QGraphicsRectItem signal

[kovacadam]

Hello!

How can I use signals of QGraphicsRectItem?

I tried this:

m_rectangle = new QGraphicsRectItem( 0, 0, 40, 40 );
m_rectangle->setPen( QPen(Qt::black) );
m_rectangle->setBrush( QBrush( QColor( 0, 200, 0, 200 ) ) );

connect(m_rectangle, SIGNAL(MousePressEvent), this, SLOT(animate()));

but I get this:
no matching function for call to 'GraphicsViewDemo::connect(QGraphicsRectItem*&, const char*, GraphicsViewDemo* const, const char*)'

[Lykurg]

Where does a QGraphicsRectItem has a signal called MousePressEvent or any other signal at all? And items do not inherit QObject which is needed for connections.

So you have to do it yourself. Just create a custom item which inherits QObject.

class MyItem : public QObject, public QGraphicsRectItem

class MyItem : public QObject, public QGraphicsRectItem

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[Urthas]

That is one way to do it.

Or you could sidestep connections altogether and write a custom class that inherits from QGraphicsRectItem only, re-implementing mousePressEvent().

[kovacadam]

Sorry for my easy questions, but I'm new in qt. So if I have a class inherit QObject and QGraphicsRectItem why has it mousePressEvent signal? QObject and QGraphicsRectItem haven't mousePressEvent signal, have they?

[qlands]

Hi,

QGraphicsRectItem does not have a signal MousePressEvent. It has the protected function virtual void mousePressEvent ( QGraphicsSceneMouseEvent * event ).

If you want to emit a signal when mousePressEvent is called, then you need to subclass your m_rectangle from QObject and QGraphicsRectItem.

class MyItem : public QObject, public QGraphicsRectItem

For example:
myrectangle.h

#ifndef MYRECTANGLE_H
#define MYRECTANGLE_H
 
#include <QtGui>
#include <QGraphicsRectItem>
 
class myrectangle : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    myrectangle();
 
signals:
    void myPressSignal();
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};
 
#endif // MYRECTANGLE_H

#ifndef MYRECTANGLE_H
#define MYRECTANGLE_H

#include <QtGui>
#include <QGraphicsRectItem>

class myrectangle : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    myrectangle();

signals:
    void myPressSignal();
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};

#endif // MYRECTANGLE_H

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myrectangle.cpp

#include "myrectangle.h"
 
myrectangle::myrectangle()
{
}
 
 
void myrectangle::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor
 
    emit myPressSignal(); //Emits the signal
}

#include "myrectangle.h"

myrectangle::myrectangle()
{
}


void myrectangle::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor

    emit myPressSignal(); //Emits the signal
}

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[kovacadam]

Thank you. Now I have just one problem:
I created my QGraphicsRectItem int this way: m_rectangle = new QGraphicsRectItem( 0, 0, 40, 40 );
and I tried this in my class: QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent) : QGraphicsRectItem(x, y, w, h, parent)
but I got this error: undefined reference to `QMyButton::QMyButton(float, float, float, float, QGraphicsItem*)'

[Lykurg]

Well you have to declare that constructor as well or use the standard constructor and set the geometry with a member method.

[kovacadam]

It's my qmybutton.h

#ifndef QMYBUTTON_H
#define QMYBUTTON_H
 
#include <QGraphicsRectItem>
 
class QMyButton : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0);
 
signals:
    void myPressSignal();
 
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};
 
#endif // QMYBUTTON_H

#ifndef QMYBUTTON_H
#define QMYBUTTON_H

#include <QGraphicsRectItem>

class QMyButton : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0);

signals:
    void myPressSignal();

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};

#endif // QMYBUTTON_H

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This line declare the constructor: QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0); ?
I already have this.

[qlands]

So the qmybutton.cpp should be something like:

QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent)
{
 
}
 
 
void QMyButton::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor
 
    emit myPressSignal(); //Emits the signal
}

QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent)
{
    
}


void QMyButton::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor

    emit myPressSignal(); //Emits the signal
}

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Why do you need this constructor?

You can also have a simple constructor like: QMyButton();

then:

//Create the button
QMyButton mybutton = new  QMyButton();
//Sets the rect
mybutton->setRect(0,0,100,100);
//Add it the scene.
myScene->addItem(mybutton);
//Or add it to a parent item:
mybutton->setParentItem(myParent);

//Create the button
QMyButton mybutton = new  QMyButton();
//Sets the rect
mybutton->setRect(0,0,100,100);
//Add it the scene.
myScene->addItem(mybutton);
//Or add it to a parent item:
mybutton->setParentItem(myParent);

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[Lykurg]

Maybe you need to rerun qmake.

[kovacadam]

I know I can use simple constructor, but I used this constructor of QGraphicsRectItem: QGraphicsRectItem(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0)
and I want create a constructor like: QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent)
And I want to know why it doesn't work.

I use Qt Creator, so if I'm right qmake run ever when I build the project.

this is my qmybutton.h:

#ifndef QMYBUTTON_H
#define QMYBUTTON_H
 
#include <QGraphicsRectItem>
 
class QMyButton : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    QMyButton();
    QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0);
 
signals:
    void myPressSignal();
 
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};
 
#endif // QMYBUTTON_H

#ifndef QMYBUTTON_H
#define QMYBUTTON_H

#include <QGraphicsRectItem>

class QMyButton : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    QMyButton();
    QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent = 0);

signals:
    void myPressSignal();

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent * event);
};

#endif // QMYBUTTON_H

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and this is my qmybutton.cpp:

#include "qmybutton.h"
 
QMyButton::QMyButton()
{
}
 
QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent) :
        QGraphicsRectItem(x, y, w, h, parent)
{
}
 
void QMyButton::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor
 
    emit myPressSignal(); //Emits the signal
}

#include "qmybutton.h"

QMyButton::QMyButton()
{
}

QMyButton::QMyButton(qreal x, qreal y, qreal w, qreal h, QGraphicsItem *parent) :
        QGraphicsRectItem(x, y, w, h, parent)
{
}

void QMyButton::mousePressEvent(QGraphicsSceneMouseEvent * event)
{
    QGraphicsItem::mousePressEvent(event); //Call the ancertor

    emit myPressSignal(); //Emits the signal
}

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[qlands]

Can you provide the error message?

[Lykurg]

Sorry for my easy questions, but I'm new in qt.

Then why don't you use the Newbie section? (-> moved)

Do you want only a "mouse click on item" signal or other signals as well. If you only need to be notified if a item was clicked then maybe following is a better solution than subclassing the item:

• Make your items selectable and use QGraphicsScene::selectionChanged() to get notified.
• Reimp QGraphicsScene::mousePressEvent() and detect the underlaying item with QGraphicsScene::itemAt().