[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 {
+
+// Stop flinging if the speed drops below this.
+// 1px per 16ms. i.e. 1px/frame.
+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;
+
+// Multiplied with the speed. This is used to prevent abruptness at the
+// beginning.
+// TODO(yuweih): This may need to be normalized with the screen DPI.
+const float kAmplituteFactor = 0.75f;
+
+float GetDisplacement(float initial_speed_rate,
+                      float time_constant,
+                      float time_delta) {
+  // x = v0 * (1 - e^(-t / T))
+  // This comes from a solution to the linear drag equation F=-kv
+  float exp_deceleration = std::exp(-time_delta / time_constant);
+  return initial_speed_rate * (1.f - exp_deceleration);
+}
+
+float GetSpeed(float initial_speed_rate,
+               float time_constant,
+               float time_delta) {
+  // v = (1 / T) * v0 * e^(-t / T).
+  // Derivative of the displacement.
+  float exp_deceleration = std::exp(-time_delta / time_constant);
+  return (1.f / time_constant) * initial_speed_rate * exp_deceleration;
+}
+
+}  // 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_, kMinFlingTime) <
+      kMinTrackSpeed) {
+    StopFling();
+    return;
+  }
+
+  velocity_ratio_x_ = velocity_x / initial_speed_rate_;
+  velocity_ratio_y_ = velocity_y / initial_speed_rate_;
+
+  initial_speed_rate_ *= kAmplituteFactor;

[nicholss, 2017/05/10 19:34:05]

Why start with attenuation?

[Yuwei, 2017/05/10 23:28:24]

I was basically inspired by this article:

https://ariya.io/2013/11/javascript-kinetic-scrolling-part-2

It multiplies the velocity with 0.8 before flinging.

The viewport feels a little bit light so I multiplied it with 0.75.

Anyway tweaking these numbers feel like black magic and the feeling is totally
subjective. Let's change it back to one and later see how we can optimize it.

+}
+
+void FlingTracker::StopFling() {
+  initial_speed_rate_ = 0.f;
+}
+
+bool FlingTracker::IsFlingInProgress() const {
+  return initial_speed_rate_ > 0;
+}
+
+bool FlingTracker::TrackPositions(base::TimeTicks time, float* x, float* y) {
+  if (!IsFlingInProgress()) {
+    return false;
+  }
+
+  float time_delta_ms = (time - start_time_).InMilliseconds();
+
+  float speed = GetSpeed(initial_speed_rate_, time_constant_, time_delta_ms);
+
+  if (speed < kMinTrackSpeed) {
+    StopFling();
+    return false;
+  }
+
+  float displacement =
+      GetDisplacement(initial_speed_rate_, time_constant_, time_delta_ms);
+
+  *x = displacement * velocity_ratio_x_;
+  *y = displacement * velocity_ratio_y_;
+
+  return true;
+}
+
+}  // namespace remoting