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, 27 insertions, 11 deletions

diff --git a/src/gui/itemviews/qtableview.cpp b/src/gui/itemviews/qtableview.cpp
index 702a8bb..43445b4 100644
--- a/src/gui/itemviews/qtableview.cpp
+++ b/src/gui/itemviews/qtableview.cpp
@@ -1858,6 +1858,9 @@ void QTableView::setSelection(const QRect &rect, QItemSelectionModel::SelectionF
 
     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 QTableView::visualRegionForSelection(const QItemSelection &selection) const
 {
@@ -1867,6 +1870,7 @@ QRegion QTableView::visualRegionForSelection(const QItemSelection &selection) co
         return QRegion();
 
     QRegion selectionRegion;
+    const QRect &viewportRect = d->viewport->rect();
     bool verticalMoved = verticalHeader()->sectionsMoved();
     bool horizontalMoved = horizontalHeader()->sectionsMoved();
 
@@ -1876,8 +1880,11 @@ QRegion QTableView::visualRegionForSelection(const QItemSelection &selection) co
             if (range.parent() != d->root || !range.isValid())
                 continue;
             for (int r = range.top(); r <= range.bottom(); ++r)
-                for (int c = range.left(); c <= range.right(); ++c)
-                    selectionRegion += QRegion(visualRect(d->model->index(r, c, d->root)));
+                for (int c = range.left(); c <= range.right(); ++c) {
+                    const QRect &rangeRect = visualRect(d->model->index(r, c, d->root));
+                    if (viewportRect.intersects(rangeRect))
+                        selectionRegion += rangeRect;
+                }
         }
     } else if (horizontalMoved) {
         for (int i = 0; i < selection.count(); ++i) {
@@ -1889,9 +1896,11 @@ QRegion QTableView::visualRegionForSelection(const QItemSelection &selection) co
             if (top > bottom)
                 qSwap<int>(top, bottom);
             int height = bottom - top;
-            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 if (verticalMoved) {
         for (int i = 0; i < selection.count(); ++i) {
@@ -1903,9 +1912,11 @@ QRegion QTableView::visualRegionForSelection(const QItemSelection &selection) co
             if (left > right)
                 qSwap<int>(left, right);
             int width = right - left;
-            for (int r = range.top(); r <= range.bottom(); ++r)
-                selectionRegion += QRegion(QRect(left, rowViewportPosition(r),
-                                                 width, rowHeight(r)));
+            for (int r = range.top(); r <= range.bottom(); ++r) {
+                const QRect rangeRect(left, rowViewportPosition(r), width, rowHeight(r));
+                if (viewportRect.intersects(rangeRect))
+                    selectionRegion += rangeRect;
+            }
         }
     } else { // nothing moved
         const int gridAdjust = showGrid() ? 1 : 0;
@@ -1926,12 +1937,17 @@ QRegion QTableView::visualRegionForSelection(const QItemSelection &selection) co
                 rleft = columnViewportPosition(range.right());
                 rright = columnViewportPosition(range.left()) + columnWidth(range.left());
             }
-            selectionRegion += QRect(QPoint(rleft, rtop), QPoint(rright - 1 - gridAdjust, rbottom - 1 - gridAdjust));
+            const QRect rangeRect(QPoint(rleft, rtop), QPoint(rright - 1 - gridAdjust, rbottom - 1 - gridAdjust));
+            if (viewportRect.intersects(rangeRect))
+                selectionRegion += rangeRect;
             if (d->hasSpans()) {
                 foreach (QSpanCollection::Span *s,
                          d->spans.spansInRect(range.left(), range.top(), range.width(), range.height())) {
-                    if (range.contains(s->top(), s->left(), range.parent()))
-                        selectionRegion += d->visualSpanRect(*s);
+                    if (range.contains(s->top(), s->left(), range.parent())) {
+                        const QRect &visualSpanRect = d->visualSpanRect(*s);
+                        if (viewportRect.intersects(visualSpanRect))
+                            selectionRegion += visualSpanRect;
+                    }
                 }
             }
         }