Move the Widget hierarchy to the Oilpan heap.

Source/platform/scroll/Scrollbar.h

 /*
  * Copyright (C) 2004, 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 10 matching lines @@
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef Scrollbar_h
 #define Scrollbar_h
 
 #include "platform/Timer.h"
 #include "platform/Widget.h"
+#include "platform/heap/Handle.h"
 #include "platform/scroll/ScrollTypes.h"
 #include "platform/scroll/ScrollbarThemeClient.h"
 #include "wtf/MathExtras.h"
 #include "wtf/PassRefPtr.h"
 
 namespace blink {
 
 class GraphicsContext;
 class IntRect;
 class PlatformGestureEvent;
 class PlatformMouseEvent;
+class ScrollAnimator;
 class ScrollableArea;
 class ScrollbarTheme;
 class ScrollView;
 
-class PLATFORM_EXPORT Scrollbar : public Widget,
-                  public ScrollbarThemeClient {
+class PLATFORM_EXPORT Scrollbar : public Widget, public ScrollbarThemeClient {
 
 public:
-    static PassRefPtr<Scrollbar> create(ScrollableArea*, ScrollbarOrientation, ScrollbarControlSize);
+    static PassRefPtrWillBeRawPtr<Scrollbar> create(ScrollableArea*, ScrollbarOrientation, ScrollbarControlSize);
 
     virtual ~Scrollbar();
 
     // ScrollbarThemeClient implementation.
     virtual int x() const OVERRIDE { return Widget::x(); }
     virtual int y() const OVERRIDE { return Widget::y(); }
     virtual int width() const OVERRIDE { return Widget::width(); }
     virtual int height() const OVERRIDE { return Widget::height(); }
     virtual IntSize size() const OVERRIDE { return Widget::size(); }
     virtual IntPoint location() const OVERRIDE { return Widget::location(); }
@@ skipping 32 matching lines @@
     virtual ScrollbarPart hoveredPart() const OVERRIDE { return m_hoveredPart; }
 
     virtual void styleChanged() OVERRIDE { }
 
     virtual bool enabled() const OVERRIDE { return m_enabled; }
     virtual void setEnabled(bool) OVERRIDE;
 
     // Called by the ScrollableArea when the scroll offset changes.
     void offsetDidChange();
 
-    void disconnectFromScrollableArea() { m_scrollableArea = 0; }
+    void disconnectFromScrollableArea();
     ScrollableArea* scrollableArea() const { return m_scrollableArea; }
 
     int pressedPos() const { return m_pressedPos; }
 
     virtual void setHoveredPart(ScrollbarPart);
     virtual void setPressedPart(ScrollbarPart);
 
     void setProportion(int visibleSize, int totalSize);
     void setPressedPos(int p) { m_pressedPos = p; }
 
@@ skipping 42 matching lines @@
     virtual void updateThumbPosition();
     virtual void updateThumbProportion();
 
     void autoscrollTimerFired(Timer<Scrollbar>*);
     void startTimerIfNeeded(double delay);
     void stopTimerIfNeeded();
     void autoscrollPressedPart(double delay);
     ScrollDirection pressedPartScrollDirection();
     ScrollGranularity pressedPartScrollGranularity();
 
     ScrollableArea* m_scrollableArea;

[haraken, 2014/09/29 14:16:36]

Given that ScrollableArea is on the heap, can we make this a Member?

[sof, 2014/10/09 07:35:34]

The objects that implement ScrollableArea are not all on the heap. In
particular, RenderLayerScrollableArea isn't.

The ScrollableAreas do control the Scrollbars created, and keep them referenced,
so they'll either die together, or in the case of RenderLayerScrollableArea, a
Scrollbar is explicitly disconnected before the Persistent reference to it is
dropped. Hence, the raw pointer is safe.

Moving RenderLayerScrollableArea to the heap will require some care (cf. its
finalizer), which is the only major obstacle to making ScrollableArea a GC
mixin. That can perhaps be tackled at some later point.

     ScrollbarOrientation m_orientation;
     ScrollbarControlSize m_controlSize;
     ScrollbarTheme* m_theme;

[haraken, 2014/09/29 14:16:36]

Are you sure that this raw pointer doesn't become a stale pointer?

[sof, 2014/10/09 07:35:34]

It is always a reference to a statically allocated ScrollbarTheme
(RenderScrollbarTheme::renderScrollbarTheme(), ScrollbarTheme::theme(), or
ScrollbarTheme::nativeTheme()), so that should be fine.

We explicitly unregister when finalizing, so stale pointers in the other cannot
arise either. (Not clear that they're ever kept, but still.)

 
+#if ENABLE(OILPAN)
+    // To simplify Oilpan finalization, keep a copy of the ScrollableArea's
+    // scroll animator. Scrollbar is responsible for notifying the animator
+    // when it is destructed.
+    RefPtr<ScrollAnimator> m_animator;
+#endif
+
     int m_visibleSize;
     int m_totalSize;
     float m_currentPos;
     float m_dragOrigin;
 
     ScrollbarPart m_hoveredPart;
     ScrollbarPart m_pressedPart;
     int m_pressedPos;
     float m_scrollPos;
     bool m_draggingDocument;
@@ skipping 12 matching lines @@
     virtual bool isScrollbar() const OVERRIDE { return true; }
 
     float scrollableAreaCurrentPos() const;
 };
 
 DEFINE_TYPE_CASTS(Scrollbar, Widget, widget, widget->isScrollbar(), widget.isScrollbar());
 
 } // namespace blink
 
 #endif // Scrollbar_h