QGraphicsView with QGraphicsPixmapItem that resizes on resize of QGraphicsView

Hey Guys,

I subclassed QGraphicsView and an QGraphicsPixmapItem...
What i want: I want to add a Pixmap to the GraphicsView and resize it (if the pixmap does not have it's desired maximum size) on resizing the GraphicsView..

What i receive: an unhandled exception in QPaintDevice::paintingActive()

Code:

---

inline bool QPaintDevice::paintingActive() const
{ return painters != 0; } // seems i get the exception here

---

Code:

---

BackgroundItem::BackgroundItem(const QPixmap &pixmap)
	: QGraphicsPixmapItem(pixmap)
{
	myOriginalPixmap = pixmap;
}
 
QPixmap BackgroundItem::getOriginalPixmap(){
	return myOriginalPixmap;
}

---

Code:

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void FloorPlanGraphicsView::init()
{
	canvas = new QGraphicsScene(this);
	//scene->setSceneRect(0,0,this->geometry().width(),this->geometry().height();
	setScene(canvas);
	setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
}
 
void FloorPlanGraphicsView::setFloorPlan(const QPixmap &image){
	BackgroundItem *backImg = new BackgroundItem(image);//.scaled(geometry().width(),geometry().height(),Qt::KeepAspectRatio));
    scene()->addItem(backImg);
    backImg->setPos(0,0);
    backImg->setZValue(0);
	backImg->setPixmap(image.scaled(geometry().width(),geometry().height(),Qt::KeepAspectRatio));
}
void FloorPlanGraphicsView::resizeEvent(QResizeEvent *event){
	scene()->setSceneRect(0,0,this->geometry().width(),this->geometry().height());
 
	QList<QGraphicsItem*> graphicItems = scene()->items();
	for (int i = 0; i < graphicItems.size(); ++i){
 
		if (graphicItems.at(i)->type() == FloorPlanGraphicsView::BackgroundType){
			BackgroundItem *myBackItem = qgraphicsitem_cast<BackgroundItem*>(graphicItems.at(i));
 
			if(this->geometry().width() <= myBackItem->getOriginalPixmap().size().width() && this->geometry().height() <= myBackItem->getOriginalPixmap().size().height())
				// i get the error immediately here on resize!
myBackItem->setPixmap(myBackItem->getOriginalPixmap().scaled(this->geometry().width(),this->geometry().height(),Qt::KeepAspectRatio)); 
			else
				myBackItem->setPixmap(myBackItem->getOriginalPixmap());
		}
	}
}

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The invocation to setFloorPlan() looks like the following:

Code:

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ui.floorPlan->setFloorPlan(QPixmap::fromImage(QImage::fromData(query.value(0).toByteArray())));

---

Do i have to reimplement a paint() function or something similar here?
Anbody any idea for me?

Has really nobody an idea for me? =/

Maybe the following is your problem.

From the docs (link):

Quote:

---

T qgraphicsitem_cast ( QGraphicsItem * item )

Returns the given item cast to type T if item is of type T; otherwise, 0 is returned.
Note: To make this function work correctly with custom items, reimplement the type() function for each custom QGraphicsItem subclass.

---

Check out the diagramscene and elasticnodes examples.

In backgrounditem.h add:

Code:

---

enum { Type = UserType +1 };
int type() const { return Type; }

---

And in FloorPlanGraphicsView::resizeEvent() change:

Code:

---

//if (graphicItems.at(i)->type() == FloorPlanGraphicsView::BackgroundType){
if (graphicItems.at(i)->type() == BackgroundItem::Type{

---

Ney norobro!

Thanks for your reply, i have not posted that part of code cauze i thought it might not be useful for a reader - but i am aware of that ..
Here is the code from the header:

Code:

---

class FloorPlanGraphicsView : public QGraphicsView
{
    Q_OBJECT
 
public:
	FloorPlanGraphicsView(QWidget * parent = 0);
	FloorPlanGraphicsView(QGraphicsScene * scene, QWidget * parent = 0);
	~FloorPlanGraphicsView();
 
	void setFloorPlan(const QPixmap & image);
	void setIndicator(const QImage & image);
 
	static const int IndicatorType	= QGraphicsItem::UserType + 1;
	static const int BackgroundType	= QGraphicsItem::UserType + 2;
 
protected:
	virtual void resizeEvent(QResizeEvent *);
	virtual void mouseDoubleClickEvent(QMouseEvent *event);
 
private:
	void init();
	void placeIndicator(const QPointF & point);
	void removeIndicator(const QPointF & point);
	QGraphicsScene *canvas;
    QImage m_indicator;
 
};
 
class ImageItem: public QGraphicsRectItem
{
public:
    ImageItem( QImage img );
 
	int type() const { return FloorPlanGraphicsView::IndicatorType; }
protected:
    void paint( QPainter *, const QStyleOptionGraphicsItem *option, QWidget *widget );
private:
    QImage image;
    QPixmap pixmap;
};
 
class BackgroundItem: public QGraphicsPixmapItem
{
public:
    BackgroundItem(const QPixmap &pixmap);
 
	virtual int type() const { return FloorPlanGraphicsView::BackgroundType; }
	QPixmap getOriginalPixmap();
private:
    QPixmap myOriginalPixmap;
};

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I don't think qgraphicsitem_cast will work the way you are using it.

Check your pointer in debug mode:

Code:

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BackgroundItem *myBackItem = qgraphicsitem_cast<BackgroundItem*>(graphicItems.at(i));
Q_ASSERT(myBackItem);

---

Try static_cast instead.

I changed the stuff a little bit and got another error facing the same problem as it seems ... as found out that the qgraphicsitem_cast isn't working i wanted to reply here -> then i saw your reply ;)

Anyways .. you're absolutely right.. and static_cast works as i want it to work... (in my new case - i will reconstruct the old one with the old files from SVN)

Could you give me an explanation why "qgraphicsitem_cast" is not working as i expected? Is that my fault or what's wrong here?

I'm not sure what the advantage of qgraphicsitem_cast is. Maybe someone can enlighten us.

Anyway, here's the source code for qgraphicsitem_cast from qgraphicsitem.h:

Code:

---

template <class T> inline T qgraphicsitem_cast(QGraphicsItem *item)
{
    return int(static_cast<T>(0)->Type) == int(QGraphicsItem::Type)
        || (item && int(static_cast<T>(0)->Type) == item->type()) ? static_cast<T>(item) : 0;
}

---

Since you don't have "Type" defined in your BackgroundItem class, int(static_cast<BackgroundItem *>(0)->Type) returns 7 which is the parent Type (QGraphicsPixmapItem).
Thus:
The first part of the return statement above becomes: 7==1 which is false;
The conditional operator expression (true && 7 == 65538) is false;
Therefore zero is returned and you end up with a null pointer.

Hope this makes sense.

Hmm what you said makes sense but i do NOT get the sense of that graphicsitem_cast .. looks complete senseless ...

Code:

---

template <class T> inline T qgraphicsitem_cast(QGraphicsItem *item)
    {
    return int(static_cast<T>(0)->Type) == int(QGraphicsItem::Type)
    || (item && int(static_cast<T>(0)->Type) == item->type()) ? static_cast<T>(item) : 0;
    }

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I do not really get that part: "int(static_cast<T>(0)->Type) == item->type()"
What should that comparison be useful for?