root/ui/gfx/android/scroller.cc

/* [<][>][^][v][top][bottom][index][help] */

DEFINITIONS

This source file includes following definitions.
  1. ApproxEquals
  2. ComputeDeceleration
  3. Signum
  4. Clamped
  5. flywheel_enabled
  6. tuning_coeff_
  7. StartScroll
  8. StartScroll
  9. Fling
  10. ComputeScrollOffset
  11. ExtendDuration
  12. SetFinalX
  13. SetFinalY
  14. AbortAnimation
  15. ForceFinished
  16. IsFinished
  17. GetTimePassed
  18. GetDuration
  19. GetCurrX
  20. GetCurrY
  21. GetCurrVelocity
  22. GetCurrVelocityX
  23. GetCurrVelocityY
  24. GetStartX
  25. GetStartY
  26. GetFinalX
  27. GetFinalY
  28. IsScrollingInDirection
  29. RecomputeDeltas
  30. GetSplineDeceleration
  31. GetSplineFlingDuration
  32. GetSplineFlingDistance

// 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/gfx/android/scroller.h"

#include <cmath>

#include "base/lazy_instance.h"

namespace gfx {
namespace {

// Default scroll duration from android.widget.Scroller.
const int kDefaultDurationMs = 250;

// Default friction constant in android.view.ViewConfiguration.
const float kDefaultFriction = 0.015f;

// == std::log(0.78f) / std::log(0.9f)
const float kDecelerationRate = 2.3582018f;

// Tension lines cross at (kInflexion, 1).
const float kInflexion = 0.35f;

const float kEpsilon = 1e-5f;

bool ApproxEquals(float a, float b) {
  return std::abs(a - b) < kEpsilon;
}

struct ViscosityConstants {
  ViscosityConstants()
      : viscous_fluid_scale_(8.f), viscous_fluid_normalize_(1.f) {
    viscous_fluid_normalize_ = 1.0f / ApplyViscosity(1.0f);
  }

  float ApplyViscosity(float x) {
    x *= viscous_fluid_scale_;
    if (x < 1.0f) {
      x -= (1.0f - std::exp(-x));
    } else {
      float start = 0.36787944117f;  // 1/e == exp(-1)
      x = 1.0f - std::exp(1.0f - x);
      x = start + x * (1.0f - start);
    }
    x *= viscous_fluid_normalize_;
    return x;
  }

 private:
  // This controls the intensity of the viscous fluid effect.
  float viscous_fluid_scale_;
  float viscous_fluid_normalize_;

  DISALLOW_COPY_AND_ASSIGN(ViscosityConstants);
};

struct SplineConstants {
  SplineConstants() {
    const float kStartTension = 0.5f;
    const float kEndTension = 1.0f;
    const float kP1 = kStartTension * kInflexion;
    const float kP2 = 1.0f - kEndTension * (1.0f - kInflexion);

    float x_min = 0.0f;
    float y_min = 0.0f;
    for (int i = 0; i < NUM_SAMPLES; i++) {
      const float alpha = static_cast<float>(i) / NUM_SAMPLES;

      float x_max = 1.0f;
      float x, tx, coef;
      while (true) {
        x = x_min + (x_max - x_min) / 2.0f;
        coef = 3.0f * x * (1.0f - x);
        tx = coef * ((1.0f - x) * kP1 + x * kP2) + x * x * x;
        if (ApproxEquals(tx, alpha))
          break;
        if (tx > alpha)
          x_max = x;
        else
          x_min = x;
      }
      spline_position_[i] = coef * ((1.0f - x) * kStartTension + x) + x * x * x;

      float y_max = 1.0f;
      float y, dy;
      while (true) {
        y = y_min + (y_max - y_min) / 2.0f;
        coef = 3.0f * y * (1.0f - y);
        dy = coef * ((1.0f - y) * kStartTension + y) + y * y * y;
        if (ApproxEquals(dy, alpha))
          break;
        if (dy > alpha)
          y_max = y;
        else
          y_min = y;
      }
      spline_time_[i] = coef * ((1.0f - y) * kP1 + y * kP2) + y * y * y;
    }
    spline_position_[NUM_SAMPLES] = spline_time_[NUM_SAMPLES] = 1.0f;
  }

  void CalculateCoefficients(float t,
                             float* distance_coef,
                             float* velocity_coef) {
    *distance_coef = 1.f;
    *velocity_coef = 0.f;
    const int index = static_cast<int>(NUM_SAMPLES * t);
    if (index < NUM_SAMPLES) {
      const float t_inf = static_cast<float>(index) / NUM_SAMPLES;
      const float t_sup = static_cast<float>(index + 1) / NUM_SAMPLES;
      const float d_inf = spline_position_[index];
      const float d_sup = spline_position_[index + 1];
      *velocity_coef = (d_sup - d_inf) / (t_sup - t_inf);
      *distance_coef = d_inf + (t - t_inf) * *velocity_coef;
    }
  }

 private:
  enum {
    NUM_SAMPLES = 100
  };

  float spline_position_[NUM_SAMPLES + 1];
  float spline_time_[NUM_SAMPLES + 1];

