Get rid of QWidgetBackingStoreTracker

It was added for Symbian almost 10 years ago (d7057e7c1f1a), for a somewhat
dubious use-case. The Symbian code is since long gone (ae30d7141), so the
remaining pieces are just adding complexity to the already intricate workings
of the QtWidgets backingstore/painting logic.

Task-number: QTBUG-8697
Change-Id: I82af610a8ac26719c588ac63f06b4501f59b400d
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -146,91 +146,6 @@ static inline bool qRectIntersects(const QRect &r1, const QRect &r2)
 extern bool qt_sendSpontaneousEvent(QObject*, QEvent*); // qapplication.cpp
 extern QDesktopWidget *qt_desktopWidget; // qapplication.cpp
 
-/*!
-    \internal
-    \class QWidgetBackingStoreTracker
-    \brief Class which allows tracking of which widgets are using a given backing store
-
-    QWidgetBackingStoreTracker is a thin wrapper around a QWidgetBackingStore pointer,
-    which maintains a list of the QWidgets which are currently using the backing
-    store.  This list is modified via the registerWidget and unregisterWidget functions.
- */
-
-QWidgetBackingStoreTracker::QWidgetBackingStoreTracker()
-    :   m_ptr(0)
-{
-
-}
-
-QWidgetBackingStoreTracker::~QWidgetBackingStoreTracker()
-{
-    delete m_ptr;
-}
-
-/*!
-    \internal
-    Destroy the contained QWidgetBackingStore, if not null, and clear the list of
-    widgets using the backing store, then create a new QWidgetBackingStore, providing
-    the QWidget.
- */
-void QWidgetBackingStoreTracker::create(QWidget *widget)
-{
-    destroy();
-    m_ptr = new QWidgetBackingStore(widget);
-}
-
-/*!
-    \internal
-    Destroy the contained QWidgetBackingStore, if not null, and clear the list of
-    widgets using the backing store.
- */
-void QWidgetBackingStoreTracker::destroy()
-{
-    delete m_ptr;
-    m_ptr = 0;
-    m_widgets.clear();
-}
-
-/*!
-    \internal
-    Add the widget to the list of widgets currently using the backing store.
-    If the widget was already in the list, this function is a no-op.
- */
-void QWidgetBackingStoreTracker::registerWidget(QWidget *w)
-{
-    Q_ASSERT(m_ptr);
-    Q_ASSERT(w->internalWinId());
-    Q_ASSERT(qt_widget_private(w)->maybeBackingStore() == m_ptr);
-    m_widgets.insert(w);
-}
-
-/*!
-    \internal
-    Remove the widget from the list of widgets currently using the backing store.
-    If the widget was in the list, and removing it causes the list to be empty,
-    the backing store is deleted.
-    If the widget was not in the list, this function is a no-op.
- */
-void QWidgetBackingStoreTracker::unregisterWidget(QWidget *w)
-{
-    if (m_widgets.remove(w) && m_widgets.isEmpty()) {
-        delete m_ptr;
-        m_ptr = 0;
-    }
-}
-
-/*!
-    \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)
       , extra(0)
@@ -1364,10 +1279,8 @@ void QWidget::create(WId window, bool initializeWindow, bool destroyOldWindow)
     d->create();
 
     // a real toplevel window needs a backing store
-    if (isWindow() && windowType() != Qt::Desktop) {
+    if (isWindow() && windowType() != Qt::Desktop)
-        d->topData()->backingStoreTracker.destroy();
+        d->topData()->widgetBackingStore.reset(new QWidgetBackingStore(this));
-        d->topData()->backingStoreTracker.create(this);
-    }
 
     d->setModal_sys();
 
@@ -1891,7 +1804,7 @@ void QWidgetPrivate::deleteTLSysExtra()
         //the qplatformbackingstore may hold a reference to the window, so the backingstore
         //needs to be deleted first.
 
