Added reference counting to QWidgetBackingStore

On Symbian, the top-level widget's backing store must be destroyed when
it is no longer required, in order to conserve memory.  The criteria
for destroying the backing store is when neither the TLW nor any of
its native descendents (which share the backing store) are visible.
In order to implement this requirement, a count must be kept of the
number of native widgets which are using the TLW's backing store.

This patch provides the mechanism for maintaining this count, and for
destroying the backing store when the count is decremented to zero.
No calls to either the increment nor decrement functions are made,
however, by this code included in this patch; this code will be added
to only the Symbian backend by a subsequent patch.

Task-number: QTBUG-8697
Reviewed-by: Bjørn Erik Nilsen
Reviewed-by: Jason Barron

8 files changed, 102 insertions, 29 deletions

diff --git a/src/gui/embedded/qwsmanager_qws.cpp b/src/gui/embedded/qwsmanager_qws.cpp
index d6ef14858b..ac6d36eb25 100644
--- a/src/gui/embedded/qwsmanager_qws.cpp
+++ b/src/gui/embedded/qwsmanager_qws.cpp
@@ -392,7 +392,7 @@ void QWSManagerPrivate::dirtyRegion(int decorationRegion,
                                     const QRegion &clip)
 {
     QTLWExtra *topextra = managed->d_func()->extra->topextra;
-    QWidgetBackingStore *bs = topextra->backingStore;
+    QWidgetBackingStore *bs = topextra->backingStore.data();
     const bool pendingUpdateRequest = bs->isDirty();
 
     if (decorationRegion == QDecoration::All) {
diff --git a/src/gui/kernel/qapplication_s60.cpp b/src/gui/kernel/qapplication_s60.cpp
index b1e0f63d0f..6997326bdd 100644
--- a/src/gui/kernel/qapplication_s60.cpp
+++ b/src/gui/kernel/qapplication_s60.cpp
@@ -1706,14 +1706,13 @@ int QApplicationPrivate::symbianProcessWsEvent(const QSymbianEvent *symbianEvent
             if (!w->d_func()->maybeTopData())
                 break;
             if (visChangedEvent->iFlags & TWsVisibilityChangedEvent::ENotVisible) {
-                delete w->d_func()->topData()->backingStore;
-                w->d_func()->topData()->backingStore = 0;
+                w->d_func()->topData()->backingStore.destroy();
                 // In order to ensure that any resources used by the window surface
                 // are immediately freed, we flush the WSERV command buffer.
                 S60->wsSession().Flush();
             } else if ((visChangedEvent->iFlags & TWsVisibilityChangedEvent::EPartiallyVisible)
                        && !w->d_func()->maybeBackingStore()) {
-                w->d_func()->topData()->backingStore = new QWidgetBackingStore(w);
+                w->d_func()->topData()->backingStore.create(w);
                 w->d_func()->invalidateBuffer(w->rect());
                 w->repaint();
             }
diff --git a/src/gui/kernel/qapplication_x11.cpp b/src/gui/kernel/qapplication_x11.cpp
index 25a7750d96..304cd173e9 100644
--- a/src/gui/kernel/qapplication_x11.cpp
+++ b/src/gui/kernel/qapplication_x11.cpp
@@ -5245,7 +5245,7 @@ bool QETWidget::translateConfigEvent(const XEvent *event)
 
         if (isVisible() && data->crect.size() != oldSize) {
             Q_ASSERT(d->extra->topextra);
-            QWidgetBackingStore *bs = d->extra->topextra->backingStore;
+            QWidgetBackingStore *bs = d->extra->topextra->backingStore.data();
             const bool hasStaticContents = bs && bs->hasStaticContents();
             // If we have a backing store with static contents, we have to disable the top-level
             // resize optimization in order to get invalidated regions for resized widgets.
diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index e180001291..a9ea9ac563 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -161,6 +161,51 @@ static inline bool hasBackingStoreSupport()
 extern bool qt_sendSpontaneousEvent(QObject*, QEvent*); // qapplication.cpp
 extern QDesktopWidget *qt_desktopWidget; // qapplication.cpp
 
