[Android] Port Scroller.java to C++ and use for fling animations

ui/gfx/android/scroller.cc

Index: ui/gfx/android/scroller.cc
diff --git a/ui/gfx/android/scroller.cc b/ui/gfx/android/scroller.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c125be2a64011fc1fda5a807f03c27496e1c4eed
--- /dev/null
+++ b/ui/gfx/android/scroller.cc
@@ -0,0 +1,418 @@
+// 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 <cmath>
+
+#include "base/lazy_instance.h"
+#include "ui/gfx/android/scroller.h"
+#include "ui/gfx/android/view_configuration.h"
+
+namespace gfx {
+namespace {
+
+const base::TimeDelta kDefaultDuration = base::TimeDelta::FromMilliseconds(250);

[aelias_OOO_until_Jul13, 2014/02/21 02:42:35]

Hmm, I'm concerned these will end up as static initializers which are
discouraged.  Could you make this and the deceleration rate member variables
initialized in the constructor?

[jdduke (slow), 2014/02/21 23:33:02]

I'll just compute the constant in kDecelerationRate, and use an int milliseconds
value for kDefaultDuration.

+
+const float kDecelerationRate = std::log(0.78f) / std::log(0.9f);
+
+// Tension lines cross at (kInflexion, 1)
+const float kInflexion = 0.35f;
+
+const float kEpsilon = 1e-5;

[aelias_OOO_until_Jul13, 2014/02/21 02:42:35]

1e-5f instead?

[jdduke (slow), 2014/02/21 23:33:02]

Done.

+
+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 viscous fluid effect (how much of it)
+  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 (std::abs(tx - alpha) < 1E-5)

[aelias_OOO_until_Jul13, 2014/02/21 02:42:35]

kEpsilon?

[jdduke (slow), 2014/02/21 23:33:02]

Done.

+          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 (std::abs(dy - alpha) < 1E-5)

[aelias_OOO_until_Jul13, 2014/02/21 02:42:35]

kEpsilon?

[jdduke (slow), 2014/02/21 23:33:02]

Done.

+          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::Scroller(bool enable_flywheel)
+    : 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_(enable_flywheel),
+      velocity_(0),
+      curr_velocity_(0),
+      distance_(0),
+      fling_friction_(ViewConfiguration::GetScrollFriction()),
+      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, kDefaultDuration);
+}
+
+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;
+  start_x_ = start_x;
+  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_;
+  start_x_ = start_x;
+  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;
+
+  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() << "Invalid scroll mode when computing scroll offset.";
+      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. / 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_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 {
+  return mode_ == FLING_MODE
+             ? curr_velocity_
+             : 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