It is time.

ui/views/animation/scroll_animator.cc

-// Copyright (c) 2012 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/views/animation/scroll_animator.h"
-
-#include <algorithm>
-#include <cmath>
-
-#include "base/logging.h"
-#include "ui/gfx/animation/slide_animation.h"
-
-namespace {
-const float kDefaultAcceleration = -1500.0f; // in pixels per second^2
-
-// Assumes that d0 == 0.0f
-float GetPosition(float v0, float a, float t) {
-  float max_t = -v0 / a;
-  if (t > max_t)
-    t = max_t;
-  return t * (v0 + 0.5f * a * t);
-}
-
-float GetDelta(float v0, float a, float t1, float t2) {
-  return GetPosition(v0, a, t2) - GetPosition(v0, a, t1);
-}
-
-}  // namespace
-
-namespace views {
-
-ScrollAnimator::ScrollAnimator(ScrollDelegate* delegate)
-  : delegate_(delegate),
-    velocity_x_(0.0f),
-    velocity_y_(0.0f),
-    last_t_(0.0f),
-    duration_(0.0f),
-    acceleration_(kDefaultAcceleration) {
-  DCHECK(delegate);
-}
-
-ScrollAnimator::~ScrollAnimator() {
-  Stop();
-}
-
-void ScrollAnimator::Start(float velocity_x, float velocity_y) {
-  if (acceleration_ >= 0.0f)
-    acceleration_ = kDefaultAcceleration;
-  float v = std::max(fabs(velocity_x), fabs(velocity_y));
-  last_t_ = 0.0f;
-  velocity_x_ = velocity_x;
-  velocity_y_ = velocity_y;
-  duration_ = -v / acceleration_; // in seconds
-  animation_.reset(new gfx::SlideAnimation(this));
-  animation_->SetSlideDuration(static_cast<int>(duration_ * 1000));
-  animation_->Show();
-}
-
-void ScrollAnimator::Stop() {
-  velocity_x_ = velocity_y_ = last_t_ = duration_ = 0.0f;
-  animation_.reset();
-}
-
-void ScrollAnimator::AnimationEnded(const gfx::Animation* animation) {
-  Stop();
-}
-
-void ScrollAnimator::AnimationProgressed(const gfx::Animation* animation) {
-  float t = static_cast<float>(animation->GetCurrentValue()) * duration_;
-  float a_x = velocity_x_ > 0 ? acceleration_ : -acceleration_;
-  float a_y = velocity_y_ > 0 ? acceleration_ : -acceleration_;
-  float dx = GetDelta(velocity_x_, a_x, last_t_, t);
-  float dy = GetDelta(velocity_y_, a_y, last_t_, t);
-  last_t_ = t;
-  delegate_->OnScroll(dx, dy);
-}
-
-void ScrollAnimator::AnimationCanceled(const gfx::Animation* animation) {
-  Stop();
-}
-
-}  // namespace views