Disable mouse tracking in QGraphicsView if possible.

We don't need mouse tracking unless there are items in the
scene that either accept hover events or have a cursor set.
This cut-off is extremely efficient in the common case since
all items ignore hover events and use the standard cursor
by default.

We no longer dig for items and do lots of intersection and
calculating just for fun :-) We even get rid of the overhead
of 2 x QCoreApplication::sendEvent!

The next step is to optimize the items(*) functions to
simply check for hasCursor()/acceptsHoverEvents() before
we do complex checks like intersects/collidesWithPath() etc.

Auto test included.

Reviewed-by: Andreas

5 files changed, 67 insertions, 2 deletions

diff --git a/src/gui/graphicsview/qgraphicsitem.cpp b/src/gui/graphicsview/qgraphicsitem.cpp
index 5c29e9c..b8ff5b4 100644
--- a/src/gui/graphicsview/qgraphicsitem.cpp
+++ b/src/gui/graphicsview/qgraphicsitem.cpp
@@ -1421,7 +1421,9 @@ void QGraphicsItem::setCursor(const QCursor &cursor)
     d_ptr->setExtra(QGraphicsItemPrivate::ExtraCursor, qVariantValue<QCursor>(cursorVariant));
     d_ptr->hasCursor = 1;
     if (d_ptr->scene) {
+        d_ptr->scene->d_func()->allItemsUseDefaultCursor = false;
         foreach (QGraphicsView *view, d_ptr->scene->views()) {
+            view->viewport()->setMouseTracking(true);
             // Note: Some of this logic is duplicated in QGraphicsView's mouse events.
             if (view->underMouse()) {
                 foreach (QGraphicsItem *itemUnderCursor, view->items(view->mapFromGlobal(QCursor::pos()))) {
@@ -2053,7 +2055,13 @@ bool QGraphicsItem::acceptsHoverEvents() const
 */
 void QGraphicsItem::setAcceptHoverEvents(bool enabled)
 {
+    if (d_ptr->acceptsHover == quint32(enabled))
+        return;
     d_ptr->acceptsHover = quint32(enabled);
+    if (d_ptr->acceptsHover && d_ptr->scene && d_ptr->scene->d_func()->allItemsIgnoreHoverEvents) {
+        d_ptr->scene->d_func()->allItemsIgnoreHoverEvents = false;
+        d_ptr->scene->d_func()->enableMouseTrackingOnViews();
+    }
 }
 
 /*!
@@ -2063,7 +2071,7 @@ void QGraphicsItem::setAcceptHoverEvents(bool enabled)
 */
 void QGraphicsItem::setAcceptsHoverEvents(bool enabled)
 {
-    d_ptr->acceptsHover = quint32(enabled);
+    setAcceptHoverEvents(enabled);
 }
 
 /*!
diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 0439847..e885238 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -351,6 +351,8 @@ QGraphicsScenePrivate::QGraphicsScenePrivate()
       dragDropItem(0),
       enterWidget(0),
       lastDropAction(Qt::IgnoreAction),
+      allItemsIgnoreHoverEvents(true),
+      allItemsUseDefaultCursor(true),
       painterStateProtection(true),
       sortCacheEnabled(false),
       updatingSortCache(false),
@@ -1047,6 +1049,12 @@ void QGraphicsScenePrivate::clearKeyboardGrabber()
         ungrabKeyboard(keyboardGrabberItems.first());
 }
 
+void QGraphicsScenePrivate::enableMouseTrackingOnViews()
+{
+    foreach (QGraphicsView *view, views)
+        view->viewport()->setMouseTracking(true);
+}
+
 /*!
     Returns all items for the screen position in \a event.
 */
@@ -2763,6 +2771,8 @@ void QGraphicsScene::clear()
     d->lastItemCount = 0;
     d->bspTree.clear();
     d->largestUntransformableItem = QRectF();
+    d->allItemsIgnoreHoverEvents = true;
+    d->allItemsUseDefaultCursor = true;
 }
 
 /*!
@@ -2927,6 +2937,17 @@ void QGraphicsScene::addItem(QGraphicsItem *item)
     ++d->selectionChanging;
     int oldSelectedItemSize = d->selectedItems.size();
 
