Replaced backing store reference count with list of visible widgets

Previously, the following sequence:
  1. Widget is hidden
  2. Widget partially revealed
  3. Widget fully revealed
resulted in the reference count of the backing store owned by the
widget's window() being incremented twice.

This patch replaces the simple reference count with a QSet which
stores pointers to the native widgets which are descendents of
the backing store owner, and which are currently visible.  The
sequence above therefore results in just a single insertion at
step (2), with step (3) having no effect on the backing store.

The QRefCountedWidgetBackingStore class has been renamed
QWidgetBackingStoreTracker to better reflect its purpose.

Task-number: QTBUG-12800
Task-number: QTBUG-12817
Reviewed-by: Jason Barron

1 files changed, 45 insertions, 16 deletions

diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index aaa29a1..dad4848 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -162,47 +162,76 @@ static inline bool hasBackingStoreSupport()
 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
 
-QRefCountedWidgetBackingStore::QRefCountedWidgetBackingStore()
+    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)
-    ,   m_count(0)
 {
 
 }
 
-QRefCountedWidgetBackingStore::~QRefCountedWidgetBackingStore()
+QWidgetBackingStoreTracker::~QWidgetBackingStoreTracker()
 {
     delete m_ptr;
 }
 
-void QRefCountedWidgetBackingStore::create(QWidget *widget)
+/*!
+    \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);
-    m_count = 0;
 }
 
-void QRefCountedWidgetBackingStore::destroy()
+/*!
+    \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_count = 0;
+    m_widgets.clear();
 }
 
-void QRefCountedWidgetBackingStore::ref()
+/*!
+    \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);
-    ++m_count;
+    Q_ASSERT(w->internalWinId());
+    Q_ASSERT(qt_widget_private(w)->maybeBackingStore() == m_ptr);
+    m_widgets.insert(w);
 }
 
-void QRefCountedWidgetBackingStore::deref()
+/*!
+    \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_count) {
-        Q_ASSERT(m_ptr);
-        if (0 == --m_count) {
-            delete m_ptr;
-            m_ptr = 0;
-        }
+    if (m_widgets.remove(w) && m_widgets.isEmpty()) {
+        delete m_ptr;
+        m_ptr = 0;
     }
 }