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 Point;
+class Rect;
+}
+
+namespace ui {
+class GestureEvent;
+class ScrollEvent;

[calamity, 2014/09/01 01:49:30]

Unnecessary forward declaration.

[Matt Giuca, 2014/09/01 03:02:26]

Done.

+}
+
+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.
+  bool OnScroll(const gfx::Point& offset);

[calamity, 2014/09/01 01:49:30]

This API seems kinda weird. The fact that OnGestureEvent takes an Event and
OnScroll doesn't means that the responsibility of marking the event as handled
is inconsistent across the class.

It would be better to use the same API style across these two methods.

[Matt Giuca, 2014/09/01 03:02:26]

I agree that this should return a bool instead of calling SetHandled(), for
consistency. But I think OnGestureEvent should still take an Event rather than
taking the pieces -- because it's fundamentally a more specialized and complex
thing (it's specifically for handling this set of event types).

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

Ack. 

+
+  // Handles a touch gesture event in the area represented by the
+  // PaginationModel.
+  void OnGestureEvent(ui::GestureEvent* event, const gfx::Rect& bounds);
+
+ private:
+  PaginationModel* pagination_model_;  // Not owned.
+  ScrollAxis scroll_axis_;
+};
+
+}  // namespace app_list
+
+#endif  // UI_APP_LIST_PAGINATION_CONTROLLER_H_