Remove widget subtree from backing store tracker when reparented

When a native widget is reparented such that the value of
maybeBackingStore() changes, it and all of its native descendents
must be removed from the old QWidgetBackingStoreTracker.

If this is not done, the backing store may not be deleted when the
top-level widget is hidden, thereby consuming memory unnecessarily.

Task-number: MOBILITY-1315
Reviewed-by: Jason Barron
Reviewed-by: bnilsen

2 files changed, 21 insertions, 0 deletions

diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index 9b44f15..6c64ffc 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -236,6 +236,17 @@ void QWidgetBackingStoreTracker::unregisterWidget(QWidget *w)
     }
 }
 
+/*!
+    \internal
+    Recursively remove widget and all of its descendents.
+ */
+void QWidgetBackingStoreTracker::unregisterWidgetSubtree(QWidget *widget)
+{
+    unregisterWidget(widget);
+    foreach (QObject *child, widget->children())
+        if (QWidget *childWidget = qobject_cast<QWidget *>(child))
+            unregisterWidgetSubtree(childWidget);
+}
 
 QWidgetPrivate::QWidgetPrivate(int version)
     : QObjectPrivate(version)
@@ -10031,7 +10042,16 @@ void QWidget::setParent(QWidget *parent, Qt::WindowFlags f)
     if (newParent && isAncestorOf(focusWidget()))
         focusWidget()->clearFocus();
 
+    QTLWExtra *oldTopExtra = window()->d_func()->maybeTopData();
+    QWidgetBackingStoreTracker *oldBsTracker = oldTopExtra ? &oldTopExtra->backingStore : 0;
+
     d->setParent_sys(parent, f);
+
+    QTLWExtra *topExtra = window()->d_func()->maybeTopData();
+    QWidgetBackingStoreTracker *bsTracker = topExtra ? &topExtra->backingStore : 0;
+    if (oldBsTracker && oldBsTracker != bsTracker)
+        oldBsTracker->unregisterWidgetSubtree(this);
+
     if (desktopWidget)
         parent = 0;
 
diff --git a/src/gui/kernel/qwidget_p.h b/src/gui/kernel/qwidget_p.h
index 6c89659..ca1e3fc 100644
--- a/src/gui/kernel/qwidget_p.h
+++ b/src/gui/kernel/qwidget_p.h
@@ -122,6 +122,7 @@ public:
 
     void registerWidget(QWidget *w);
     void unregisterWidget(QWidget *w);
+    void unregisterWidgetSubtree(QWidget *w);
 
     inline QWidgetBackingStore* data()
     {