+    // Enable mouse tracking if the item accepts hover events or has a cursor set.
+    if (d->allItemsIgnoreHoverEvents && d->itemAcceptsHoverEvents_helper(item)) {
+        d->allItemsIgnoreHoverEvents = false;
+        d->enableMouseTrackingOnViews();
+    }
+    if (d->allItemsUseDefaultCursor && item->hasCursor()) {
+        d->allItemsUseDefaultCursor = false;
+        if (d->allItemsIgnoreHoverEvents) // already enabled otherwise
+            d->enableMouseTrackingOnViews();
+    }
+
     // Update selection lists
     if (item->isSelected())
         d->selectedItems << item;
@@ -4219,6 +4240,9 @@ bool QGraphicsScenePrivate::itemAcceptsHoverEvents_helper(const QGraphicsItem *i
 */
 bool QGraphicsScenePrivate::dispatchHoverEvent(QGraphicsSceneHoverEvent *hoverEvent)
 {
+    if (allItemsIgnoreHoverEvents)
+        return false;
+
     // Find the first item that accepts hover events, reusing earlier
     // calculated data is possible.
     if (cachedItemsUnderMouse.isEmpty()) {
diff --git a/src/gui/graphicsview/qgraphicsscene_p.h b/src/gui/graphicsview/qgraphicsscene_p.h
index 8af8b28..9ace725 100644
--- a/src/gui/graphicsview/qgraphicsscene_p.h
+++ b/src/gui/graphicsview/qgraphicsscene_p.h
@@ -168,6 +168,9 @@ public:
     Qt::DropAction lastDropAction;
     QList<QGraphicsItem *> cachedItemsUnderMouse;
     QList<QGraphicsItem *> hoverItems;
+    bool allItemsIgnoreHoverEvents;
+    bool allItemsUseDefaultCursor;
+    void enableMouseTrackingOnViews();
     QMap<Qt::MouseButton, QPointF> mouseGrabberButtonDownPos;
     QMap<Qt::MouseButton, QPointF> mouseGrabberButtonDownScenePos;
     QMap<Qt::MouseButton, QPoint> mouseGrabberButtonDownScreenPos;
diff --git a/src/gui/graphicsview/qgraphicsview.cpp b/src/gui/graphicsview/qgraphicsview.cpp
index b41dc66..797c895 100644
--- a/src/gui/graphicsview/qgraphicsview.cpp
+++ b/src/gui/graphicsview/qgraphicsview.cpp
@@ -587,6 +587,10 @@ void QGraphicsViewPrivate::mouseMoveEventHandler(QMouseEvent *event)
         return;
     if (!scene)
         return;
+    if (scene->d_func()->allItemsIgnoreHoverEvents && scene->d_func()->allItemsUseDefaultCursor
+        && !event->buttons()) { // forward event to the scene if something is pressed.
+        return; // No need to process this event further.
+    }
 
     QGraphicsSceneMouseEvent mouseEvent(QEvent::GraphicsSceneMouseMove);
     mouseEvent.setWidget(q->viewport());
@@ -614,6 +618,16 @@ void QGraphicsViewPrivate::mouseMoveEventHandler(QMouseEvent *event)
     }
 
 #ifndef QT_NO_CURSOR
+    // If all the items ignore hover events, we don't look-up any items
+    // in QGraphicsScenePrivate::dispatchHoverEvent, hence the
+    // cachedItemsUnderMouse list will be empty. We therefore do the look-up
+    // for cursor items here if not all items use the default cursor.
+    if (scene->d_func()->allItemsIgnoreHoverEvents && !scene->d_func()->allItemsUseDefaultCursor
+        && scene->d_func()->cachedItemsUnderMouse.isEmpty()) {
+        scene->d_func()->cachedItemsUnderMouse = scene->d_func()->itemsAtPosition(mouseEvent.screenPos(),
+                                                                                  mouseEvent.scenePos(),
+                                                                                  mouseEvent.widget());
+    }
     // Find the topmost item under the mouse with a cursor.
     foreach (QGraphicsItem *item, scene->d_func()->cachedItemsUnderMouse) {
         if (item->hasCursor()) {
@@ -1688,6 +1702,12 @@ void QGraphicsView::setScene(QGraphicsScene *scene)
         d->recalculateContentSize();
         d->lastCenterPoint = sceneRect().center();
         d->keepLastCenterPoint = true;
+        // We are only interested in mouse tracking if items accept
+        // hover events or use non-default cursors.
+        if (!d->scene->d_func()->allItemsIgnoreHoverEvents
+            || !d->scene->d_func()->allItemsUseDefaultCursor) {
+            d->viewport->setMouseTracking(true);
+        }
     } else {
         d->recalculateContentSize();
     }
@@ -2799,7 +2819,12 @@ void QGraphicsView::setupViewport(QWidget *widget)
         widget->setAutoFillBackground(true);
     }
 
-    widget->setMouseTracking(true);
+    // We are only interested in mouse tracking if items
+    // accept hover events or use non-default cursors.
+    if (d->scene && (!d->scene->d_func()->allItemsIgnoreHoverEvents
+                     || !d->scene->d_func()->allItemsUseDefaultCursor)) {
+        widget->setMouseTracking(true);
+    }
     widget->setAcceptDrops(acceptDrops());
 }
 
diff --git a/src/gui/graphicsview/qgraphicswidget.cpp b/src/gui/graphicsview/qgraphicswidget.cpp
index 7f02fb9..2e7d82a 100644
--- a/src/gui/graphicsview/qgraphicswidget.cpp
+++ b/src/gui/graphicsview/qgraphicswidget.cpp
@@ -1635,6 +1635,11 @@ void QGraphicsWidget::setWindowFlags(Qt::WindowFlags wFlags)
         else
             d->scene->d_func()->addPopup(this);
     }
+
+    if (d->scene && d->scene->d_func()->allItemsIgnoreHoverEvents && d->hasDecoration()) {
+        d->scene->d_func()->allItemsIgnoreHoverEvents = false;
+        d->scene->d_func()->enableMouseTrackingOnViews();
+    }
 }
 
 /*!