Avoid unnecessary detach / deep copy in QGraphicsItem::scroll().

QGraphicsItem::scroll() scrolls the cache pixmap when cacheMode is
enabled (for ItemCoordinateCache only). Because the pixmap exists both
in the cache and in a temp QPixmap copy, the ref count is 2, so the
scroll operation has to do a deep copy. To avoid this, we remove the
pixmap from the cache before and reinsert it again after calling
QPixmap::scroll().

Reviewed-by: Alexis Menard <alexis.menard@nokia.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/gui/graphicsview/qgraphicsitem.cpp b/src/gui/graphicsview/qgraphicsitem.cpp
index 36d21a6..d284a29 100644
--- a/src/gui/graphicsview/qgraphicsitem.cpp
+++ b/src/gui/graphicsview/qgraphicsitem.cpp
@@ -5687,11 +5687,13 @@ void QGraphicsItem::scroll(qreal dx, qreal dy, const QRectF &rect)
         return;
     }
 
+    // Find pixmap in cache, then remove to avoid deep copy when modifying.s
     QPixmap cachedPixmap;
     if (!QPixmapCache::find(cache->key, &cachedPixmap)) {
         update(rect);
         return;
     }
+    QPixmapCache::remove(cache->key);
 
     QRegion exposed;
     const bool scrollEntirePixmap = rect.isNull();
@@ -5707,7 +5709,8 @@ void QGraphicsItem::scroll(qreal dx, qreal dy, const QRectF &rect)
         cachedPixmap.scroll(dx, dy, scrollRect.translated(-cache->boundingRect.topLeft()), &exposed);
     }
 
-    QPixmapCache::replace(cache->key, cachedPixmap);
+    // Reinsert into cache.
+    cache->key = QPixmapCache::insert(cachedPixmap);
 
     // Translate the existing expose.
     for (int i = 0; i < cache->exposed.size(); ++i) {