Moving contentY by large values in List/GridView is slow

We needed to create/destroy every delegate between the current position
and the new position.  Now we estimate element at the new position and
just create the elements from that item.

Change-Id: I9da1354cbadb4e44fafc1a0bee619d058d1e06a2
Task-number: QTBUG-14974
Reviewed-by: Bea Lam

2 files changed, 42 insertions, 0 deletions

diff --git a/src/declarative/graphicsitems/qdeclarativegridview.cpp b/src/declarative/graphicsitems/qdeclarativegridview.cpp
index 3ec29c1..21bfc59 100644
--- a/src/declarative/graphicsitems/qdeclarativegridview.cpp
+++ b/src/declarative/graphicsitems/qdeclarativegridview.cpp
@@ -584,6 +584,26 @@ void QDeclarativeGridViewPrivate::refill(qreal from, qreal to, bool doBuffer)
             --i;
         modelIndex = visibleItems.at(i)->index + 1;
     }
+
+    if (visibleItems.count() && (fillFrom > rowPos + rowSize()*2
+        || fillTo < rowPosAt(visibleIndex) - rowSize())) {
+        // We've jumped more than a page.  Estimate which items are now
+        // visible and fill from there.
+        int count = (fillFrom - (rowPos + rowSize())) / (rowSize()) * columns;
+        for (int i = 0; i < visibleItems.count(); ++i)
+            releaseItem(visibleItems.at(i));
+        visibleItems.clear();
+        modelIndex += count;
+        if (modelIndex >= model->count())
+            modelIndex = model->count() - 1;
+        else if (modelIndex < 0)
+            modelIndex = 0;
+        modelIndex = modelIndex / columns * columns;
+        visibleIndex = modelIndex;
+        colPos = colPosAt(visibleIndex);
+        rowPos = rowPosAt(visibleIndex);
+    }
+
     int colNum = colPos / colSize();
 
     FxGridItem *item = 0;
diff --git a/src/declarative/graphicsitems/qdeclarativelistview.cpp b/src/declarative/graphicsitems/qdeclarativelistview.cpp
index cb751f6..7c01293 100644
--- a/src/declarative/graphicsitems/qdeclarativelistview.cpp
+++ b/src/declarative/graphicsitems/qdeclarativelistview.cpp
@@ -744,6 +744,28 @@ void QDeclarativeListViewPrivate::refill(qreal from, qreal to, bool doBuffer)
         if (visibleItems.at(i)->index != -1)
             modelIndex = visibleItems.at(i)->index + 1;
     }
+
+    if (visibleItems.count() && (fillFrom > itemEnd+averageSize+spacing
+        || fillTo < visiblePos - averageSize - spacing)) {
+        // We've jumped more than a page.  Estimate which items are now
+        // visible and fill from there.
+        int count = (fillFrom - itemEnd) / (averageSize + spacing);
+        for (int i = 0; i < visibleItems.count(); ++i)
+            releaseItem(visibleItems.at(i));
+        visibleItems.clear();
+        modelIndex += count;
+        if (modelIndex >= model->count()) {
+            count -= modelIndex - model->count() + 1;
+            modelIndex = model->count() - 1;
+        } else if (modelIndex < 0) {
+            count -= modelIndex;
+            modelIndex = 0;
+        }
+        visibleIndex = modelIndex;
+        visiblePos = itemEnd + count * (averageSize + spacing) + 1;
+        itemEnd = visiblePos-1;
+    }
+
     bool changed = false;
     FxListItem *item = 0;
     qreal pos = itemEnd + 1;