Implementing document.setRootScroller API for main thread scrolling.

third_party/WebKit/Source/core/page/scrolling/RootScroller.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 RootScroller_h
+#define RootScroller_h
+
+#include "core/CoreExport.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class Element;
+class FrameHost;
+class ViewportScrollCallback;
+
+// This class manages the root scroller associated with the top-level document.
+// The root scroller causes top controls movement, overscroll effects and
+// prevents chaining up further in the DOM. It can be set using
+// document.setRootScroller. By default, the rootScroller is the
+// documentElement.
+//
+// Not all elements can be set as the root scroller. A valid element must be
+// in the top-level document and must have a LayoutObject that is at least a
+// LayoutBlockFlow or LayoutIFrame. In addition, only the top-level document can
+// set a non-default root scroller.
+class CORE_EXPORT RootScroller : public GarbageCollected<RootScroller> {
+public:
+    static RootScroller* create(FrameHost& frameHost)
+    {
+        return new RootScroller(frameHost);
+    }
+
+    // This method returns true if the element was set as the root scroller. If
+    // the element isn't eligible to be the root scroller or we're in some bad
+    // state, the method returns false without changing the current root
+    // scroller.
+    bool set(Element&);
+
+    // Returns the element currently set as the root scroller.
+    Element* get() const;
+
+    // This class needs to be informed of changes in layout so that it can
+    // determine if the current scroller is still valid or if it must be
+    // replaced by the defualt root scroller.
+    void didUpdateTopDocumentLayout();
+
+    DECLARE_TRACE();
+
+private:
+    RootScroller(FrameHost&);
+
+    bool isValid(Element&) const;
+    void resetToDefault();
+    void createApplyScrollIfNeeded();
+
+    WeakMember<FrameHost> m_frameHost;
+    Member<ViewportScrollCallback> m_viewportApplyScroll;
+
+    WeakMember<Element> m_rootScroller;
+};
+
+} // namespace blink
+
+#endif // RootScroller_h