Implementing document.setRootScroller API for main thread scrolling.

third_party/WebKit/Source/core/page/scrolling/RootScroller.cpp

+// 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.
+
+#include "core/page/scrolling/RootScroller.h"
+
+#include "core/dom/Element.h"
+#include "core/frame/FrameHost.h"
+#include "core/frame/FrameView.h"
+#include "core/layout/LayoutBox.h"
+#include "core/page/Page.h"
+#include "core/page/scrolling/ViewportScrollCallback.h"
+#include "core/paint/PaintLayerScrollableArea.h"
+#include "platform/scroll/ScrollableArea.h"
+
+namespace blink {
+
+namespace {
+
+Document* topDocument(FrameHost* frameHost)
+{
+    DCHECK(frameHost);
+    if (!frameHost->page().mainFrame())
+        return nullptr;
+
+    DCHECK(frameHost->page().mainFrame()->isLocalFrame());
+    return toLocalFrame(frameHost->page().mainFrame())->document();
+}
+
+ScrollableArea* scrollableAreaFor(Element& element)
+{
+    if (!element.layoutObject() || !element.layoutObject()->isBox())
+        return nullptr;
+
+    LayoutBox* box = toLayoutBox(element.layoutObject());
+
+    if (box->isDocumentElement() && element.document().view())
+        return element.document().view()->getScrollableArea();
+
+    return static_cast<PaintInvalidationCapableScrollableArea*>(
+        box->getScrollableArea());
+}
+
+} // namespace
+
+RootScroller::RootScroller(FrameHost& frameHost)
+    : m_frameHost(&frameHost)
+{
+}
+
+DEFINE_TRACE(RootScroller)
+{
+    visitor->trace(m_frameHost);
+    visitor->trace(m_viewportApplyScroll);
+    visitor->trace(m_rootScroller);
+}
+
+bool RootScroller::set(Element& newRootScroller)
+{
+    if (!isValid(newRootScroller))
+        return false;
+
+    DCHECK(m_frameHost);
+
+    Document* document = topDocument(m_frameHost);
+    if (!document)
+        return false;
+
+    ScrollableArea* newRootScrollableArea = scrollableAreaFor(newRootScroller);
+    if (!newRootScrollableArea)
+        return false;
+
+    if (m_rootScroller)
+        m_rootScroller->removeApplyScroll();
+
+    m_rootScroller = &newRootScroller;
+
+    createApplyScrollIfNeeded();
+
+    // Ideally, the scrolling infrastructure would pass this to the callback but
+    // we don't get that today so we set it manually.
+    m_viewportApplyScroll->setScroller(*newRootScrollableArea);
+
+    // Installs the viewport scrolling callback (the "applyScroll" in Scroll
+    // Customization lingo) on the given element. This callback is
+    // responsible for viewport related scroll actions like top controls
+    // movement and overscroll glow as well as actually scrolling the element.
+    // Use disable-native-scroll since the ViewportScrollCallback needs to
+    // apply scroll actions before (TopControls) and after (overscroll)
+    // scrolling the element so it applies scroll to the element itself.
+    m_rootScroller->setApplyScroll(
+        m_viewportApplyScroll,
+        "disable-native-scroll");
+
+    return true;
+}
+
+Element* RootScroller::get() const
+{
+    if (!m_frameHost)
+        return nullptr;
+
+    return m_rootScroller;
+}
+
+void RootScroller::resetToDefault()
+{
+    if (!m_frameHost)
+        return;
+
+    Document* document = topDocument(m_frameHost);
+    if (!document)
+        return;
+
+    if (Element* defaultRootElement = document->documentElement())
+        set(*defaultRootElement);
+}
+
+void RootScroller::didUpdateTopDocumentLayout()
+{
+    if (m_rootScroller && isValid(*m_rootScroller))
+        return;
+
+    resetToDefault();
+}
+
+bool RootScroller::isValid(Element& element) const
+{
+    if (!m_frameHost)
+        return false;
+
+    if (element.document() != topDocument(m_frameHost))
+        return false;
+
+    if (!element.isInTreeScope())
+        return false;
+
+    if (!element.layoutObject())
+        return false;
+
+    if (!element.layoutObject()->isLayoutBlockFlow()
+        && !element.layoutObject()->isLayoutIFrame())
+        return false;
+
+    return true;
+}
+
+void RootScroller::createApplyScrollIfNeeded()
+{
+    if (!m_viewportApplyScroll) {
+        TopControls& topControls = m_frameHost->topControls();
+        OverscrollController& overscrollController =
+            m_frameHost->overscrollController();
+        m_viewportApplyScroll =
+            new ViewportScrollCallback(topControls, overscrollController);
+    }
+}
+
+} // namespace blink