ui/events/gestures/fling_curve.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "ui/events/gestures/fling_curve.h"
   6
   7 #include <algorithm>
   8 #include <cmath>
   9
  10 #include "base/logging.h"
  11
  12 namespace {
  13
  14 const float kDefaultAlpha = -5.70762e+03f;
  15 const float kDefaultBeta = 1.72e+02f;
  16 const float kDefaultGamma = 3.7e+00f;
  17
  18 inline double GetPositionAtTime(double t) {
  19   return kDefaultAlpha * exp(-kDefaultGamma * t) - kDefaultBeta * t -
  20          kDefaultAlpha;
  21 }
  22
  23 inline double GetVelocityAtTime(double t) {
  24   return -kDefaultAlpha * kDefaultGamma * exp(-kDefaultGamma * t) -
  25          kDefaultBeta;
  26 }
  27
  28 inline double GetTimeAtVelocity(double v) {
  29   return -log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) /
  30          kDefaultGamma;
  31 }
  32
  33 }  // namespace
  34
  35 namespace ui {
  36
  37 FlingCurve::FlingCurve(const gfx::Vector2dF& velocity,
  38                        base::TimeTicks start_timestamp)
  39     : curve_duration_(GetTimeAtVelocity(0)),
  40       start_timestamp_(start_timestamp),
  41       time_offset_(0),
  42       position_offset_(0) {
  43   float max_start_velocity = std::max(fabs(velocity.x()), fabs(velocity.y()));
  44   if (max_start_velocity > GetVelocityAtTime(0))
  45     max_start_velocity = GetVelocityAtTime(0);
  46   CHECK_GT(max_start_velocity, 0);
  47
  48   displacement_ratio_ = gfx::Vector2dF(velocity.x() / max_start_velocity,
  49                                        velocity.y() / max_start_velocity);
  50   time_offset_ = GetTimeAtVelocity(max_start_velocity);
  51   position_offset_ = GetPositionAtTime(time_offset_);
  52   last_timestamp_ = start_timestamp_ + base::TimeDelta::FromSecondsD(
  53                                            curve_duration_ - time_offset_);
  54 }
  55
  56 FlingCurve::~FlingCurve() {
  57 }
  58
  59 gfx::Vector2dF FlingCurve::GetScrollAmountAtTime(base::TimeTicks current) {
  60   if (current < start_timestamp_)
  61     return gfx::Vector2dF();
  62
  63   float displacement = 0;
  64   if (current < last_timestamp_) {
  65     float time = time_offset_ + (current - start_timestamp_).InSecondsF();
  66     CHECK_LT(time, curve_duration_);
  67     displacement = GetPositionAtTime(time) - position_offset_;
  68   } else {
  69     displacement = GetPositionAtTime(curve_duration_) - position_offset_;
  70   }
  71
  72   gfx::Vector2dF scroll(displacement * displacement_ratio_.x(),
  73                         displacement * displacement_ratio_.y());
  74   gfx::Vector2dF scroll_increment(scroll.x() - cumulative_scroll_.x(),
  75                                   scroll.y() - cumulative_scroll_.y());
  76   cumulative_scroll_ = scroll;
  77   return scroll_increment;
  78 }
  79
  80 }  // namespace ui