Move viewport scrolling logic into separate class

cc/layers/viewport.h

+// Copyright 2013 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 CC_LAYERS_VIEWPORT_H_
+#define CC_LAYERS_VIEWPORT_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "cc/layers/layer_impl.h"
+#include "ui/gfx/geometry/size.h"
+#include "ui/gfx/geometry/vector2d_f.h"
+
+namespace cc {
+
+class LayerTreeHostImpl;
+
+class CC_EXPORT Viewport {
+ public:
+  struct ScrollResult {
+    gfx::Vector2dF applied_delta;
+    gfx::Vector2dF unused_scroll_delta;
+  };
+
+  static scoped_ptr<Viewport> Create(LayerTreeHostImpl* host_impl);
+  virtual ~Viewport();
+
+  void SetViewportLayers(LayerImpl* inner_viewport_scroll,
+                         LayerImpl* outer_viewport_scroll);
+  bool IsViewportLayer(LayerImpl* layer) const;
+  bool IsViewportScrollingLayer(LayerImpl* layer) const;
+
+  ScrollResult ScrollBy(const gfx::Vector2dF& delta,
+                          const gfx::Point& viewport_point,
+                          bool is_wheel_scroll);
+
+ private:
+  explicit Viewport(LayerTreeHostImpl* host_impl);
+
+  bool ShouldTopControlsConsumeScroll(const gfx::Vector2dF& scroll_delta) const;
+  gfx::Vector2dF ScrollTopControls(const gfx::Vector2dF& delta);
+  gfx::Vector2dF ScrollViewport(const gfx::Vector2dF& delta,
+                                const gfx::Point& viewport_point,
+                                bool is_wheel_scroll);
+  gfx::Vector2dF ScrollLayer(LayerImpl* layer_impl,
+                             const gfx::Vector2dF& delta,
+                             const gfx::Point& viewport_point,
+                             bool is_wheel_scroll);
+
+  gfx::ScrollOffset MaxTotalScrollOffset() const;
+  gfx::ScrollOffset TotalScrollOffset() const;
+
+  LayerTreeHostImpl* host_impl_;
+  LayerImpl* inner_scroll_layer_;

[aelias_OOO_until_Jul13, 2015/03/06 04:19:37]

I'd rather this call out to the active tree every time to avoid duplicate state
we need to sync around.

+  LayerImpl* outer_scroll_layer_;
+
+  DISALLOW_COPY_AND_ASSIGN(Viewport);
+};
+
+}  // namespace cc
+
+#endif  // CC_LAYERS_VIEWPORT_H_