Refactor app list scrolling: introduce the PaginationController class.

ui/app_list/pagination_controller.h

+// Copyright 2014 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 UI_APP_LIST_PAGINATION_CONTROLLER_H_
+#define UI_APP_LIST_PAGINATION_CONTROLLER_H_
+
+#include "ui/app_list/app_list_export.h"
+
+namespace gfx {
+class Vector2d;
+class Rect;
+}
+
+namespace ui {
+class GestureEvent;
+}
+
+namespace app_list {
+
+class PaginationModel;
+
+// Receives user scroll events from various sources (mouse wheel, touchpad,
+// touch gestures) and manipulates a PaginationModel as necessary.
+class APP_LIST_EXPORT PaginationController {
+ public:
+  enum ScrollAxis { SCROLL_AXIS_HORIZONTAL, SCROLL_AXIS_VERTICAL };
+
+  // Creates a PaginationController. Does not take ownership of |model|. The
+  // |model| is required to outlive this PaginationController. |scroll_axis|
+  // specifies the axis in which the pages will scroll.
+  PaginationController(PaginationModel* model, ScrollAxis scroll_axis);
+
+  ScrollAxis scroll_axis() const { return scroll_axis_; }
+
+  // Handles a mouse wheel or touchpad scroll event in the area represented by
+  // the PaginationModel. |offset| is the number of units scrolled in each axis.
+  // Returns true if the event was handled and there was some room to scroll.
+  bool OnScroll(const gfx::Vector2d& offset);
+
+  // Handles a touch gesture event in the area represented by the
+  // PaginationModel. Returns true if the event was handled.

[calamity, 2014/09/01 05:50:32]

'event was captured'? I'm nervous about using the word handled since
event->SetHandled() is a thing.

[Matt Giuca, 2014/09/01 06:03:07]

Done.

+  bool OnGestureEvent(ui::GestureEvent* event, const gfx::Rect& bounds);

[calamity, 2014/09/01 05:50:32]

event can be a const ref now.

[Matt Giuca, 2014/09/01 06:03:07]

Done.

+
+ private:
+  PaginationModel* pagination_model_;  // Not owned.
+  ScrollAxis scroll_axis_;
+};
+
+}  // namespace app_list
+
+#endif  // UI_APP_LIST_PAGINATION_CONTROLLER_H_