[SOLVED]Crash on ensureVisible ?



  • I have a problem with ensureVisible and I didn't find an answer why this thing behaves like that. So, I have a view, this view is displaying on a scene where I added a movable rect item, and I'm trying to do some auto-scrolling when I'm moving the item but when the view has his height smaller than a value the app crashes. I did a small code example to show the behavior:

    @//testview.h

    #ifndef TESTVIEW_H
    #define TESTVIEW_H

    #include<QGraphicsView>
    #include<QGraphicsScene>
    #include<QGraphicsRectItem>

    class MyView : public QGraphicsView
    {
    QGraphicsScene* myScene;

    public:
    MyView();
    };

    class MyItem : public QGraphicsRectItem
    {
    public:
    MyItem();
    //events
    QVariant itemChange( GraphicsItemChange change, const QVariant &value );
    };

    #endif // TESTVIEW_H

    //testview.cpp

    #include "testview.h"

    MyView::MyView() : QGraphicsView()
    {
    myScene = new QGraphicsScene;
    setFixedHeight( 100 );
    setScene( myScene );
    myScene->setSceneRect( 0, 0, 2000, 2000 );
    MyItem* item = new MyItem;
    myScene->addItem( item );
    }

    MyItem::MyItem() : QGraphicsRectItem()
    {
    setRect( 0,0, 50, 50 );
    setFlag(ItemSendsGeometryChanges,true);
    setFlag(ItemIsMovable, true );
    setZValue( 4 );
    }

    QVariant MyItem::itemChange(GraphicsItemChange change, const QVariant &value)
    {
    if( change == ItemPositionChange )
    {
    scene()->views()[0]->ensureVisible( this );
    }
    return QGraphicsRectItem::itemChange( change, value );
    }

    //main.cpp

    #include<QApplication>
    #include"testview.h"

    int main( int argc, char* argv[] )
    {
    QApplication app( argc, argv );

    MyView* testView = new MyView;
    testView->show();
    
    
    return app.exec&#40;&#41;
    

    }@

    When I'm removing this line : setFixedHeight( 100 ); the view will have a bigger height and it will work as it should but in my app I really need a smaller view and that is not a solution for me. Or maybe I'm doing something wrong in my code. Please take a look and let me know.

    Thanks!



  • no one had this problem before?...



  • What did your debugger tell you at the crash?



  • The inferior stopped because it received a signal from the OS. Signal name: SIGSEGV, Signal Meaning : Segmentation fault . The call stack looks like this:

    0 QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData> >::operator-> qscopedpointer.h 112 0x00f18c23
    1 QObject::parent qobject.h 250 0x00f4b39c
    2 QWidget::parentWidget qwidget.h 1022 0x00f51865
    3 QWidget::window qwidget.cpp 4182 0x007ea2b1
    4 QWidgetPrivate::scrollRect qbackingstore.cpp 980 0x00973662
    5 QWidgetPrivate::scroll_sys qwidget_win.cpp 1605 0x00825a3c
    6 QWidget::scroll qwidget.cpp 10138 0x007f7222
    7 QGraphicsView::scrollContentsBy qgraphicsview.cpp 3601 0x00d6dd44
    8 QAbstractScrollAreaPrivate::_q_vslide qabstractscrollarea.cpp 1319 0x00bfc246
    9 QAbstractScrollArea::qt_metacall moc_qabstractscrollarea.cpp 85 0x00bfc6bf
    10 QGraphicsView::qt_metacall moc_qgraphicsview.cpp 152 0x00d6e96a
    11 QMetaObject::metacall qmetaobject.cpp 237 0x6a20445c
    12 QMetaObject::activate qobject.cpp 3272 0x6a214bbe
    13 QAbstractSlider::valueChanged moc_qabstractslider.cpp 182 0x00db32fa
    14 QAbstractSlider::setValue qabstractslider.cpp 543 0x00b24ee4
    15 QGraphicsView::ensureVisible qgraphicsview.cpp 1905 0x00d672cc
    16 QGraphicsView::ensureVisible qgraphicsview.cpp 1930 0x00d6730d
    17 MyItem::itemChange testview.cpp 28 0x0040196c
    18 QGraphicsItem::setPos qgraphicsitem.cpp 3678 0x00d223ba
    19 QGraphicsItem::mouseMoveEvent qgraphicsitem.cpp 7126 0x00d2af36
    20 QGraphicsItem::sceneEvent qgraphicsitem.cpp 6663 0x00d29aab
    ... <More>

    My guess is that when the ensureVisible is called, itemChange is called so we're entering into an infinite loop( don't know if it's a correct behavior ), but why it's working on views with higher height?
    Thanks!



  • I think your code triggers a corner case where ensureVisible() implicitly causes QGraphicsItem::setPos() to be called. This in turn causes a new itemChange event. The Qt Documentation states that method calls that affect the item state should be avoided inside itemChange() since they might trigger endless recursion:

    http://doc.qt.nokia.com/4.7/qgraphicsitem.html#GraphicsItemChange-enum

    The following code should work:

    @
    #include <QGraphicsView>
    #include <QGraphicsScene>
    #include <QGraphicsRectItem>
    #include <QApplication>
    #include <QMouseEvent>

    class MyView : public QGraphicsView
    {

    QGraphicsScene* myScene;

    public:
    MyView();

    protected:
    void mouseMoveEvent(QMouseEvent* event);

    };

    class MyItem : public QGraphicsRectItem
    {
    public:
    MyItem();

    };

    MyView::MyView() : QGraphicsView()
    {
    myScene = new QGraphicsScene;
    setFixedHeight( 100 );
    setScene( myScene );
    myScene->setSceneRect( 0, 0, 2000, 2000 );
    MyItem* item = new MyItem;
    myScene->addItem( item );
    }

    void MyView::mouseMoveEvent(QMouseEvent* event)
    {

    QGraphicsView::mouseMoveEvent(event);

    QGraphicsItem* item = itemAt(event->pos());

    if (item) {
    ensureVisible(item);
    }

    }

    MyItem::MyItem() : QGraphicsRectItem()
    {
    setRect( 0,0, 50, 50 );
    setFlag(ItemSendsGeometryChanges,true);
    setFlag(ItemIsMovable, true );
    setZValue( 4 );
    }

    int main( int argc, char* argv[])
    {
    QApplication app( argc, argv );

    MyView* testView = new MyView;
    testView->show();

    return app.exec();

    }
    @

    This is of course no perfect solution (if such a thing exists, anyway) but works well enough to illustrate a possible workaround.



  • Yes, seems to be a good workaround, Thanks for help!


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