget
     |
     +----------+
     |          |
**Target**   Overlay

您将效果应用于Target小部件(例如,QMainWindow). ContainerWidget是一个帮助程序类,可以让子项占用窗口小部件的完整大小.这消除了对显式零边距布局的需要.

以下工作,甚至在Mac上.如果放弃了ContainerWidget,它就不会.不幸的是,这仅适用于Qt 5.在Qt 4上,你的“跨平台”支持不包括Mac :(它在Windows上使用Qt 4(4.8.5)或Qt 5正常工作.

// https://github.com/KubaO/stackoverflown/tree/master/questions/overlay-blur-19383427
#include <QtGui>
#if QT_VERSION >= QT_VERSION_CHECK(5,0)
#include <QtWidgets>
#endif

class OverlayWidget : public QWidget {
   void newParent() {
      if (!parent()) return;
      parent()->installEventFilter(this);
      raise();
   }
public:
   explicit OverlayWidget(QWidget *parent = {}) : QWidget(parent) {
      setAttribute(Qt::WA_NoSystemBackground);
      setAttribute(Qt::WA_TransparentForMouseEvents);
      newParent();
   }
protected:
   //! Catches resize and child events from the parent widget
   bool eventFilter(QObject *obj,QEvent *ev) override {
      if (obj == parent()) {
         if (ev->type() == QEvent::Resize)
            resize(static_cast<QResizeEvent*>(ev)->size());
         else if (ev->type() == QEvent::ChildAdded)
            raise();
      }
      return QWidget::eventFilter(obj,ev);
   }
   //! Tracks parent widget changes
   bool event(QEvent *ev) override {
      if (ev->type() == QEvent::ParentAbouttochange) {
         if (parent()) parent()->removeEventFilter(this);
      }
      else if (ev->type() == QEvent::ParentChange)
         newParent();
      return QWidget::event(ev);
   }
};

class ContainerWidget : public QWidget
{
public:
   explicit ContainerWidget(QWidget *parent = {}) : QWidget(parent) {}
   void setSize(QObject *obj) {
      if (obj->isWidgetType()) static_cast<QWidget*>(obj)->setGeometry(rect());
   }
protected:
   //! Resizes children to fill the extent of this widget
   bool event(QEvent *ev) override {
      if (ev->type() == QEvent::ChildAdded) {
         setSize(static_cast<QChildEvent*>(ev)->child());
      }
      return QWidget::event(ev);
   }
   //! Keeps the children appropriately sized
   void resizeEvent(QResizeEvent *) override {
      for(auto obj : children()) setSize(obj);
   }
};

class LoadingOverlay : public OverlayWidget
{
public:
   LoadingOverlay(QWidget *parent = {}) : OverlayWidget{parent} {
      setAttribute(Qt::WA_TranslucentBackground);
   }
protected:
   void paintEvent(QPaintEvent *) override {
      QPainter p{this};
      p.fillRect(rect(),{100,100,128});
      p.setPen({200,200,255});
      p.setFont({"arial,helvetica",48});
      p.drawText(rect(),"Loading...",Qt::AlignHCenter | Qt::AlignTop);
   }
};

namespace compat {
#if QT_VERSION >= QT_VERSION_CHECK(5,4,0)
using QT_PREPEND_NAMESPACE(QTimer);
#else
using Q_QTimer = QT_PREPEND_NAMESPACE(QTimer);
class QTimer : public Q_QTimer {
public:
   QTimer(QTimer *parent = nullptr) : Q_QTimer(parent) {}
   template <typename F> static void singleShot(int period,F &&fun) {
      struct Helper : public QObject {
         F fun;
         QBasicTimer timer;
         void timerEvent(QTimerEvent *event) override {
            if (event->timerId() != timer.timerId()) return;
            fun();
            deleteLater();
         }
         Helper(int period,F &&fun) : fun(std::forward<F>(fun)) {
            timer.start(period,this);
         }
      };
      new Helper(period,std::forward<F>(fun));
   }
};
#endif
}

int main(int argc,char *argv[])
{
   QApplication a{argc,argv};
   ContainerWidget base;
   QLabel label("Dewey,Cheatem and Howe,LLC.",&base);
   label.setFont({"times,times new roman",32});
   label.setAlignment(Qt::AlignCenter);
   label.setGraphicsEffect(new QGraphicsBlurEffect);
   LoadingOverlay overlay(&base);
   base.show();
   compat::QTimer::singleShot(2000,[&]{
      overlay.hide();
      label.setGraphicsEffect({});
   });
   return a.exec();
}