Subclassing QGraphicsScene

pmwalk

Hey all,

So I am trying to write a subclass of QGraphicsScene, so that I can get key and mouse press events, and send them out with custom signals to the main UI. However, I can't for the life of me find a straightforward answer on how to do it, so here I am. This is what I have so far:

@
#include <QtGui>
#include <QGraphicsScene>
class QKeyEvent;
class QGraphicsSceneMouseEvent;

class MapScene : public QGraphicsItem {
Q_OBJECT

public:
MapScene(QObject *parent = 0);
~MapScene();

signals:
void deselectPoints();
void selectPoints(QRectF);
void sendPoint(QPoint);

protected:
void keyPressEvent(QKeyEvent *event);
void mousePressEvent(QGraphicsSceneMouseEvent *event);
void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
};
@

I know this is incorrect, but no matter how I write the constructor declarations or class headers I get various compile errors- either undefined references for the constructor/deconstructor, or no know conversion to QObject, or others. Any help? I feel this is a pretty basic question which is why I'm surprised it's proving difficult.

Jake007

Do you have actual code for constructor ad destructor?
Did you take a look at examples (Diagram Scene Example has this covered pretty good)?
Q_OBJECT is not needed ( not sure if it's even possible).

Kxyu

So you have
@class MapScene : public QGraphicsItem @
QGraphics_Item_

pmwalk

It turns out the Q_OBJECT was needed (although I agree it shouldn't be). I kept getting errors saying it needed to be there if I eliminated it. But it turns out the constructor/destructor both in this and in the .cpp file were incorrect, so it basically thought they weren't there. I also needed it to extend QGraphicsScene, not QGraphicsItem. Works now though... thank you for the tip!

Asperamanca

The Q_OBJECT macro is only for QObject-based classes. QGraphicsItem is not a QObject based class.

Since it isn't, it also can't have signals. Without the Q_OBJECT macro, the C++ compiler will not understand what to do about those signals, and throw errors. With the Q_OBJECT macro, the compiler is pacified, but since you are not actually having a QObject-based class, they won't work.

If you need signals in your class, change the base class to QGraphicsObject.

pmwalk

I actually ended up making the base class QGraphicsScene, because really that's what I was subclassing anyway, which resolved the conflict.

I had still been wondering what the macro was doing, thank you for the explanation.

Asperamanca

The Q_OBJECT macro does more than I wrote. Basically, it sets everything up so Qt's meta object system can be used.