QAbstractItemView: don't calc update rect when there are to many indexes

With 8de62d34321cd827e60b0a7b6e7434070de301ae we try to avoid a complete
repaint by checking if the changed indexes are inside the visible rect.
This works fine when the amount of indexes to check is small but fails
otherwise. Therefore add a configurable, upper bound for this check.

Fixes: QTBUG-124173
Change-Id: I1aedd2cbe6e2d0090a640c8a1cf1cdb69f5b002a
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/widgets/itemviews/qabstractitemview.cpp

@@ -85,7 +85,8 @@ QAbstractItemViewPrivate::QAbstractItemViewPrivate()
         delayedPendingLayout(true),
         moveCursorUpdatedView(false),
         verticalScrollModeSet(false),
-        horizontalScrollModeSet(false)
+        horizontalScrollModeSet(false),
+        updateThreshold(200)
 {
     keyboardInputTime.invalidate();
 }
@@ -3203,6 +3204,37 @@ int QAbstractItemView::sizeHintForColumn(int column) const
     return width;
 }
 
+/*!
+    \property QAbstractItemView::updateThreshold
+    \since 6.9
+    This property holds the amount of changed indexes to directly trigger
+    a full update of the view inside dataChanged().
+
+    The algorithm inside dataChanged() tries to minimize a full update of the
+    view by calculating if the changed indexes are visible or not. For very
+    large models, with a lot of large changes, this might take longer than the
+    actual update so it's counter-productive. This property gives the ability
+    to control the algorithm to skip the check and directly trigger a full
+    update when the amount of changed indexes exceeds the given value.
+
+    The default value is 200.
+
+    \sa dataChanged()
+*/
+uint32_t QAbstractItemView::updateThreshold() const
+{
+    Q_D(const QAbstractItemView);
+    return d->updateThreshold;
+}
+
+void QAbstractItemView::setUpdateThreshold(uint32_t threshold)
+{
+    Q_D(QAbstractItemView);
+    if (d->updateThreshold == threshold)
+        return;
+    d->updateThreshold = threshold;
+}
+
 /*!
     Opens a persistent editor on the item at the given \a index.
     If no editor exists, the delegate will create a new editor.
@@ -3397,6 +3429,11 @@ void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelInde
                 topLeft.row() > bottomRight.row() ||
                 topLeft.column() > bottomRight.column()) {
                 // invalid parameter - call update() to redraw all
+                Q_ASSERT(false);
+                d->viewport->update();
+            } else if ((bottomRight.row() - topLeft.row() + 1ULL) *
+                       (bottomRight.column() - topLeft.column() + 1ULL) > d->updateThreshold) {
+                // too many indices to check - force full update
                 d->viewport->update();
             } else {
                 const QRect updateRect = d->intersectedRect(d->viewport->rect(), topLeft, bottomRight);

src/widgets/itemviews/qabstractitemview.h

@@ -46,6 +46,8 @@ class Q_WIDGETS_EXPORT QAbstractItemView : public QAbstractScrollArea
                RESET resetVerticalScrollMode)
     Q_PROPERTY(ScrollMode horizontalScrollMode READ horizontalScrollMode
                WRITE setHorizontalScrollMode RESET resetHorizontalScrollMode)
+    Q_PROPERTY(uint32_t updateThreshold READ updateThreshold
+               WRITE setUpdateThreshold)
 
 public:
     enum SelectionMode {
@@ -177,6 +179,9 @@ public:
     virtual int sizeHintForRow(int row) const;
     virtual int sizeHintForColumn(int column) const;
 
+    uint32_t updateThreshold() const;
+    void setUpdateThreshold(uint32_t threshold);
+
     void openPersistentEditor(const QModelIndex &index);
     void closePersistentEditor(const QModelIndex &index);
     bool isPersistentEditorOpen(const QModelIndex &index) const;

src/widgets/itemviews/qabstractitemview_p.h

@@ -421,6 +421,8 @@ public:
     bool verticalScrollModeSet;
     bool horizontalScrollModeSet;
 
+    uint32_t updateThreshold;
+
     virtual QRect visualRect(const QModelIndex &index) const { return q_func()->visualRect(index); }
 
     std::array<QMetaObject::Connection, 14> modelConnections;

tests/auto/widgets/itemviews/qabstractitemview/tst_qabstractitemview.cpp

@@ -295,6 +295,10 @@ void tst_QAbstractItemView::getSetCheck()
     QCOMPARE(20, obj1->autoScrollMargin());
     obj1->setAutoScrollMargin(16);
     QCOMPARE(16, obj1->autoScrollMargin());
+
+    QCOMPARE(200U, obj1->updateThreshold());
+    obj1->setUpdateThreshold(4711);
+    QCOMPARE(4711U, obj1->updateThreshold());
 }
 
 void tst_QAbstractItemView::cleanup()