+
+QRefCountedWidgetBackingStore::QRefCountedWidgetBackingStore()
+    :   m_ptr(0)
+    ,   m_count(0)
+{
+
+}
+
+QRefCountedWidgetBackingStore::~QRefCountedWidgetBackingStore()
+{
+    delete m_ptr;
+}
+
+void QRefCountedWidgetBackingStore::create(QWidget *widget)
+{
+    destroy();
+    m_ptr = new QWidgetBackingStore(widget);
+    m_count = 0;
+}
+
+void QRefCountedWidgetBackingStore::destroy()
+{
+    delete m_ptr;
+    m_ptr = 0;
+    m_count = 0;
+}
+
+void QRefCountedWidgetBackingStore::ref()
+{
+    Q_ASSERT(m_ptr);
+    ++m_count;
+}
+
+void QRefCountedWidgetBackingStore::deref()
+{
+    if (m_count) {
+        Q_ASSERT(m_ptr);
+        if (0 == --m_count) {
+            delete m_ptr;
+            m_ptr = 0;
+        }
+    }
+}
+
+
 QWidgetPrivate::QWidgetPrivate(int version)
     : QObjectPrivate(version)
       , extra(0)
@@ -1324,11 +1369,9 @@ void QWidget::create(WId window, bool initializeWindow, bool destroyOldWindow)
 
     // a real toplevel window needs a backing store
     if (isWindow() && windowType() != Qt::Desktop) {
-        delete d->topData()->backingStore;
-        // QWidgetBackingStore will check this variable, hence it must be 0
-        d->topData()->backingStore = 0;
+        d->topData()->backingStore.destroy();
         if (hasBackingStoreSupport())
-            d->topData()->backingStore = new QWidgetBackingStore(this);
+            d->topData()->backingStore.create(this);
     }
 
     d->setModal_sys();
@@ -1451,8 +1494,7 @@ QWidget::~QWidget()
         // the backing store will delete its window surface, which may or may
         // not have a reference to this widget that will be used later to
         // notify the window it no longer has a surface.
-        delete d->extra->topextra->backingStore;
-        d->extra->topextra->backingStore = 0;
+        d->extra->topextra->backingStore.destroy();
     }
 #endif
     if (QWidgetBackingStore *bs = d->maybeBackingStore()) {
@@ -1540,7 +1582,6 @@ void QWidgetPrivate::createTLExtra()
         QTLWExtra* x = extra->topextra = new QTLWExtra;
         x->icon = 0;
         x->iconPixmap = 0;
-        x->backingStore = 0;
         x->windowSurface = 0;
         x->sharedPainter = 0;
         x->incw = x->inch = 0;
@@ -1619,7 +1660,7 @@ void QWidgetPrivate::deleteExtra()
 #endif
         if (extra->topextra) {
             deleteTLSysExtra();
-            delete extra->topextra->backingStore;
+            extra->topextra->backingStore.destroy();
             delete extra->topextra->icon;
             delete extra->topextra->iconPixmap;
 #if defined(Q_WS_QWS) && !defined(QT_NO_QWS_MANAGER)
diff --git a/src/gui/kernel/qwidget_p.h b/src/gui/kernel/qwidget_p.h
index 555647c1df..9f7ad95aac 100644
--- a/src/gui/kernel/qwidget_p.h
+++ b/src/gui/kernel/qwidget_p.h
@@ -110,13 +110,53 @@ class QWidgetItemV2;
 
 class QStyle;
 
+class Q_AUTOTEST_EXPORT QRefCountedWidgetBackingStore
+{
+public:
+    QRefCountedWidgetBackingStore();
+    ~QRefCountedWidgetBackingStore();
+
+    void create(QWidget *tlw);
+    void destroy();
+
+    void ref();
+    void deref();
+
+    inline QWidgetBackingStore* data()
+    {
+        return m_ptr;
+    }
+
+    inline QWidgetBackingStore* operator->()
+    {
+        return m_ptr;
+    }
+
+    inline QWidgetBackingStore& operator*()
+    {
+        return *m_ptr;
+    }
+
+    inline operator bool() const
+    {
+        return (0 != m_ptr);
+    }
+
+private:
+    Q_DISABLE_COPY(QRefCountedWidgetBackingStore)
+
+private:
+    QWidgetBackingStore* m_ptr;
+    int m_count;
+};
+
 struct QTLWExtra {
     // *************************** Cross-platform variables *****************************
 
     // Regular pointers (keep them together to avoid gaps on 64 bits architectures).
     QIcon *icon; // widget icon
     QPixmap *iconPixmap;
-    QWidgetBackingStore *backingStore;
+    QRefCountedWidgetBackingStore backingStore;
     QWindowSurface *windowSurface;
     QPainter *sharedPainter;
 
