QGraphicsItem: optimize setCursor() for large number of graphic items

This patch optimizes setCursor() by invoking view->items(position) only
if the current graphics item is under the mouse cursor. If it is not,
setting the cursor for this item should not have any effect on the
actually currently visible mouse cursor, so there should be no
reason to call _q_setViewportCursor() at all in this case, so
it is not necessary to query for all the items under the cursor
position.
In my use case this gives a significant performance improvement
from slow behavior to almost immediately (Linux with X11).

My scenario is that I have many rectangular graphic items next to each
other in a graphics view, and there is the functionality to add one
more graphics item (a "handle") to one of those rectangular graphics
items, and then for this new "handle" a new graphics item will be
added on each of the other existing rectangular graphics items.
The ctor of the "handle" graphics item calls
setCursor(Qt::OpenHandCursor). QGraphicsItem::setCursor() calls
view->item(cursorPosition), and then updates the visible cursor
according to the top most graphics item under the cursor.

So assuming that there are e.g. 1000 rectangular graphics items next
to each other, each showing e.g. 5 "handle" graphic items, adding one
more "handle" graphics item will add one graphics item on each of the
1000 rectangular items, so 1000 times view->items(cursorPos) will be
called, at the beginning with 6000 graphic items (1000 rectangles and
5000 handles) already in the scene, for the last one with 6999 items
in the scene (999 "handles" have been already added), from which the
ones under the cursor have to be found.
This is basically O^2 complexity. With the patch it changes to
linear regarding the number of rectangular graphics items.

Change-Id: I9836fc710a8f11d01a94930ea64c6c946e0db282
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 11 insertions, 6 deletions

diff --git a/src/widgets/graphicsview/qgraphicsitem.cpp b/src/widgets/graphicsview/qgraphicsitem.cpp
index a2120e53fe..38cab60fc2 100644
--- a/src/widgets/graphicsview/qgraphicsitem.cpp
+++ b/src/widgets/graphicsview/qgraphicsitem.cpp
@@ -2331,12 +2331,17 @@ void QGraphicsItem::setCursor(const QCursor &cursor)
             view->viewport()->setMouseTracking(true);
             // Note: Some of this logic is duplicated in QGraphicsView's mouse events.
             if (view->underMouse()) {
-                const auto itemsUnderCursor = view->items(view->mapFromGlobal(QCursor::pos()));
-                for (QGraphicsItem *itemUnderCursor : itemsUnderCursor) {
-                    if (itemUnderCursor->hasCursor()) {
-                        QMetaObject::invokeMethod(view, "_q_setViewportCursor",
-                                                  Q_ARG(QCursor, itemUnderCursor->cursor()));
-                        break;
+                const QPoint viewPoint = view->mapFromGlobal(QCursor::pos());
+                const QPointF cursorPos = mapFromScene(view->mapToScene(viewPoint));
+                // the cursor can only change if the current item is under the mouse
+                if (boundingRect().contains(cursorPos)) {
+                    const auto itemsUnderCursor = view->items(viewPoint);
+                    for (QGraphicsItem *itemUnderCursor : itemsUnderCursor) {
+                        if (itemUnderCursor->hasCursor()) {
+                            QMetaObject::invokeMethod(view, "_q_setViewportCursor",
+                                                      Q_ARG(QCursor, itemUnderCursor->cursor()));
+                            break;
+                        }
                     }
                 }
                 break;