Slot is not being used

Question

I'm learning the signals/slots in Qt and I have found a problem. I need to create my own slot that is called when items on QGraphicsScene (in QGraphicsView) are moved or selected.

I'm starting with a simple app that has one widget and on it is graphicsView and label. I've created a slot in my window and connected it to QGraphicsScene's signal, but it is not being used. Where is my mistake?

Here is the code:

//MainWindow.h
//as generated by QtCreator, just added one slot to it
...omitted for brevity...

public slots:
       void selectedItemChanged(QGraphicsItem * newItem, QgraphicsItem * oldItem);

..omitted for brevity...

//------------------------------------------------------------------

//MainWindow.cpp
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
   ui->setupUi(this);
   QGraphicsScene * scene = new QGraphicsScene();
   scene->setBackgroundBrush (QBrush(Qt::gray));
   ui->graphicsView->setScene (scene);

   for(int x = 10; x < 250; x+=20)
   {
      QGraphicsEllipseItem * item = scene->addEllipse (x,x,5,5,QPen(Qt::darkGreen),QBrush(Qt::darkGreen));
     item->setFlag (QGraphicsItem::ItemIsFocusable,true);
   }

   QObject::connect (scene,SIGNAL(focusItemChanged),this,SLOT(selectedItemChanged));
}

void MainWindow::selectedItemChanged (QGraphicsItem *newItem, QGraphicsItem *oldItem)
{
   qDebug()<<"called";
   if(newItem == 0)
   {
      ui->label->setText ("Není vybrán bod");
   }
   else
   {
      ui->label->setText (QString::number (newItem->scenePos ().x ()) + "," + QString::number (newItem->scenePos ().y ()));
   }
}

Now, when I run the probram it rins ok, but I cannot set Focus on the circles(ellipses) drawn on the scene and the slot is not used. I tried setting IsSelectable flag, but it does not help. Is there any other preferred way to get this done or solution to my problem?

Have you tried using the full signature? `void QGraphicsScene::focusItemChanged(QGraphicsItem * newFocus, QGraphicsItem * oldFocus, Qt::FocusReason reason)` — Marco A., Oct 17 '14 at 08:23
http://stackoverflow.com/questions/26422154/code-compiles-by-slot-is-not-invoked-called-used-working-executed will probably help you. — Silicomancer, Oct 17 '14 at 09:44
Yup, thanks siliconmancer. Your answer was helpful. Although my mistake was even more basic level. Maybe you can add the bit about the function signature in the SLOT/SIGNAL, not just the name of it. — mishan, Oct 17 '14 at 10:17

score 3 · Answer 1 · answered Oct 17 '14 at 08:32

You're not linking against the signal's right signature, according to the documentation:

void QGraphicsScene::focusItemChanged( QGraphicsItem * newFocus, QGraphicsItem * oldFocus,
                                       Qt::FocusReason reason)

and also notice that you can check the connection's success/failure status via the bool return type of the QObject::connect method

score 0 · Accepted Answer · answered Oct 17 '14 at 10:13

So, in the end i found the answer to my own question. It was a mistake on my side.

in the connect() i used the slots without parenthesis/parameters. It should have looked like:

QObject::connect (scene,
                  SIGNAL(focusItemChanged(QGraphicsItem*,QGraphicsItem*,Qt::FocusReason)),
                  this,
                  SLOT(selectedItemChanged(QGraphicsItem*,QGraphicsItem*)));

Slot is not being used

2 Answers2