sky/framework/fling-curve.sky

<!--
// Copyright 2015 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.
-->
<script>
const kDefaultAlpha = -5707.62;
const kDefaultBeta = 172;
const kDefaultGamma = 3.7;

function positionAtTime(t) {
  return kDefaultAlpha * Math.exp(-kDefaultGamma * t) - kDefaultBeta * t - kDefaultAlpha;
}

function velocityAtTime(t) {
  return -kDefaultAlpha * kDefaultGamma * Math.exp(-kDefaultGamma * t) - kDefaultBeta;
}

function timeAtVelocity(v) {
  return -Math.log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) / kDefaultGamma;
}

var kMaxVelocity = velocityAtTime(0);
var kCurveDuration = timeAtVelocity(0);

module.exports = class FlingCurve {
  constructor(velocity, startTime) {
    var startingVelocity = Math.min(kMaxVelocity, Math.abs(velocity));
    this.timeOffset_ = timeAtVelocity(startingVelocity);
    this.positionOffset_ = positionAtTime(this.timeOffset_);
    this.startTime_ = startTime / 1000;
    this.previousPosition_ = 0;
    this.direction_ = Math.sign(velocity);
    Object.preventExtensions(this);
  }

  update(timeStamp) {
    var t = timeStamp / 1000 - this.startTime_ + this.timeOffset_;
    if (t >= kCurveDuration)
      return 0;
    var position = positionAtTime(t) - this.positionOffset_;
    var positionDelta = position - this.previousPosition_;
    this.previousPosition_ = position;
    return this.direction_ * Math.max(0, positionDelta);
  }
};
</script>