Move viewport scrolling logic into separate class

cc/layers/viewport.h

+// Copyright 2015 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 "cc/layers/layer_impl.h"
+#include "ui/gfx/geometry/vector2d_f.h"
+
+namespace cc {
+
+class LayerTreeHostImpl;
+
+// Encapsulates gesture handling logic on the viewport layers. The "viewport"
+// is made up of two scrolling layers, the inner viewport (visual) and the
+// outer viewport (layout) scroll layers. These layers have different scroll
+// bubbling behavior from the rest of the layer tree which is encoded in this
+// class.
+class CC_EXPORT Viewport {
+ public:
+  struct ScrollResult {
+    gfx::Vector2dF applied_delta;
+    gfx::Vector2dF unused_scroll_delta;
+    gfx::Vector2dF top_controls_applied_delta;
+  };
+
+  static scoped_ptr<Viewport> Create(LayerTreeHostImpl* host_impl);
+
+  // Scrolls the viewport, applying the unique bubbling between the inner and
+  // outer viewport. Scrolls can be consumed by top controls.
+  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 AdjustOverscroll(const gfx::Vector2dF& delta) const;
+
+  // Sends the delta to the top controls, returns the amount applied.
+  gfx::Vector2dF ScrollTopControls(const gfx::Vector2dF& delta);
+
+  gfx::ScrollOffset MaxTotalScrollOffset() const;
+  gfx::ScrollOffset TotalScrollOffset() const;
+
+  LayerImpl* InnerScrollLayer() const;
+  LayerImpl* OuterScrollLayer() const;
+
+  LayerTreeHostImpl* host_impl_;
+
+  DISALLOW_COPY_AND_ASSIGN(Viewport);
+};
+
+}  // namespace cc
+
+#endif  // CC_LAYERS_VIEWPORT_H_