QGraphicsView ensureVisible() and centerOn() - c++

Question

I am going to do pan/scale stuff on QGraphicsView.
So I read the documentation of QGraphicsView and see some utility functions like ensureVisible() and centerOn().
I think I understand what the documentation says but I can' t manage to write a working example.
Could you please write/suggest me an example code to understand the issue.

Answer 1

Ton pan the view by a certain amount (for example in your view's mouseMoveEvent()), assuming MyView is a subclass of QGraphicsView (all the following code was ported from Python, I didn't test it):
void MyView::moveBy(QPoint &delta)
{
QScrollBar *horiz_scroll = horizontalScrollBar();
QScrollBar *vert_scroll = verticalScrollBar();
horiz_scroll->setValue(horiz_scroll.value() - delta.x());
vert_scroll->setValue(vert_scroll.value() - delta.y());
}
To fit a rectangle specified in scene coordinates by zooming and panning:
void MyView::fit(QRectF &rect)
{
setSceneRect(rect);
fitInView(rect, Qt::KeepAspectRatio);
}
Note that if your scene contains non transformable items (with the QGraphicsItem::ItemIgnoresTransformations flag set), you'll have to take extra steps to compute their correct bounding box:
/**
* Compute the bounding box of an item in scene space, handling non
* transformable items.
*/
QRectF sceneBbox(QGraphicsItem *item, QGraphicsItemView *view=NULL)
{
QRectF bbox = item->boundingRect();
QTransform vp_trans, item_to_vp_trans;
if (!(item->flags() & QGraphicsItem::ItemIgnoresTransformations)) {
// Normal item, simply map its bounding box to scene space
bbox = item->mapRectToScene(bbox);
} else {
// Item with the ItemIgnoresTransformations flag, need to compute its
// bounding box with deviceTransform()
if (view) {
vp_trans = view->viewportTransform();
} else {
vp_trans = QTransform();
}
item_to_vp_trans = item->deviceTransform(vp_trans);
// Map bbox to viewport space
bbox = item_to_vp_trans.mapRect(bbox);
// Map bbox back to scene space
bbox = vp_trans.inverted().mapRect(bbox);
}
return bbox;
}
In that case the bounding rect of your objects becomes dependent on the view's zoom level, meaning that sometimes MyView::fit() won't fit exactly your objects (for example when fitting a selection of objects from a largely zoomed out view). A quick and dirty solution is to call MyView::fit() repeatedly until the bounding rect naturally "stabilizes" itself.