Hi all,

I'm trying to write a thin Qt wrapper which allows to scroll a canvas rendered by a 3rd-party class Foo.

This seems to work fine:
Qt Code: 
  1. class MyWrapper : public QScrollArea
  2. //class MyWrapper : public QAbstractScrollArea
  3. {
  4.     Q_OBJECT
  5.  
  6. private:
  7.     Foo _foo;
  8.     QLabel _canvas;
  9.  
  10. public:
  11.     MyWrapper()
  12.     {
  13.         setWidget(&_canvas);     //QScrollArea
  14. //      setViewport(&_canvas);   //QAbstractScrollArea
  15.         _canvas.setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
  16.         _foo.setHook(hook, this);
  17.     }
  18.  
  19.     //3rd-party class Foo signals "render finished" through this callback
  20.     friend void hook(void *data, bool canceled);
  21.  
  22. public slots:
  23.     void showRendered()
  24.     {
  25.         QImage img(reinterpret_cast<const uchar *>(_foo.buffer()),
  26.                            _pdfView->bufferWidth(),
  27.                            _pdfView->bufferHeight(),
  28.                            QImage::Format_ARGB32);
  29.         _canvas.setPixmap(QPixmap::fromImage(img));
  30.         _canvas.adjustSize();
  31.     }
  32.  
  33. protected:
  34.     void paintEvent(QPaintEvent *event)
  35.     {
  36.         _foo.update();
  37.     }
  38.  
  39.     void resizeEvent(QResizeEvent *event)
  40.     {
  41.         _foo.resize(viewport()->width(), viewport()->height());
  42.     }
  43. };
  44.  
  45. void hook(void *data, bool canceled)
  46. {
  47.     MyWrapper*wrapper = (PdfWidget *)data;
  48.     //Foo::update() spawns a sparate thread
  49.     QMetaObject::invokeMethod(wrapper, "showRendered", Qt::AutoConnection);
  50. }
To copy to clipboard, switch view to plain text mode 

As Foo provides everything needed to move/zoom/resize/pan the canvas, QAbstractScrollArea seems more appropriate to inherit from.
However, if I apply the changes indicated in the above code snippet, nothing is painted at all in the viewport.

What am I missing? Any help appreciated.