  DISALLOW_COPY_AND_ASSIGN(SplineConstants);
};

float ComputeDeceleration(float friction) {
  const float kGravityEarth = 9.80665f;
  return kGravityEarth  // g (m/s^2)
         * 39.37f       // inch/meter
         * 160.f        // pixels/inch
         * friction;
}

template <typename T>
int Signum(T t) {
  return (T(0) < t) - (t < T(0));
}

template <typename T>
T Clamped(T t, T a, T b) {
  return t < a ? a : (t > b ? b : t);
}

// Leaky to allow access from the impl thread.
base::LazyInstance<ViscosityConstants>::Leaky g_viscosity_constants =
    LAZY_INSTANCE_INITIALIZER;

base::LazyInstance<SplineConstants>::Leaky g_spline_constants =
    LAZY_INSTANCE_INITIALIZER;

}  // namespace

Scroller::Config::Config()
    : fling_friction(kDefaultFriction),
      flywheel_enabled(false) {}

Scroller::Scroller(const Config& config)
    : mode_(UNDEFINED),
      start_x_(0),
      start_y_(0),
      final_x_(0),
      final_y_(0),
      min_x_(0),
      max_x_(0),
      min_y_(0),
      max_y_(0),
      curr_x_(0),
      curr_y_(0),
      duration_seconds_reciprocal_(1),
      delta_x_(0),
      delta_x_norm_(1),
      delta_y_(0),
      delta_y_norm_(1),
      finished_(true),
      flywheel_enabled_(config.flywheel_enabled),
      velocity_(0),
      curr_velocity_(0),
      distance_(0),
      fling_friction_(config.fling_friction),
      deceleration_(ComputeDeceleration(fling_friction_)),
      tuning_coeff_(ComputeDeceleration(0.84f)) {}

Scroller::~Scroller() {}

void Scroller::StartScroll(float start_x,
                           float start_y,
                           float dx,
                           float dy,
                           base::TimeTicks start_time) {
  StartScroll(start_x,
              start_y,
              dx,
              dy,
              start_time,
              base::TimeDelta::FromMilliseconds(kDefaultDurationMs));
}

void Scroller::StartScroll(float start_x,
                           float start_y,
                           float dx,
                           float dy,
                           base::TimeTicks start_time,
                           base::TimeDelta duration) {
  mode_ = SCROLL_MODE;
  finished_ = false;
  duration_ = duration;
  duration_seconds_reciprocal_ = 1.0 / duration_.InSecondsF();
  start_time_ = start_time;
  curr_x_ = start_x_ = start_x;
  curr_y_ = start_y_ = start_y;
  final_x_ = start_x + dx;
  final_y_ = start_y + dy;
  RecomputeDeltas();
  curr_time_ = start_time_;
}

void Scroller::Fling(float start_x,
                     float start_y,
                     float velocity_x,
                     float velocity_y,
                     float min_x,
                     float max_x,
                     float min_y,
                     float max_y,
                     base::TimeTicks start_time) {
  // Continue a scroll or fling in progress.
  if (flywheel_enabled_ && !finished_) {
    float old_velocity_x = GetCurrVelocityX();
    float old_velocity_y = GetCurrVelocityY();
    if (Signum(velocity_x) == Signum(old_velocity_x) &&
        Signum(velocity_y) == Signum(old_velocity_y)) {
      velocity_x += old_velocity_x;
      velocity_y += old_velocity_y;
    }
  }

  mode_ = FLING_MODE;
  finished_ = false;

  float velocity = std::sqrt(velocity_x * velocity_x + velocity_y * velocity_y);

  velocity_ = velocity;
  duration_ = GetSplineFlingDuration(velocity);
  duration_seconds_reciprocal_ = 1.0 / duration_.InSecondsF();
  start_time_ = start_time;
  curr_time_ = start_time_;
  curr_x_ = start_x_ = start_x;
  curr_y_ = start_y_ = start_y;

  float coeff_x = velocity == 0 ? 1.0f : velocity_x / velocity;
  float coeff_y = velocity == 0 ? 1.0f : velocity_y / velocity;

  double total_distance = GetSplineFlingDistance(velocity);
  distance_ = total_distance * Signum(velocity);

  min_x_ = min_x;
  max_x_ = max_x;
  min_y_ = min_y;
  max_y_ = max_y;

  final_x_ = start_x + total_distance * coeff_x;
  final_x_ = Clamped(final_x_, min_x_, max_x_);

  final_y_ = start_y + total_distance * coeff_y;
  final_y_ = Clamped(final_y_, min_y_, max_y_);

