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Connecting a signal from a QGraphicsItem inside QGraphicsView



  • In my main window, I have called a subclass of QGraphicsView. And inside my custom QGraphicsView class, I have added a subclass of QGraphicsItem. In the custom QGraphicsItem class, I have a function that emits a signal. I want to catch this signal back in my main window, but I don't know how to connect to it. I tried using the graphicsView object as the signal sender but it doesn't work.

    main_window.cpp

    MyGraphicsView myView = new MyGraphicsView();
    
    connect(m_ui.myView, &MyGraphicsView::mySignal, this, &MyMainWindow::mySlot)
    

    MyGraphicsView .cpp

    class MyGraphicsView : public QGraphicsView
    {
    ...
        scene = new QGraphicsScene(this);
        myItem = new MyGraphicsItem();
        scene->addItem(myItem);
    }
    

    MyGraphicsItem.cpp

    class MyGraphicsItem: public QGraphicsItem
    {
        Q_OBJECT
    ...
        void myFunction() {
            emit mySignal();
        }
    }
    

    I'm getting the error "no member named mySignal in MyGraphicsView". I don't know how to point to the graphicsItem as the signal sender from the main window.


  • Lifetime Qt Champion

    Hi,
    @lansing said in Connecting a signal from a QGraphicsItem inside QGraphicsView:

    MyGraphicsView myView = new MyGraphicsView();

    connect(m_ui.myView, &MyGraphicsView::mySignal, this, &MyMainWindow::mySlot)

    myView and m_ui.myView are not the same object and likely not from the same class.



  • @SGaist said in Connecting a signal from a QGraphicsItem inside QGraphicsView:

    Hi,
    @lansing said in Connecting a signal from a QGraphicsItem inside QGraphicsView:

    MyGraphicsView myView = new MyGraphicsView();

    connect(m_ui.myView, &MyGraphicsView::mySignal, this, &MyMainWindow::mySlot)

    myView and m_ui.myView are not the same object and likely not from the same class.

    Hi they are the same, I just removed all the other extra stuff to simplify my problem. In my project, myView was promoted to MyGraphicsView class in the ui instead of declaring it in the main_window.cpp



  • Hi I figured it out.

    First I should subclass QGraphicsObject for my item instead of QGraphicsItem, because it was made specifically for use of signal and slots inside graphicsitem.

    Then when the item object fired a signal, catch it in the graphicsView object and then forward it to the main_window at the top. Just create a signal with the same name for each of the item and view class.

    MyGraphicsItem.cpp

    class MyGraphicsItem: public QGraphicsObject
    {
        Q_OBJECT
        ...
        void myFunction() {
            emit mySignal();
        }    
    }
    

    MyGraphicsView .cpp

    class MyGraphicsView : public QGraphicsView
    {
        ...
        scene = new QGraphicsScene(this);    
        myItem = new MyGraphicsItem();    
        scene->addItem(myItem);
    
        connect(myItem, &MyGraphicsItem::mySignal, this, &MyGraphicsView::mySignal);
    }
    

    main_window.cpp

    connect(m_ui.myView, &MyGraphicsView::mySignal, this, &MyMainWindow::mySlot)
    

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