[CRD iOS] Viewport fling animation

remoting/client/fling_tracker.cc

+// Copyright 2017 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 "remoting/client/fling_tracker.h"
+
+#include <cmath>
+
+namespace remoting {
+
+namespace {
+
+// TODO(yuweih): May need to tweak these numbers to get better smoothness.
+
+// Stop flinging if the speed drops below this.
+// 1px per 16ms. i.e. 1px/frame.
+// TODO(yuweih): The screen unit may not be in pixel. This needs to be
+//               normalized with the DPI.
+const float kMinTrackSpeed = 0.0625f;
+
+// The minimum fling duration (ms) needed to trigger the fling animation. This
+// is to prevent unintentional fling with low velocity.
+const float kMinFlingTime = 500.f;
+
+float GetExpFactor(float time_constant, float time_delta) {
+  return std::exp(-time_delta / time_constant);
+}
+
+float GetDisplacement(float initial_speed_rate, float exp_factor) {
+  // x = v0 * (1 - e^(-t / T))
+  // This comes from a solution to the linear drag equation F=-kv
+  return initial_speed_rate * (1.f - exp_factor);
+}
+
+float GetSpeed(float initial_speed_rate,
+               float time_constant,
+               float exp_factor) {
+  // v = (1 / T) * v0 * e^(-t / T).
+  // Derivative of the displacement.
+  return (1.f / time_constant) * initial_speed_rate * exp_factor;
+}
+
+}  // namespace
+
+FlingTracker::FlingTracker(float time_constant)
+    : time_constant_(time_constant) {}
+
+FlingTracker::~FlingTracker() {}
+
+void FlingTracker::StartFling(float velocity_x, float velocity_y) {
+  start_time_ = base::TimeTicks::Now();
+
+  initial_speed_rate_ =
+      std::sqrt(velocity_x * velocity_x + velocity_y * velocity_y);
+
+  if (GetSpeed(initial_speed_rate_, time_constant_,
+               GetExpFactor(time_constant_, kMinFlingTime)) < kMinTrackSpeed) {
+    StopFling();
+    return;
+  }
+
+  velocity_ratio_x_ = velocity_x / initial_speed_rate_;
+  velocity_ratio_y_ = velocity_y / initial_speed_rate_;
+
+  previous_position_x_ = 0;
+  previous_position_y_ = 0;
+}
+
+void FlingTracker::StopFling() {
+  initial_speed_rate_ = 0.f;
+}
+
+bool FlingTracker::IsFlingInProgress() const {
+  return initial_speed_rate_ > 0;
+}
+
+bool FlingTracker::TrackMovement(base::TimeDelta time_delta,
+                                 float* dx,
+                                 float* dy) {
+  if (!IsFlingInProgress()) {
+    return false;
+  }
+
+  float time_delta_ms = time_delta.InMilliseconds();
+
+  float exp_factor = GetExpFactor(time_constant_, time_delta_ms);
+
+  float speed = GetSpeed(initial_speed_rate_, time_constant_, exp_factor);
+
+  if (speed < kMinTrackSpeed) {
+    StopFling();
+    return false;
+  }
+
+  float displacement = GetDisplacement(initial_speed_rate_, exp_factor);
+
+  float position_x = displacement * velocity_ratio_x_;
+  float position_y = displacement * velocity_ratio_y_;
+
+  *dx = position_x - previous_position_x_;

[nicholss, 2017/05/11 15:13:50]

It seems to me you also do not need previous_position_* if you have the
time_delta. I don't think you need to change this at the moment, I think it can
work out, but we might want to change this up when we integrate with scroll
velocity which only acts in one direction.

[Yuwei, 2017/05/11 18:26:27]

I just renamed |time_delta| to |time_elapsed|. It means the time elapsed since
the animation has started...

I don't feel like we can use time_delta to calculate the change in position
directly given that f(t)-f(t-time_delta) here is dependent on t... Lets see
whether we can simplify this when we get to that

+  *dy = position_y - previous_position_y_;
+
+  previous_position_x_ = position_x;
+  previous_position_y_ = position_y;
+
+  return true;
+}
+
+}  // namespace remoting