  RecomputeDeltas();
}

bool Scroller::ComputeScrollOffset(base::TimeTicks time) {
  if (finished_)
    return false;

  if (time == curr_time_)
    return true;

  base::TimeDelta time_passed = time - start_time_;

  if (time_passed < base::TimeDelta()) {
    time_passed = base::TimeDelta();
  }

  if (time_passed >= duration_) {
    curr_x_ = final_x_;
    curr_y_ = final_y_;
    curr_time_ = start_time_ + duration_;
    finished_ = true;
    return true;
  }

  curr_time_ = time;

  const float t = time_passed.InSecondsF() * duration_seconds_reciprocal_;

  switch (mode_) {
    case UNDEFINED:
      NOTREACHED() << "|StartScroll()| or |Fling()| must be called prior to "
                      "scroll offset computation.";
      return false;

    case SCROLL_MODE: {
      float x = g_viscosity_constants.Get().ApplyViscosity(t);

      curr_x_ = start_x_ + x * delta_x_;
      curr_y_ = start_y_ + x * delta_y_;
    } break;

    case FLING_MODE: {
      float distance_coef = 1.f;
      float velocity_coef = 0.f;
      g_spline_constants.Get().CalculateCoefficients(
          t, &distance_coef, &velocity_coef);

      curr_velocity_ = velocity_coef * distance_ * duration_seconds_reciprocal_;

      curr_x_ = start_x_ + distance_coef * delta_x_;
      curr_x_ = Clamped(curr_x_, min_x_, max_x_);

      curr_y_ = start_y_ + distance_coef * delta_y_;
      curr_y_ = Clamped(curr_y_, min_y_, max_y_);

      if (ApproxEquals(curr_x_, final_x_) && ApproxEquals(curr_y_, final_y_)) {
        finished_ = true;
      }
    } break;
  }

  return true;
}

void Scroller::ExtendDuration(base::TimeDelta extend) {
  base::TimeDelta passed = GetTimePassed();
  duration_ = passed + extend;
  duration_seconds_reciprocal_ = 1.0 / duration_.InSecondsF();
  finished_ = false;
}

void Scroller::SetFinalX(float new_x) {
  final_x_ = new_x;
  finished_ = false;
  RecomputeDeltas();
}

void Scroller::SetFinalY(float new_y) {
  final_y_ = new_y;
  finished_ = false;
  RecomputeDeltas();
}

void Scroller::AbortAnimation() {
  curr_x_ = final_x_;
  curr_y_ = final_y_;
  curr_velocity_ = 0;
  curr_time_ = start_time_ + duration_;
  finished_ = true;
}

void Scroller::ForceFinished(bool finished) { finished_ = finished; }

bool Scroller::IsFinished() const { return finished_; }

base::TimeDelta Scroller::GetTimePassed() const {
  return curr_time_ - start_time_;
}

base::TimeDelta Scroller::GetDuration() const { return duration_; }

float Scroller::GetCurrX() const { return curr_x_; }

float Scroller::GetCurrY() const { return curr_y_; }

float Scroller::GetCurrVelocity() const {
  if (finished_)
    return 0;
  if (mode_ == FLING_MODE)
    return curr_velocity_;
  return velocity_ - deceleration_ * GetTimePassed().InSecondsF() * 0.5f;
}

float Scroller::GetCurrVelocityX() const {
  return delta_x_norm_ * GetCurrVelocity();
}

float Scroller::GetCurrVelocityY() const {
  return delta_y_norm_ * GetCurrVelocity();
}

float Scroller::GetStartX() const { return start_x_; }

float Scroller::GetStartY() const { return start_y_; }

float Scroller::GetFinalX() const { return final_x_; }

float Scroller::GetFinalY() const { return final_y_; }

bool Scroller::IsScrollingInDirection(float xvel, float yvel) const {
  return !finished_ && Signum(xvel) == Signum(delta_x_) &&
         Signum(yvel) == Signum(delta_y_);
}

void Scroller::RecomputeDeltas() {
  delta_x_ = final_x_ - start_x_;
  delta_y_ = final_y_ - start_y_;

  const float hyp = std::sqrt(delta_x_ * delta_x_ + delta_y_ * delta_y_);
  if (hyp > kEpsilon) {
    delta_x_norm_ = delta_x_ / hyp;
    delta_y_norm_ = delta_y_ / hyp;
  } else {
    delta_x_norm_ = delta_y_norm_ = 1;
  }
}

double Scroller::GetSplineDeceleration(float velocity) const {
  return std::log(kInflexion * std::abs(velocity) /
                  (fling_friction_ * tuning_coeff_));
}

base::TimeDelta Scroller::GetSplineFlingDuration(float velocity) const {
  const double l = GetSplineDeceleration(velocity);
  const double decel_minus_one = kDecelerationRate - 1.0;
  const double time_seconds = std::exp(l / decel_minus_one);
  return base::TimeDelta::FromMicroseconds(time_seconds *
                                           base::Time::kMicrosecondsPerSecond);
}

double Scroller::GetSplineFlingDistance(float velocity) const {
  const double l = GetSplineDeceleration(velocity);
  const double decel_minus_one = kDecelerationRate - 1.0;
  return fling_friction_ * tuning_coeff_ *
         std::exp(kDecelerationRate / decel_minus_one * l);
}

}  // namespace gfx

/* [<][>][^][v][top][bottom][index][help] */