Have subclassed QGraphicsScene (GridScene) and QGraphicsView (GridView). In GridScene drawBackGround method draw grid with text lables on the hor and ver axis. Must zoom in ,out using QToolButtons, (no scrollbars) on dlg, and pan left, right, up, down with mouse. Cannot get clear view of how to use QGraphicsScene with QGraphicsView.
1. Do override the mouse events in GridView and GridScene to execute the neccessary actions?
a. If do What do in the GridView and what in the GridScene methods?
b. If not, how do propagate the changes to the QGraphicsScene?
2. When is it necessary to subclass QGraphicsScene, will it be better to draw the grid on the scene in the GridView class?
3. If also override QGraphicsView:: drawBackGround, what needs to be drawn here and what in GridScene:: drawBackGround ?
4. Do I need to override paintevent in either or both GridView and GridScene?

Qt Code: 
  1. class GridView : public QGraphicsView
  2. {
  3.     Q_OBJECT
  4.  
  5. public:
  6.     GridView(QWidget* parent = 0);
  7.  
  8. private:
  9. ...
  10. public slots:
  11.     void zoomIn ();
  12.     void zoomOut();
  13.  
  14. private:
  15.     GridScene* m_scene;
  16.     int m_minScaleFactor;
  17.     int m_maxScaleFactor;
  18.     int m_curScaleFactor;
  19.  
  20. };
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Qt Code: 
  1. GridView::GridView(QWidget* parent)
  2.     : QGraphicsView(parent)
  3. {
  4.     setGeometry(QRect(0, 0, 474, 621));
  5.     setHorizontalScrollBarPolicy (Qt::ScrollBarAlwaysOff);
  6.     setVerticalScrollBarPolicy (Qt::ScrollBarAlwaysOff);
  7.     setDragMode (QGraphicsView::ScrollHandDrag);
  8.     m_minScaleFactor = 0;
  9.     m_maxScaleFactor = 500;
  10.     m_curScaleFactor = 250;
  11.     m_curPanX = 0;
  12.     m_curPanY = 0;
  13.  
  14.     m_scene = new GridScene;
  15.     this->setScene (m_scene);
  16.     m_scene->setSceneRect(QRectF(0, 0, 1500, 1500));
  17.     setBackgroundRole(QPalette::Base);
  18.     setAutoFillBackground(true);
  19.  
  20. //    setViewportUpdateMode(QGraphicsView::FullViewPortUpdate);
  21. }
  22.  
  23. void GridView::zoomIn ()
  24. {
  25.     qreal scale = qPow(qreal(2), (m_curScaleFactor - 250) / qreal(50));
  26.     QTransform transform;
  27.     transform.scale(scale, scale);
  28.     m_curScaleFactor = transform.m11();
  29. }
  30.  
  31. void GridView::mousePressEvent(QMouseEvent *event)
  32. {
  33.      if (event->button() == Qt::LeftButton)
  34.     {
  35.         m_pan = true;
  36.         m_panStartX = event->x();
  37.         m_panStartY = event->y();
  38.         setCursor(Qt::ClosedHandCursor);
  39.         event->accept();
  40.         return;
  41.     }
  42.     event->ignore();
  43. }
  44.  
  45. void GridView::mouseMoveEvent(QMouseEvent *event)
  46. {
  47.      if (m_pan)
  48.     {
  49.         m_curPanX = (event->x() - m_panStartX);
  50.         m_curPanY = (event->y() - m_panStartY);
  51.         m_panStartX = event->x();
  52.         m_panStartY = event->y();
  53.        event->accept();
  54.         return;
  55.     }
  56.     event->ignore();
  57. }
  58.  
  59. void GridView::mouseReleaseEvent(QMouseEvent *event)
  60. {
  61.     if (event->button() == Qt::LeftButton)
  62.     {
  63.         m_pan = false;
  64.         setCursor(Qt::ArrowCursor);
  65.  
  66.         m_curPanX = event->x();
  67.         m_curPanY = event->y();
  68.         setCursor(Qt::OpenHandCursor);
  69.         event->accept();
  70.         return;
  71.     }
  72.     event->ignore();
  73. }
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Qt Code: 
  1. #include <QGraphicsScene>
  2.  
  3. class GridScene : public QGraphicsScene
  4. {
  5.  
  6. public:
  7.     GridScene();
  8.  
  9. protected:
  10.     void drawBackground(QPainter *painter, const QRectF &rect);
  11.  
  12. private:
  13.  ...
  14.  
  15. };
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Qt Code: 
  1. GridScene::GridScene()
  2. {
  3.     minX = 0.0;
  4.     maxX = 10.0;
  5.     numXTicks = 12;
  6.  
  7.     minY = 0.0;
  8.     maxY = 10.0;
  9.     numYTicks = 12;
  10. }
  11.  
  12. void GridScene::drawBackground(QPainter *painter, const QRectF &rect)
  13. {
  14.     QRect rect1(Margin, Margin,
  15.                width() - 2 * Margin, height() - 2 * Margin);
  16.     if (!rect1.isValid())
  17.         return;
  18.  
  19.      for (int i = 0; i <= numXTicks; ++i) {
  20.         int x = rect1.left() + (i * (rect1.width() - 1) / numXTicks);
  21.         double label = minX + (i * spanX() / numXTicks);
  22.         painter->setPen(QPen(QColor(Qt::darkBlue)));
  23.         painter->drawLine(x, rect1.top(), x, rect1.bottom());
  24.         painter->setPen(QPen(QColor(Qt::darkBlue)));
  25.         painter->drawLine(x, rect1.bottom(), x, rect1.bottom() + 5);
  26.         painter->drawText(x - 50, rect1.bottom() + 5, 100, 20,
  27.                           Qt::AlignHCenter | Qt::AlignTop,
  28.                           QString::number(label));
  29.     }
  30.     for (int j = 0; j <= numYTicks; ++j) {
  31.         int y = rect1.bottom() - (j * (rect1.height() - 1)/ numYTicks);
  32.         double label = minY + (j * spanY()
  33.                                           / numYTicks);
  34.         painter->setPen(QPen(QColor(Qt::darkBlue)));
  35.         painter->drawLine(rect1.left(), y, rect1.right(), y);
  36.         painter->setPen(QPen(QColor(Qt::darkBlue)));
  37.         painter->drawLine(rect1.left() - 5, y, rect1.left(), y);
  38.         painter->drawText(rect1.left() - Margin, y - 10, Margin - 5, 20,
  39.                           Qt::AlignRight | Qt::AlignVCenter,
  40.                           QString::number(label));
  41.     }
  42.     painter->drawRect(rect1.adjusted(0, 0, -1, -1));
  43. }
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