QAbstractItemView::dataChanged(): optimize call to QWidget::update()

When topLeft and bottomRight are different in QAIV::dataChanged(), the
current implementation simply calls QWidget::update() without checking
if the affected cells are visible. This results in a big performance hit
when cells are updated frequently.
Now try to compute the exact update rect by iterating through the
modified indexes.

Fixes: QTBUG-58580
Change-Id: I97de567d494e40ed8cdb1ea1f5b3cf3a2f60455e
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>

1 files changed, 26 insertions, 2 deletions

diff --git a/src/widgets/itemviews/qabstractitemview.cpp b/src/widgets/itemviews/qabstractitemview.cpp
index ebef36d033..b8c30321ff 100644
--- a/src/widgets/itemviews/qabstractitemview.cpp
+++ b/src/widgets/itemviews/qabstractitemview.cpp
@@ -3335,8 +3335,19 @@ void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelInde
         }
     } else {
         d->updateEditorData(topLeft, bottomRight);
-        if (isVisible() && !d->delayedPendingLayout)
-            d->viewport->update();
+        if (isVisible() && !d->delayedPendingLayout) {
+            if (!topLeft.isValid() ||
+                topLeft.parent() != bottomRight.parent() ||
+                topLeft.row() > bottomRight.row() ||
+                topLeft.column() > bottomRight.column()) {
+                // invalid parameter - call update() to redraw all
+                d->viewport->update();
+            } else {
+                const QRect updateRect = d->intersectedRect(d->viewport->rect(), topLeft, bottomRight);
+                if (!updateRect.isEmpty())
+                    d->viewport->update(updateRect);
+            }
+        }
     }
 
 #ifndef QT_NO_ACCESSIBILITY
@@ -3620,6 +3631,19 @@ void QAbstractItemViewPrivate::_q_columnsMoved(const QModelIndex &, int, int, co
   _q_layoutChanged();
 }
 
+QRect QAbstractItemViewPrivate::intersectedRect(const QRect rect, const QModelIndex &topLeft, const QModelIndex &bottomRight) const
+{
+    Q_Q(const QAbstractItemView);
+
+    const auto parentIdx = topLeft.parent();
+    QRect updateRect;
+    for (int r = topLeft.row(); r <= bottomRight.row(); ++r) {
+        for (int c = topLeft.column(); c <= bottomRight.column(); ++c)
+            updateRect |= q->visualRect(model->index(r, c, parentIdx));
+    }
+    return rect.intersected(updateRect);
+}
+
 /*!
     This slot is called when the selection is changed. The previous
     selection (which may be empty), is specified by \a deselected, and the