Question about the MouseMoveEvent of qtdemo's TextButton

[yzdying2]

I would like to use MouseMoveEvent event of Textbutton in qtdemo, so that I can drag the item to the specified location.
I add the flag "QGraphicsItem::ItemIsMovable" to item in menumanager.cpp
@
void MenuManager::createMenu(const QDomElement &category, BUTTON_TYPE type)
{
.........

        // create normal menu button
        QString label = currentNode.toElement().attribute("name");
        item = new TextButton(label, TextButton::LEFT, type, this->window->scene, this->window->mainSceneRoot);
        currentNode = currentNode.nextSibling();

        item->setFlags(QGraphicsItem::ItemIsMovable);

.........
}
@
First, Drag a item to specified location . When I click the leftbutton and start to drag the item,it will jump to (0 ,0) before the mouse moving.

For example:
I drag dowm a item from the location of (59, 233) to the specified location of (59, 236);
drag item pos output:
QPointF(59, 233)
QPointF(0, 1)
QPointF(0, 2)
QPointF(0, 3)

As you can see, the moving graber jump from (59, 233) to (0, 1).

So I don't know what cause this result.