Add QGraphicsItem::itemChangeEnabled().

This function allows the user to disable notifications for item changes.
In QSimpleCanvasItem, all notifications are disabled by default, and
you can enable them one at a time. QGraphicsItem's behavior is to by
default enable all notifications.

A side effect of this change is that I've removed one optimization in
favor of another: by not constructing a QVariant at all when there's no
notification we get maximum performance, at the expense of constructing
it twice if there is a notification.

1 files changed, 20 insertions, 10 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 8641fe2..96800a3 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -666,8 +666,10 @@ void QGraphicsScenePrivate::_q_polishItems()
     const QVariant booleanTrueVariant(true);
     foreach (QGraphicsItem *item, unpolishedItems) {
         if (!item->d_ptr->explicitlyHidden) {
-            item->itemChange(QGraphicsItem::ItemVisibleChange, booleanTrueVariant);
-            item->itemChange(QGraphicsItem::ItemVisibleHasChanged, booleanTrueVariant);
+            if (item->itemChangeEnabled(QGraphicsItem::ItemVisibleChange)) {
+                item->itemChange(QGraphicsItem::ItemVisibleChange, booleanTrueVariant);
+                item->itemChange(QGraphicsItem::ItemVisibleHasChanged, booleanTrueVariant);
+            }
         }
         if (item->isWidget()) {
             QEvent event(QEvent::Polish);
@@ -2950,9 +2952,12 @@ void QGraphicsScene::addItem(QGraphicsItem *item)
 
     // Notify the item that its scene is changing, and allow the item to
     // react.
-    const QVariant newSceneVariant(item->itemChange(QGraphicsItem::ItemSceneChange,
-                                                    qVariantFromValue<QGraphicsScene *>(this)));
-    QGraphicsScene *targetScene = qVariantValue<QGraphicsScene *>(newSceneVariant);
+    QGraphicsScene *targetScene = this;
+    bool notify = item->itemChangeEnabled(QGraphicsItem::ItemSceneChange);
+    if (notify) {
+        targetScene = qVariantValue<QGraphicsScene *>(item->itemChange(QGraphicsItem::ItemSceneChange,
+                                                                       qVariantFromValue<QGraphicsScene *>(this)));
+    }
     if (targetScene != this) {
         if (targetScene && item->scene() != targetScene)
             targetScene->addItem(item);
@@ -3063,7 +3068,8 @@ void QGraphicsScene::addItem(QGraphicsItem *item)
         emit selectionChanged();
 
     // Deliver post-change notification
-    item->itemChange(QGraphicsItem::ItemSceneHasChanged, newSceneVariant);
+    if (notify)
+        item->itemChange(QGraphicsItem::ItemSceneHasChanged, qVariantFromValue<QGraphicsScene *>(targetScene));
 }
 
 /*!
@@ -3327,9 +3333,12 @@ void QGraphicsScene::removeItem(QGraphicsItem *item)
 
     // Notify the item that it's scene is changing to 0, allowing the item to
     // react.
-    const QVariant newSceneVariant(item->itemChange(QGraphicsItem::ItemSceneChange,
-                                                    qVariantFromValue<QGraphicsScene *>(0)));
-    QGraphicsScene *targetScene = qVariantValue<QGraphicsScene *>(newSceneVariant);
+    QGraphicsScene *targetScene = 0;
+    bool notify = item->itemChangeEnabled(QGraphicsItem::ItemSceneChange);
+    if (notify) {
+        targetScene = qVariantValue<QGraphicsScene *>(item->itemChange(QGraphicsItem::ItemSceneChange,
+                                                                       qVariantFromValue<QGraphicsScene *>(0)));
+    }
     if (targetScene != 0 && targetScene != this) {
         targetScene->addItem(item);
         return;
@@ -3425,7 +3434,8 @@ void QGraphicsScene::removeItem(QGraphicsItem *item)
         emit selectionChanged();
 
     // Deliver post-change notification
-    item->itemChange(QGraphicsItem::ItemSceneHasChanged, newSceneVariant);
+    if (notify)
+        item->itemChange(QGraphicsItem::ItemSceneHasChanged, qVariantFromValue<QGraphicsScene *>(targetScene));
 }
 
 /*!