Split RootScrollerController into top-document and child-document classes

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 ScrollableArea;
 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
+// 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
 // scroller this will be nullptr. The "effective" root scroller is the current
-// element we're using internally to apply viewport scroll actions. i.e It's the
-// element with the ViewportScrollCallback set as its apply-scroll callback.
-// The effective root scroller will only be null during document initialization.
+// element we're using internally to apply viewport scrolling actions.  The
+// effective root scroller will only be null during document initialization.
 //
-// If the root scroller element is a valid element to become the root scroller,
-// it will be promoted to the effective root scroller. If it is not valid, the
-// effective root scroller will fall back to a default Element (see
+// If the currently set m_rootScroller is a valid element to become the root
+// scroller, it will be promoted to the effective root scroller. If it is not
+// valid, the effective root scroller will fall back to a default Element (see
 // defaultEffectiveRootScroller()). The rules for what makes an element a valid
 // root scroller are set in isValidRootScroller(). The validity of the current
 // root scroller is re-checked after each layout.
 class CORE_EXPORT RootScrollerController
     : public GarbageCollected<RootScrollerController> {
 public:
-    // Creates a RootScrollerController for the given document. You should use
-    // setViewportScrollCallback to provide this class with a scroll callback
-    // that RootScrollerController will keep applied to the current RootScroller
-    // so that special actions can occur on scrolling.
-    static RootScrollerController* create(Document& document)
-    {
-        return new RootScrollerController(document);
-    }
+    // Creates a RootScrollerController for the given document. Note, instances
+    // of this class need to be made aware of lifecycle events, see the
+    // didUpdateLayout, didUpdateCompositing, etc. type methods below.
+    static RootScrollerController* create(Document&);
 
-    DECLARE_TRACE();
+    DECLARE_VIRTUAL_TRACE();
 
     // Sets the element that will be used as the root scroller. This can be
     // nullptr, in which case we'll use the default element (documentElement) as
     // the effective root scroller.
     void set(Element*);
 
     // Returns the element currently set as the root scroller from script. This
     // differs from the effective root scroller since the set Element may not
     // currently be a valid root scroller. e.g. If the page sets an Element
     // with `display: none`, get() will return that element, even though the
     // effective root scroller will remain the element returned by
     // defaultEffectiveRootScroller().
     Element* get() const;
 
     // This returns the Element that's actually being used to control viewport
     // actions right now. This is different from get() if a root scroller hasn't
     // been set, or if the set root scroller isn't currently a valid scroller.
     Element* effectiveRootScroller() const;
 
     // 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();
+    virtual 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();
+    virtual void didAttachDocument();
 
+    // Returns the GraphicsLayer for the current effective root scroller
+    // element.
     GraphicsLayer* rootScrollerLayer();
 
+    // Returns true if the given ScrollStateCallback is the ViewportScrollCallback managed
+    // by this class.
     // TODO(bokan): Temporarily needed to allow ScrollCustomization to
     // differentiate between real custom callback and the built-in viewport
-    // apply scroll.
-    bool isViewportScrollCallback(const ScrollStateCallback*) const;
+    // apply scroll. crbug.com/623079.
+    virtual bool isViewportScrollCallback(const ScrollStateCallback*) const;
 
-private:
+protected:
     RootScrollerController(Document&);
 
-    Element* defaultEffectiveRootScroller();
-
     // Ensures the effective root scroller is currently valid and replaces it
     // with the default if not.
-    void updateEffectiveRootScroller();
+    virtual void updateEffectiveRootScroller();
 
-    // 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();
+    // Returns the ScrollableArea to use to scroll the given Element.
+    ScrollableArea* scrollableAreaFor(const Element&) const;
 
+    // The owning Document whose root scroller this object manages.
     WeakMember<Document> m_document;
-    Member<ViewportScrollCallback> m_viewportApplyScroll;
 
+private:
+
+    // Determines whether the given element meets the criteria to become the
+    // effective root scroller.
+    bool isValidRootScroller(const Element&) const;
+
+    // Returns the Element that should be used if the currently set
+    // m_rootScroller isn't valid to be a root scroller.
+    Element* defaultEffectiveRootScroller();
+
+    // The Element that was set from script as rootScroller. Depending on its
+    // validity to be the root scroller (e.g. a display: none element isn't a
+    // valid root scroller), this may not actually be the Element being used as
+    // the root scroller.
     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