@@ -907,7 +947,7 @@ inline QWidgetBackingStore *QWidgetPrivate::maybeBackingStore() const
 {
     Q_Q(const QWidget);
     QTLWExtra *x = q->window()->d_func()->maybeTopData();
-    return x ? x->backingStore : 0;
+    return x ? x->backingStore.data() : 0;
 }
 
 QT_END_NAMESPACE
diff --git a/src/gui/kernel/qwidget_s60.cpp b/src/gui/kernel/qwidget_s60.cpp
index 438c2a303a..6b91dff2f7 100644
--- a/src/gui/kernel/qwidget_s60.cpp
+++ b/src/gui/kernel/qwidget_s60.cpp
@@ -908,14 +908,12 @@ void QWidgetPrivate::registerDropSite(bool /* on */)
 
 void QWidgetPrivate::createTLSysExtra()
 {
-    extra->topextra->backingStore = 0;
     extra->topextra->inExpose = 0;
 }
 
 void QWidgetPrivate::deleteTLSysExtra()
 {
-    delete extra->topextra->backingStore;
-    extra->topextra->backingStore = 0;
+    extra->topextra->backingStore.destroy();
 }
 
 void QWidgetPrivate::createSysExtra()
diff --git a/src/gui/painting/qbackingstore.cpp b/src/gui/painting/qbackingstore.cpp
index 8de9eaa5c8..3ddffef7c2 100644
--- a/src/gui/painting/qbackingstore.cpp
+++ b/src/gui/painting/qbackingstore.cpp
@@ -914,7 +914,7 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
         invalidateBuffer((newRect & clipR).translated(-data.crect.topLeft()));
     } else {
 
-        QWidgetBackingStore *wbs = x->backingStore;
+        QWidgetBackingStore *wbs = x->backingStore.data();
         QRegion childExpose(newRect & clipR);
 
         if (sourceRect.isValid() && wbs->bltRect(sourceRect, dx, dy, pw))
@@ -957,7 +957,7 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
     if (x->inTopLevelResize)
         return;
 
-    QWidgetBackingStore *wbs = x->backingStore;
+    QWidgetBackingStore *wbs = x->backingStore.data();
     if (!wbs)
         return;
 
diff --git a/tests/auto/qwidget/tst_qwidget.cpp b/tests/auto/qwidget/tst_qwidget.cpp
index f4add5a8b7..9369f4a774 100644
--- a/tests/auto/qwidget/tst_qwidget.cpp
+++ b/tests/auto/qwidget/tst_qwidget.cpp
@@ -9500,9 +9500,7 @@ void tst_QWidget::destroyBackingStore()
     QTRY_VERIFY(w.numPaintEvents > 0);
     w.reset();
     w.update();
-    delete qt_widget_private(&w)->topData()->backingStore;
-    qt_widget_private(&w)->topData()->backingStore = 0;
-    qt_widget_private(&w)->topData()->backingStore = new QWidgetBackingStore(&w);
+    qt_widget_private(&w)->topData()->backingStore.create(&w);
 
     w.update();
     QApplication::processEvents();
@@ -9524,7 +9522,7 @@ QWidgetBackingStore* backingStore(QWidget &widget)
     QWidgetBackingStore *backingStore = 0;
 #ifdef QT_BUILD_INTERNAL
     if (QTLWExtra *topExtra = qt_widget_private(&widget)->maybeTopData())
-        backingStore = topExtra->backingStore;
+        backingStore = topExtra->backingStore.data();
 #endif
     return backingStore;
 }
@@ -10244,15 +10242,12 @@ class scrollWidgetWBS : public QWidget
 public:
     void deleteBackingStore()
     {
-        if (static_cast<QWidgetPrivate*>(d_ptr.data())->maybeBackingStore()) {
-            delete static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore;
-            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore = 0;
-        }
+        static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore.destroy();
     }
     void enableBackingStore()
     {
         if (!static_cast<QWidgetPrivate*>(d_ptr.data())->maybeBackingStore()) {
-            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore = new QWidgetBackingStore(this);
+            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore.create(this);
             static_cast<QWidgetPrivate*>(d_ptr.data())->invalidateBuffer(this->rect());
             repaint();
         }