Signal / Slot from QGraphicsItem

[steg90]

Hi,

I have the following code declared in my QGraphicsScene derived class:

connect( pItem, SIGNAL(ShowOptions()),this,SLOT(ShowCanOptions()) );

connect( pItem, SIGNAL(ShowOptions()),this,SLOT(ShowCanOptions()) );

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Where pItem is an object derived from QGraphicsItem.

I get a conversion error on pItem - does this have to be derived from QObject?

Regards,
Steve

[QCasper]

Yes, your item class have to be derived from QObject too. Note that you have to inherit from QObject first and then from QGraphicsIiem:

class MyGraphicsItem : public QObject, public QGraphicsItem {
Q_OBJECT
...
};

[marcel]

Yes, and make sure QObject is first in the base class list.
Also don't forget to add the Q_OBJECT macro.

[steg90]

Thanks Marcel, I did try that but still get the type conversion error.

This is my derived graphics item class :

class CCanGraphicsItem : public QObject, public QGraphicsItem
{
	Q_OBJECT
 
public:
	CCanGraphicsItem( int x, int y, QString strText ) : m_rect( 10, 10, 200, 100 ), m_color( QColor(255,255,255,255) )
	{
		m_strText = strText;
		setPos( x, y );
		m_pFont = new QFont("Times", 20, QFont::Bold );
		setFlag(QGraphicsItem::ItemIsMovable, true);
		setFlag(QGraphicsItem::ItemIsSelectable, true);
	}
	void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) 
	{
		//  painter->setRenderHint(QPainter::Antialiasing, true);
		setFlag(QGraphicsItem::ItemIsMovable, true);
		painter->setFont( *m_pFont );
		painter->setPen(Qt::SolidLine);
		painter->setBrush(m_color);
		painter->drawRoundRect( m_rect );//10, 10, 100, 100);
		painter->drawText( 20, 20, 190, 90, Qt::AlignCenter, m_strText );//, m_rect );
	}
 
	QRectF boundingRect() const { return m_rect; }
 
signals:
	void ShowOptions();
 
protected:
	void mouseDoubleClickEvent ( QGraphicsSceneMouseEvent * event ) 
	{
		QMessageBox::information(0, "This", m_strText );
		emit ShowOptions();
	}
 
    void contextMenuEvent(QContextMenuEvent *event)
	{
	}
 
	QRectF m_rect;
	QColor m_color;
	QString m_strText;
	QFont* m_pFont;
};

class CCanGraphicsItem : public QObject, public QGraphicsItem
{
	Q_OBJECT

public:
	CCanGraphicsItem( int x, int y, QString strText ) : m_rect( 10, 10, 200, 100 ), m_color( QColor(255,255,255,255) )
	{
		m_strText = strText;
		setPos( x, y );
		m_pFont = new QFont("Times", 20, QFont::Bold );
		setFlag(QGraphicsItem::ItemIsMovable, true);
		setFlag(QGraphicsItem::ItemIsSelectable, true);
	}
	void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) 
	{
		//  painter->setRenderHint(QPainter::Antialiasing, true);
		setFlag(QGraphicsItem::ItemIsMovable, true);
		painter->setFont( *m_pFont );
		painter->setPen(Qt::SolidLine);
		painter->setBrush(m_color);
		painter->drawRoundRect( m_rect );//10, 10, 100, 100);
		painter->drawText( 20, 20, 190, 90, Qt::AlignCenter, m_strText );//, m_rect );
	}

	QRectF boundingRect() const { return m_rect; }
 
signals:
	void ShowOptions();

protected:
	void mouseDoubleClickEvent ( QGraphicsSceneMouseEvent * event ) 
	{
		QMessageBox::information(0, "This", m_strText );
		emit ShowOptions();
	}

    void contextMenuEvent(QContextMenuEvent *event)
	{
	}

	QRectF m_rect;
	QColor m_color;
	QString m_strText;
	QFont* m_pFont;
};

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and the full code that tries to do the signal/slot connection :

void CanScene::AddBoxItem( int x, int y, QString strText, ITEMS item )
{
	QGraphicsItem* pItem = NULL;
 
	switch( item )
	{
	case CAN :
		addItem( pItem = new CCanGraphicsItem( x, y, strText ) );
		QObject::connect( pItem, SIGNAL(ShowOptions()),this,SLOT(ShowCanOptions()) );
		break;
// etc...
	}
}

void CanScene::AddBoxItem( int x, int y, QString strText, ITEMS item )
{
	QGraphicsItem* pItem = NULL;

	switch( item )
	{
	case CAN :
		addItem( pItem = new CCanGraphicsItem( x, y, strText ) );
		QObject::connect( pItem, SIGNAL(ShowOptions()),this,SLOT(ShowCanOptions()) );
		break;
// etc...
	}
}

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Something I'm obviously doing wrong?!

Regards,
Steve

[marcel]

Because the pItem is actually only a QGraphicsItem. It does not now about your signal.
Change the type of pItem to CCanGraphicsItem.

[steg90]

Ooops

How I didn't notice that?!

Regards,
Steve

[wysota]

I don't think this is a problem, at least not during compile time.

steg90: What exactly is the error?

[marcel]

The error was most likely QObject::connect no such signal...etc

[wysota]

I wouldn't call that a conversion error I expected a "Cannot convert ..." message.

[steg90]

Hi guys,

Exact error was :

\CanScene.cpp(17) : error C2664: 'bool QObject::connect(const QObject *,const char *,const QObject *,const char *,Qt::ConnectionType)' : cannot convert parameter 1 from 'QGraphicsItem *' to 'const QObject *'
1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

Regards,
Steve

[wysota]

Oh, yes in that case changing the type will help, because QGraphicsItem is not a QObject thus it can't be used with connect().