Cleanup and refactor RootScrollerController.

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef RootScrollerController_h
 #define RootScrollerController_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Document;
 class Element;
 class GraphicsLayer;
+class ScrollStateCallback;
 class ViewportScrollCallback;
 
 // Manages the root scroller associated with a given document. The root scroller
 // causes top controls movement, overscroll effects and prevents chaining
 // scrolls up further in the DOM. It can be set from script using
 // document.setRootScroller.
 //
 // There are two notions of a root scroller in this class: m_rootScroller and
 // m_effectiveRootScroller. The former is the Element that was set as the root
 // scroller using document.setRootScroller. If the page didn't set a root
@@ skipping 42 matching lines @@
 
     // This class needs to be informed of changes in layout so that it can
     // determine if the current root scroller is still valid or if it must be
     // replaced by the defualt root scroller.
     void didUpdateLayout();
 
     // This class needs to be informed of changes to compositing so that it can
     // update the compositor when the effective root scroller changes.
     void didUpdateCompositing();
 
+    // This class needs to be informed when the document has been attached to a
+    // FrameView so that we can initialize the viewport scroll callback.
+    void didAttachDocument();
+
     GraphicsLayer* rootScrollerLayer();
 
-    // TODO(bokan): Temporarily exposed to allow ScrollCustomization to
+    // TODO(bokan): Temporarily needed to allow ScrollCustomization to
     // differentiate between real custom callback and the built-in viewport
     // apply scroll.
-    const ViewportScrollCallback* viewportScrollCallback()
-    {
-        return m_viewportApplyScroll;
-    }
-
-    void setViewportScrollCallback(ViewportScrollCallback*);
+    bool isViewportScrollCallback(const ScrollStateCallback*) const;
 
 private:
     RootScrollerController(Document&);
 
     Element* defaultEffectiveRootScroller();
 
     // Ensures the effective root scroller is currently valid and replaces it
     // with the default if not.
     void updateEffectiveRootScroller();
 
-    // Returns true if the move was successful.
-    bool moveViewportApplyScroll(Element* target);
+    // Called only from the top Document's RootScrollerController. Ensures that
+    // the element that should be used as the root scroller on the page has the
+    // m_viewportApplyScroll callback set on it.
+    void setViewportApplyScrollOnRootScroller();
 
     WeakMember<Document> m_document;
     Member<ViewportScrollCallback> m_viewportApplyScroll;
 
     WeakMember<Element> m_rootScroller;
 
     // The element currently being used as the root scroller. If
     // m_viewportApplyScroll has been set, this element is guaranteed to have it
     // set as its applyScroll callback. This can be nullptr during
     // initialization and will not be set until m_viewportApplyScroll is
     // provided.
     WeakMember<Element> m_effectiveRootScroller;
+
+    // Tracks which element currently has the m_viewportApplyScroll set to it.
+    // This will only ever be set on the top Document's RootScrollerController.
+    WeakMember<Element> m_currentViewportApplyScrollHost;
 };
 
 } // namespace blink
 
 #endif // RootScrollerController_h