ItemViews: make dragging faster when lots of items are selected

QListView know exactly what they have on their viewport and we only
paint items clipped to the viewport. So we don't need to ask for each
item its visualRect.
NB: QTreeView and QTableView probably deservee the same treatment

4 files changed, 55 insertions, 11 deletions

diff --git a/src/gui/itemviews/qabstractitemview.cpp b/src/gui/itemviews/qabstractitemview.cpp
index 5f347dd..a4a69c3 100644
--- a/src/gui/itemviews/qabstractitemview.cpp
+++ b/src/gui/itemviews/qabstractitemview.cpp
@@ -3881,34 +3881,45 @@ bool QAbstractItemViewPrivate::openEditor(const QModelIndex &index, QEvent *even
     return true;
 }
 
-QPixmap QAbstractItemViewPrivate::renderToPixmap(const QModelIndexList &indexes, QRect *r) const
+/*
+  \internal
+
+  returns the pair QRect/QModelIndex that should be painted on the viewports's rect
+  */
+
+QItemViewPaintPairs QAbstractItemViewPrivate::draggablePaintPairs(const QModelIndexList &indexes, QRect *r) const
 {
     Q_ASSERT(r);
     Q_Q(const QAbstractItemView);
     QRect &rect = *r;
     const QRect viewportRect = viewport->rect();
-    QList<QRect> rects;
-    QModelIndexList paintedIndexes;
+    QItemViewPaintPairs ret;
     for (int i = 0; i < indexes.count(); ++i) {
         const QModelIndex &index = indexes.at(i);
         const QRect current = q->visualRect(index);
         if (current.intersects(viewportRect)) {
-            paintedIndexes += index;
-            rects += current;
+            ret += qMakePair(current, index);
             rect |= current;
         }
     }
-    rect = rect.intersected(viewportRect);
-    if (rect.isEmpty())
+    rect &= viewportRect;
+    return ret;
+}
+
+QPixmap QAbstractItemViewPrivate::renderToPixmap(const QModelIndexList &indexes, QRect *r) const
+{
+    Q_ASSERT(r);
+    QItemViewPaintPairs paintPairs = draggablePaintPairs(indexes, r);
+    if (paintPairs.isEmpty())
         return QPixmap();
-    QPixmap pixmap(rect.size());
+    QPixmap pixmap(r->size());
     pixmap.fill(Qt::transparent);
     QPainter painter(&pixmap);
     QStyleOptionViewItemV4 option = viewOptionsV4();
     option.state |= QStyle::State_Selected;
-    for (int j = 0; j < paintedIndexes.count(); ++j) {
-        const QModelIndex &current = paintedIndexes.at(j);
-        option.rect = QRect(rects.at(j).topLeft() - rect.topLeft(), rects.at(j).size());
+    for (int j = 0; j < paintPairs.count(); ++j) {
+        option.rect = paintPairs.at(j).first.translated(r->topLeft());
+        const QModelIndex &current = paintPairs.at(j).second;
         delegateForIndex(current)->paint(&painter, option, current);
     }
     return pixmap;
diff --git a/src/gui/itemviews/qabstractitemview_p.h b/src/gui/itemviews/qabstractitemview_p.h
index 7443d50..557e98b 100644
--- a/src/gui/itemviews/qabstractitemview_p.h
+++ b/src/gui/itemviews/qabstractitemview_p.h
@@ -86,6 +86,9 @@ struct QEditorInfo
 
 };
 
+typedef QPair<QRect, QModelIndex> QItemViewPaintPair;
+typedef QList<QItemViewPaintPair> QItemViewPaintPairs;
+
 class QEmptyModel : public QAbstractItemModel
 {
 public:
@@ -176,7 +179,9 @@ public:
             q_func()->style()->drawPrimitive(QStyle::PE_IndicatorItemViewItemDrop, &opt, painter, q_func());
         }
     }
+
 #endif
+    virtual QItemViewPaintPairs draggablePaintPairs(const QModelIndexList &indexes, QRect *r) const;
 
     inline void releaseEditor(QWidget *editor) const {
         if (editor) {
diff --git a/src/gui/itemviews/qlistview.cpp b/src/gui/itemviews/qlistview.cpp
index 6ff516a..44bcf6f 100644
--- a/src/gui/itemviews/qlistview.cpp
+++ b/src/gui/itemviews/qlistview.cpp
@@ -709,6 +709,32 @@ void QListViewPrivate::selectAll(QItemSelectionModel::SelectionFlags command)
         selectionModel->select(selection, command);
 }
 
+/*!
+  \reimp
+
+  We have a QListView way of knowing what elements are on the viewport
+  through the intersectingSet function
+*/
+QItemViewPaintPairs QListViewPrivate::draggablePaintPairs(const QModelIndexList &indexes, QRect *r) const
+{
+    Q_ASSERT(r);
+    Q_Q(const QListView);
+    QRect &rect = *r;
+    const QRect viewportRect = viewport->rect();
+    QItemViewPaintPairs ret;
+    intersectingSet(viewportRect);
+    const QSet<QModelIndex> visibleIndexes = intersectVector.toList().toSet();
+    for (int i = 0; i < indexes.count(); ++i) {
+        const QModelIndex &index = indexes.at(i);
+        if (visibleIndexes.contains(index)) {
+            const QRect current = q->visualRect(index);
+            ret += qMakePair(current, index);
+            rect |= current;
+        }
+    }
+    rect &= viewportRect;
+    return ret;
+}
 
 /*!
   \internal
diff --git a/src/gui/itemviews/qlistview_p.h b/src/gui/itemviews/qlistview_p.h
index a7a7000..16f2de9 100644
--- a/src/gui/itemviews/qlistview_p.h
+++ b/src/gui/itemviews/qlistview_p.h
@@ -351,6 +351,8 @@ public:
 
     void scrollElasticBandBy(int dx, int dy);
 
+    QItemViewPaintPairs draggablePaintPairs(const QModelIndexList &indexes, QRect *r) const;
+
     // ### FIXME: we only need one at a time
     QDynamicListViewBase *dynamicListView;
     QStaticListViewBase *staticListView;