QGraphicsPixmapItem, alternative methods of connecting to slots - qt

Question

I'm aware I need to derive from QObject in order to connect to a slot if I am using QGraphicsPixmapItem, but I am struggling to do this. I have tried alternative ways to achieve what I want, I have tried onMousePress and isSelectable i.e.
run->setFlag(QGraphicsPixmapItem::ItemIsSelectable);
if (run->isSelected())
{
qDebug() << "selected";
}
else if (!run->isSelected())
{
qDebug() << "not selected";
}
although run is selectable, the first argument is never true, it is always "not selected"
This is my code, I am working on the slot method;
mainwindow.cpp
int MainWindow::sim()
{
...
QGraphicsPixmapItem* run = new QGraphicsPixmapItem(QPixmap::fromImage(image6));
run->scale(0.3,0.3);
run->setPos(-200,-200);
run->setFlag(QGraphicsPixmapItem::ItemIsSelectable);
run->setCursor(Qt::PointingHandCursor);
connect(run, SIGNAL(selectionChanged()), this, SLOT(runClicked()));
scene->addItem(run);
//pause
QGraphicsPixmapItem* pause = new QGraphicsPixmapItem(QPixmap::fromImage(image7));
pause->scale(0.3,0.3);
pause->setPos(-160,-197);
pause->setFlag(QGraphicsPixmapItem::ItemIsSelectable);
pause->setCursor(Qt::PointingHandCursor);
connect(pause, SIGNAL(selectionChanged()), this, SLOT(pauseClicked()));
scene->addItem(pause);
...
}
void MainWindow::runClicked()
{
qDebug() << "run Clicked";
}
void MainWindow::pauseClicked()
{
qDebug() << "pause Clicked";
}
mainwindow.h
#define MAINWINDOW_H
#include <QMainWindow>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow();
int sim();
...
public slots:
void runClicked();
void pauseClicked();
...
So obviously I get the error when connecting to the slots. Could anyone help please? Thank you.

Answer 1

To find out if your item is selected, do this:
QVariant MyItem::itemChange( GraphicsItemChange change, const QVariant& value )
{
if ( change == QGraphicsItem::ItemSelectedHasChanged ) {
qDebug() << ( isSelected() ? "selected" : "not selected" );
}
return QGraphicsItem::itemChange( change, value );
}

Answer 2

If you want to use signals and slots, you need to subclass both QObject and QGraphicsPixmapItem.
Because QObject doesn't contain clicked() signal, you need to implement that, too, by re-implementing
void mousePressEvent ( QGraphicsSceneMouseEvent *e ) and void mouseReleaseEvent ( QGraphicsSceneMouseEvent *e ).
MyItem:
#pragma once
#include <QGraphicsPixmapItem>
#include <qobject.h>
#include <QMouseEvent>
#include <QGraphicsSceneMouseEvent>
class MyItem: public QObject, public QGraphicsPixmapItem
/* moc.exe requires to derive from QObject first! */
{
Q_OBJECT
public:
MyItem(QGraphicsItem *parent = 0): QObject(), QGraphicsPixmapItem(parent)
{
}
MyItem(const QPixmap & pixmap, QGraphicsItem * parent = 0 ): QObject(),
QGraphicsPixmapItem(pixmap, parent)
{
}
signals:
void clicked();
protected:
// re-implement processing of mouse events
void mouseReleaseEvent ( QGraphicsSceneMouseEvent *e )
{
// check if cursor not moved since click beginning
if ((m_mouseClick) && (e->pos() == m_lastPoint))
{
// do something: for example emit Click signal
emit clicked();
}
}
void mousePressEvent ( QGraphicsSceneMouseEvent *e )
{
// store click position
m_lastPoint = e->pos();
// set the flag meaning "click begin"
m_mouseClick = true;
}
private:
bool m_mouseClick;
QPointF m_lastPoint;
};
And simple example of usage:
#include <qgraphicsview.h>
#include <qgraphicsscene.h>
#include "reader.h"
#include <qdebug.h>
class MainAppClass: public QObject
{
Q_OBJECT
public:
MainAppClass()
{
QGraphicsScene *scene = new QGraphicsScene();;
scene->setSceneRect( -100.0, -100.0, 200.0, 200.0 );
MyItem *item = new MyItem(QPixmap("about.png"));
connect(item, SIGNAL(clicked()), this, SLOT(pixmapClicked()));
scene->addItem(item);
QGraphicsView * view = new QGraphicsView( scene );
view->setRenderHints( QPainter::Antialiasing );
view->show();
}
public slots:
void pixmapClicked()
{
qDebug() << "item clicked!" ;
}
};