Qt: restrict movable area of a QGraphicsItem insid

问题:

I think my question is similar to this post but in C++ and inside a QGraphicsItem.

I would like to fix the movable area of my object inside an other QGraphicsItem. I want my object to stay inside if I try to move it outside.

Maybe the idea will be to use setParentItem().

Does someone know how to restrict a movable area inside a QGraphicsItem please?

回答1:

Yes, you are correct. As in here you have to reimplement itemChange. From the qt documentation

QVariant Component::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange && scene()) {
        // value is the new position.
        QPointF newPos = value.toPointF();
        QRectF rect = scene()->sceneRect();
        if (!rect.contains(newPos)) {
            // Keep the item inside the scene rect.
            newPos.setX(qMin(rect.right(), qMax(newPos.x(), rect.left())));
            newPos.setY(qMin(rect.bottom(), qMax(newPos.y(), rect.top())));
            return newPos;
        }
    }
    return QGraphicsItem::itemChange(change, value);
}

where scene() refers to the QGraphicsScene in which the item reside. If you don't use a QGraphicScene you must set a QRectF appropriately (maybe from the parent item geometry).

回答2:

I solved my problem!

For this I add to redefine how to set the position of my QGraphicsItem. My item is just define by the boundingRect() like this:

QRectF MyClass::boundingRect() const
{
return QRectF( -_w/2, -_h/2, _w, _h);
}

So I wanted this QRectF to stay inside the scene. The position of my item is define by the center of this QRectF. Using the code from the Qt documentation proposed by @Salvatore Avanzo here is my code:

QVariant Aabb::itemChange(GraphicsItemChange change, const QVariant &value)
{


if (change == ItemPositionChange && scene()) {
    // value is the new position.
    QPointF newPos = value.toPointF();
    QRectF rect = scene()->sceneRect();

    if (!rect.contains(newPos.x() - _w/2, newPos.y() -     _h/2)||!rect.contains(newPos.x() - _w/2, newPos.y() + _h/2)||!rect.contains(newPos.x() + _w/2, newPos.y() + _h/2)||!rect.contains(newPos.x() + _w/2, newPos.y() - _h/2)) 
    {
        // Keep the item inside the scene rect.
        newPos.setX(qMin(rect.right() - _w/2, qMax(newPos.x() , rect.left() + _w/2)));
        newPos.setY(qMin(rect.bottom() - _h/2, qMax(newPos.y() , rect.top() + _h/2)));
        return newPos;
    }


}

return QGraphicsItem::itemChange(change, value);
}

Don't forget to set the QRectF of the scene (see comments in the question).