QGraphicsView中的事件

Question

I'm having trouble getting events in QGraphicsView working. I've subclassed QGraphicsView and tried to overload the mousePressEvent and wheelEvent . But neither mousePressEvent nor wheelEvent get called.

Here's my code (Edited a few things now):

Declaration:

#include <QGraphicsView>
#include <QGraphicsScene>
class rasImg: public QGraphicsView
{
public:
rasImg(QString file);
~rasImg(void);
    initialize();
QGraphicsView *view;
QGraphicsScene *scene;
private:
virtual void mousePressEvent (QGraphicsSceneMouseEvent  *event);
virtual void wheelEvent ( QGraphicsSceneMouseEvent  * event );
}

Definition:

#include "rasImg.h"
void rasImg::initialize()
{
view = new QGraphicsView();
scene = new QGraphicsScene(QRect(0, 0, MaxRow, MaxCol));
scene->addText("HELLO");
scene->setBackgroundBrush(QColor(100,100,100));
view->setDragMode(QGraphicsView::ScrollHandDrag);
view->setScene(scene);
}
void rasImg::mousePressEvent (QGraphicsSceneMouseEvent  *event)
{
    qDebug()<<"Mouse released";
    scene->setBackgroundBrush(QColor(100,0,0));
}
void rasImg::wheelEvent ( QGraphicsSceneMouseEvent  * event )
{
qDebug()<<"Mouse released";
    scene->setBackgroundBrush(QColor(100,0,0));
}

So, what am I doing wrong?.
Why don't I see a message or background color change when I click the view or use the mouse wheel?

Answer 1

You're not getting the events because they're being handled by the scene object you're creating, not your class.

Remove the QGraphicsScene *scene; from your rasImg and try something like this for the constructor:

rasImg::rasImg(QString file)
  : QGraphicsScene(QRect(0, 0, MaxRow, MaxCol))
{
 addText("HELLO");
 setBackgroundBrush(QColor(100,100,100));
 setDragMode(QGraphicsView::ScrollHandDrag);

 view = new QGraphicsView();
 view->setScene(this);
}

If you want that in two steps, you could do:

rasImg::rasImg(QString file)
  : QGraphicsScene()
{
  // any constructor work you need to do
}


rasImg::initialize()
{
 addText("HELLO");
 setSceneRect(QRect(0, 0, MaxRow, MaxCol));
 setBackgroundBrush(QColor(100,100,100));
 setDragMode(QGraphicsView::ScrollHandDrag);

 view = new QGraphicsView();
 view->setScene(this);
}

The point is that the scene that is displayed must be an actual instance of your rasImg , not an instance of QGraphicsScene .

If it's the view you're subclassing, do the same thing. The view you're displaying must be an instance of your class, not a plain QGraphicsView .

rasImg::rasImg(QString file)
    : QGraphicsView()
{
   // constructor work
}

void rasImg::initialize()
{
  scene = new QGraphicsScene(QRect(0, 0, MaxRow, MaxCol));
  scene->addText("HELLO");
  scene->setBackgroundBrush(QColor(100,100,100));

  setDragMode(QGraphicsView::ScrollHandDrag);
  setScene(scene);
}

Answer 2

QGraphicsView is derived from QWidget. Therefore it receives mouse events like regular widgets. If your code really is

void rasImg::mousePressEvent (QGraphicsSceneMouseEvent *event)

It can not receive events since it should be

void rasImg::mousePressEvent ( QMouseEvent *event )

QGraphicsSceneMouseEvent is for items in QGraphicsScene that receive mouse input.

Answer 3

If you would like to handle clicks on a specific GUI element rather than handle click on the whole scene, you should derive your own class either from QGraphicsItem (see example of SimpleClass here ) or derive from one of existing elements, eg QGraphicsPixmapItem .

In both cases in your derived class you can override void mousePressEvent(QGraphicsSceneMouseEvent *event);