-        extra->topextra->backingStoreTracker.destroy();
+        extra->topextra->widgetBackingStore.reset(nullptr);
         deleteBackingStore(this);
 #ifndef QT_NO_OPENGL
         extra->topextra->widgetTextures.clear();
@@ -10755,16 +10668,8 @@ void QWidget::setParent(QWidget *parent, Qt::WindowFlags f)
     if (newParent && isAncestorOf(focusWidget()))
         focusWidget()->clearFocus();
 
-    QTLWExtra *oldTopExtra = window()->d_func()->maybeTopData();
-    QWidgetBackingStoreTracker *oldBsTracker = oldTopExtra ? &oldTopExtra->backingStoreTracker : 0;
-
     d->setParent_sys(parent, f);
 
-    QTLWExtra *topExtra = window()->d_func()->maybeTopData();
-    QWidgetBackingStoreTracker *bsTracker = topExtra ? &topExtra->backingStoreTracker : 0;
-    if (oldBsTracker && oldBsTracker != bsTracker)
-        oldBsTracker->unregisterWidgetSubtree(this);
-
     if (desktopWidget)
         parent = 0;
 
@@ -11134,7 +11039,7 @@ void QWidgetPrivate::repaint(T r)
 
     QTLWExtra *tlwExtra = q->window()->d_func()->maybeTopData();
     if (tlwExtra && !tlwExtra->inTopLevelResize && tlwExtra->backingStore)
-        tlwExtra->backingStoreTracker->markDirty(r, q, QWidgetBackingStore::UpdateNow);
+        tlwExtra->widgetBackingStore->markDirty(r, q, QWidgetBackingStore::UpdateNow);
 }
 
 /*!
@@ -11209,7 +11114,7 @@ void QWidgetPrivate::update(T r)
 
     QTLWExtra *tlwExtra = q->window()->d_func()->maybeTopData();
     if (tlwExtra && !tlwExtra->inTopLevelResize && tlwExtra->backingStore)
-        tlwExtra->backingStoreTracker->markDirty(clipped, q);
+        tlwExtra->widgetBackingStore->markDirty(clipped, q);
 }
 
  /*!

src/widgets/kernel/qwidget_p.h

@@ -116,56 +116,12 @@ protected:
     QRegion m_region;
 };
 
-
-
-class Q_AUTOTEST_EXPORT QWidgetBackingStoreTracker
-{
-
-public:
-    QWidgetBackingStoreTracker();
-    ~QWidgetBackingStoreTracker();
-
-    void create(QWidget *tlw);
-    void destroy();
-
-    void registerWidget(QWidget *w);
-    void unregisterWidget(QWidget *w);
-    void unregisterWidgetSubtree(QWidget *w);
-
-    inline QWidgetBackingStore* data()
-    {
-        return m_ptr;
-    }
-
-    inline QWidgetBackingStore* operator->()
-    {
-        return m_ptr;
-    }
-
-    inline QWidgetBackingStore& operator*()
-    {
-        return *m_ptr;
-    }
-
-    inline operator bool() const
-    {
-        return (nullptr != m_ptr);
-    }
-
-private:
-    Q_DISABLE_COPY_MOVE(QWidgetBackingStoreTracker)
-
-private:
-    QWidgetBackingStore* m_ptr;
-    QSet<QWidget *> m_widgets;
-};
-
 struct QTLWExtra {
     // *************************** Cross-platform variables *****************************
 
     // Regular pointers (keep them together to avoid gaps on 64 bits architectures).
     std::unique_ptr<QIcon> icon; // widget icon
-    QWidgetBackingStoreTracker backingStoreTracker;
+    std::unique_ptr<QWidgetBackingStore> widgetBackingStore;
     QBackingStore *backingStore;
     QPainter *sharedPainter;
     QWidgetWindow *window;
@@ -1028,7 +984,7 @@ inline QWidgetBackingStore *QWidgetPrivate::maybeBackingStore() const
 {
     Q_Q(const QWidget);
     QTLWExtra *x = q->window()->d_func()->maybeTopData();
-    return x ? x->backingStoreTracker.data() : nullptr;
+    return x ? x->widgetBackingStore.get() : nullptr;
 }
 
 QT_END_NAMESPACE

src/widgets/kernel/qwidgetbackingstore.cpp

@@ -300,7 +300,7 @@ void QWidgetBackingStore::unflushPaint(QWidget *widget, const QRegion &rgn)
     if (!tlwExtra)
         return;
 
