[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;
+}
+
+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