[not for review - epic CL] Adding Elastic+Momentum+Layered scrolling to views::ScrollView

ui/compositor/overscroll/ui_scroll_input_manager.cc

+// Copyright 2016 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/compositor/overscroll/ui_scroll_input_manager.h"
+
+#include "base/logging.h"
+#include "ui/compositor/overscroll/ui_input_handler.h"
+#include "ui/events/event.h"
+
+namespace ui {
+
+UIScrollInputManager::UIScrollInputManager(
+    const base::WeakPtr<cc::InputHandler>& input_handler)
+    : input_wrapper_(new UIInputHandler(input_handler)) {}
+
+UIScrollInputManager::~UIScrollInputManager() {}
+
+bool UIScrollInputManager::OnScrollEvent(const ScrollEvent& event) {
+  // Decide whether to HandleScrollBegin() based only on whether the
+  // InputHandler last handled an "End" (i.e. do not use properties on |event|).
+  // This is because, on Mac, an event ending the non-fling portion of a scroll
+  // can't be distinguished from an event ending a scroll that won't fling. So,
+  // when handling the fling portion, HandleScrollBegin() must be invoked, even
+  // though |event| will not be a "begin" phase.
+  if (!scrolling_layer_)
+    scrolling_layer_ = input_wrapper_->HandleScrollBegin(event);
+  if (!scrolling_layer_)
+    return false;
+
+  input_wrapper_->HandleScrollUpdate(event, scrolling_layer_);
+  if (event.momentum_phase() == EventMomentumPhase::END) {
+    input_wrapper_->HandleScrollEnd(event);
+    scrolling_layer_ = cc::ElementId();
+  }
+  return true;
+}
+
+}  // namespace ui