-    qt_flush(widget, rgn, tlwExtra->backingStoreTracker->store, tlw, 0, tlw->d_func()->maybeBackingStore());
+    qt_flush(widget, rgn, tlwExtra->widgetBackingStore->store, tlw, 0, tlw->d_func()->maybeBackingStore());
 }
 #endif // QT_NO_PAINT_DEBUG
 
@@ -800,7 +800,7 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
         invalidateBackingStore((newRect & clipR).translated(-data.crect.topLeft()));
     } else {
 
-        QWidgetBackingStore *wbs = x->backingStoreTracker.data();
+        QWidgetBackingStore *wbs = x->widgetBackingStore.get();
         QRegion childExpose(newRect & clipR);
         QRegion overlappedExpose;
 
@@ -864,7 +864,7 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
     if (x->inTopLevelResize)
         return;
 
-    QWidgetBackingStore *wbs = x->backingStoreTracker.data();
+    QWidgetBackingStore *wbs = x->widgetBackingStore.get();
     if (!wbs)
         return;
 
@@ -1528,10 +1528,10 @@ void QWidgetPrivate::invalidateBackingStore(const T &r)
         if (masked.isEmpty())
             return;
 
-        tlwExtra->backingStoreTracker->markDirty(masked, q,
+        tlwExtra->widgetBackingStore->markDirty(masked, q,
             QWidgetBackingStore::UpdateLater, QWidgetBackingStore::BufferInvalid);
     } else {
-        tlwExtra->backingStoreTracker->markDirty(clipped, q,
+        tlwExtra->widgetBackingStore->markDirty(clipped, q,
             QWidgetBackingStore::UpdateLater, QWidgetBackingStore::BufferInvalid);
     }
 }

src/widgets/kernel/qwidgetwindow.cpp

@@ -770,7 +770,7 @@ void QWidgetWindow::repaintWindow()
 
     QTLWExtra *tlwExtra = m_widget->window()->d_func()->maybeTopData();
     if (tlwExtra && !tlwExtra->inTopLevelResize && tlwExtra->backingStore)
-        tlwExtra->backingStoreTracker->markDirty(m_widget->rect(), m_widget,
+        tlwExtra->widgetBackingStore->markDirty(m_widget->rect(), m_widget,
                                                  QWidgetBackingStore::UpdateNow, QWidgetBackingStore::BufferInvalid);
 }
 

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -46,6 +46,7 @@
 #include <qstylefactory.h>
 #include <qdesktopwidget.h>
 #include <private/qwidget_p.h>
+#include <private/qwidgetbackingstore_p.h>
 #include <private/qapplication_p.h>
 #include <private/qhighdpiscaling_p.h>
 #include <qcalendarwidget.h>
@@ -9564,7 +9565,7 @@ void tst_QWidget::destroyBackingStore()
     QTRY_VERIFY(w.numPaintEvents > 0);
     w.reset();
     w.update();
-    qt_widget_private(&w)->topData()->backingStoreTracker.create(&w);
+    qt_widget_private(&w)->topData()->widgetBackingStore.reset(new QWidgetBackingStore(&w));
 
     w.update();
     QApplication::processEvents();
@@ -9584,7 +9585,7 @@ QWidgetBackingStore* backingStore(QWidget &widget)
     QWidgetBackingStore *backingStore = nullptr;
 #ifdef QT_BUILD_INTERNAL
     if (QTLWExtra *topExtra = qt_widget_private(&widget)->maybeTopData())
-        backingStore = topExtra->backingStoreTracker.data();
+        backingStore = topExtra->widgetBackingStore.get();
 #endif
     return backingStore;
 }
@@ -9784,12 +9785,12 @@ class scrollWidgetWBS : public QWidget
 public:
     void deleteBackingStore()
     {
-        static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStoreTracker.destroy();
+        static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->widgetBackingStore.reset(nullptr);
     }
     void enableBackingStore()
     {
         if (!static_cast<QWidgetPrivate*>(d_ptr.data())->maybeBackingStore()) {
-            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStoreTracker.create(this);
+            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->widgetBackingStore.reset(new QWidgetBackingStore(this));
             static_cast<QWidgetPrivate*>(d_ptr.data())->invalidateBackingStore(this->rect());
             repaint();
         }