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

5 files changed, 95 insertions, 17 deletions

diff --git a/src/gui/kernel/qapplication_s60.cpp b/src/gui/kernel/qapplication_s60.cpp
index b1e0f63..6997326 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 25a7750..304cd17 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 e180001..a9ea9ac 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 555647c..9f7ad95 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 438c2a3..6b91dff 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()