Qt tracing QGraphicsScene::itemAt() back to data model

Question

I have a list of objects that I use to add objects into a QGraphicsScene:

for(int i = 0; i < levelObjects.length(); i++)
{
    QRect objRect;
    objRect = spriteSheetLocations.value(levelObjects.at(i).value("frame_name"));

    //Q_ASSERT_X(objRect != QRect(0,0,0,0), "MainWindow::loadFile()", "Could not find sprite location!");
    QImage img = spriteSheet.copy(objRect);
    int height = levelObjects.at(i).value("height").toInt();
    int width = levelObjects.at(i).value("width").toInt();
    int x = levelObjects.at(i).value("x").toInt();
    int y = levelObjects.at(i).value("y").toInt();
    img = img.scaled(QSize(width, height), Qt::IgnoreAspectRatio);
    item = scene->addPixmap(QPixmap::fromImage(img));
    int xPos = x - width/2;
    int yPos = levelPlist.value("level_height").toInt() - (y + height/2);
    item->setPos(xPos, yPos);
}

Later on, in the GraphicsScene class, I detect when the user clicks on an item and drags it to move it:

void LevelGraphicsView::mousePressEvent(QMouseEvent *event)
{
    if (QGraphicsItem *item = itemAt(event->pos())) {
        qDebug() << "You clicked on item" << item;
        draggedItem = item;
        int mouseX = draggedItem->pos().x() - mapToScene(event->pos()).x();
        int mouseY = draggedItem->pos().y() - mapToScene(event->pos()).y();
        mouseOffset = QPointF(mouseX, mouseY);
    } else {
        qDebug() << "You didn't click on an item.";
        draggedItem = NULL;
        mouseOffset = QPointF(0,0);
    }
}

void LevelGraphicsView::mouseMoveEvent(QMouseEvent *event)
{
    if(!draggedItem) // no item selected
        return;

    QPointF pos = mapToScene(event->pos()) + mouseOffset;
    draggedItem->setPos(pos);
}

This works fine for moving the items in the graphics view, but I'm having trouble tracing the QGraphicsItem back to the list item that created it.

What's the best way to link the QGraphicsItem with the list item from which it was made so that the list item can be changed to reflect the change of position?

zkunov zkunov · Accepted Answer · 2011-07-18T01:42:11

The best way would be a way that does not require to manualy sync items from your list and items on the scene.
The best way to do that depends on your design - may be your items can become pointers to the items on the scene or they can hold ones.

Qt tracing QGraphicsScene::itemAt() back to data model

2 Answers