Optimized visualRegionForSelection in various item views classes

When the number of selected items is large (a few thousands), the
computation of visualRegionForSelection can take a long time (up to a
few seconds). Analysis with valgrind shows that most of the time is
spent in miRegionOp (in qregion.cpp), which is being called by
QRegion::operator+=. The visualRegionForSelection virtual method being
called only to update the view after selection, we can safely ignore
those item's rectangles outside the viewport, thus both reducing the
number of calls to miRegionOp and the actual cost of each call.

This, however, introduces a behaviour change in
visualRegionForSelection, as the returned region will *not* contain
any rectangle *not* intersecting the viewport.

Reviewed-by: Thierry
Task-number: QTBUG-884

1 files changed, 11 insertions, 4 deletions

diff --git a/src/gui/itemviews/qtreeview.cpp b/src/gui/itemviews/qtreeview.cpp
index ada3936..101dafe 100644
--- a/src/gui/itemviews/qtreeview.cpp
+++ b/src/gui/itemviews/qtreeview.cpp
@@ -2271,6 +2271,9 @@ void QTreeView::setSelection(const QRect &rect, QItemSelectionModel::SelectionFl
 /*!
   Returns the rectangle from the viewport of the items in the given
   \a selection.
+
+  Since 4.7, the returned region only contains rectangles intersecting
+  (or included in) the viewport.
 */
 QRegion QTreeView::visualRegionForSelection(const QItemSelection &selection) const
 {
@@ -2279,6 +2282,7 @@ QRegion QTreeView::visualRegionForSelection(const QItemSelection &selection) con
         return QRegion();
 
     QRegion selectionRegion;
+    const QRect &viewportRect = d->viewport->rect();
     for (int i = 0; i < selection.count(); ++i) {
         QItemSelectionRange range = selection.at(i);
         if (!range.isValid())
@@ -2311,13 +2315,16 @@ QRegion QTreeView::visualRegionForSelection(const QItemSelection &selection) con
             qSwap<int>(top, bottom);
         int height = bottom - top + 1;
         if (d->header->sectionsMoved()) {
-            for (int c = range.left(); c <= range.right(); ++c)
-                selectionRegion += QRegion(QRect(columnViewportPosition(c), top,
-                                                 columnWidth(c), height));
+            for (int c = range.left(); c <= range.right(); ++c) {
+                const QRect rangeRect(columnViewportPosition(c), top, columnWidth(c), height);
+                if (viewportRect.intersects(rangeRect))
+                    selectionRegion += rangeRect;
+            }
         } else {
             QRect combined = leftRect|rightRect;
             combined.setX(columnViewportPosition(isRightToLeft() ? range.right() : range.left()));
-            selectionRegion += combined;
+            if (viewportRect.intersects(combined))
+                selectionRegion += combined;
         }
     }
     return selectionRegion;