Refactor app list scrolling: introduce the PaginationController class.

ui/app_list/pagination_controller.cc

+// 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.
+
+#include "ui/app_list/pagination_controller.h"
+
+#include "ui/app_list/pagination_model.h"
+#include "ui/events/event.h"
+#include "ui/gfx/geometry/point.h"
+#include "ui/gfx/geometry/rect.h"
+
+namespace app_list {
+
+namespace {
+
+// Constants for dealing with scroll events.
+const int kMinScrollToSwitchPage = 20;
+const int kMinHorizVelocityToSwitchPage = 800;
+
+const double kFinishTransitionThreshold = 0.33;
+
+}  // namespace
+
+PaginationController::PaginationController(PaginationModel* model,
+                                           ScrollAxis scroll_axis)
+    : pagination_model_(model), scroll_axis_(scroll_axis) {
+}
+
+bool PaginationController::OnScroll(const gfx::Point& offset) {
+  int offset_magnitude;
+  if (scroll_axis_ == SCROLL_AXIS_HORIZONTAL) {
+    // If the view scrolls horizontally, both horizontal and vertical scroll
+    // events are valid (since most mouse wheels only have vertical scrolling).
+    if (abs(offset.x()) > abs(offset.y()))
+      offset_magnitude = offset.x();
+    else
+      offset_magnitude = offset.y();
+  } else {
+    // If the view scrolls vertically, only vertical scroll events are valid.
+    offset_magnitude = offset.y();
+  }
+
+  if (abs(offset_magnitude) > kMinScrollToSwitchPage) {
+    if (!pagination_model_->has_transition()) {
+      pagination_model_->SelectPageRelative(offset_magnitude > 0 ? -1 : 1,
+                                            true);
+    }
+    return true;
+  }
+
+  return false;
+}
+
+void PaginationController::OnGestureEvent(ui::GestureEvent* event,
+                                          const gfx::Rect& bounds) {
+  const ui::GestureEventDetails& details = event->details();
+  switch (event->type()) {
+    case ui::ET_GESTURE_SCROLL_BEGIN:
+      pagination_model_->StartScroll();
+      event->SetHandled();
+      return;
+    case ui::ET_GESTURE_SCROLL_UPDATE: {
+      float scroll = scroll_axis_ == SCROLL_AXIS_HORIZONTAL
+                         ? details.scroll_x()
+                         : details.scroll_y();
+      int width = scroll_axis_ == SCROLL_AXIS_HORIZONTAL ? bounds.width()
+                                                         : bounds.height();
+      // scroll > 0 means moving contents right or down. That is, transitioning
+      // to the previous page.
+      pagination_model_->UpdateScroll(scroll / width);
+      event->SetHandled();
+      return;
+    }
+    case ui::ET_GESTURE_SCROLL_END:
+      pagination_model_->EndScroll(pagination_model_->transition().progress <
+                                   kFinishTransitionThreshold);
+      event->SetHandled();
+      return;
+    case ui::ET_SCROLL_FLING_START: {
+      float velocity = scroll_axis_ == SCROLL_AXIS_HORIZONTAL
+                           ? details.velocity_x()
+                           : details.velocity_y();
+      pagination_model_->EndScroll(true);
+      if (fabs(velocity) > kMinHorizVelocityToSwitchPage)
+        pagination_model_->SelectPageRelative(velocity < 0 ? 1 : -1, true);
+      event->SetHandled();
+      return;
+    }
+    default:
+      break;
+  }
+}
+
+}  // namespace app_list