sky/packages/sky/lib/animation/forces.dart

// 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.

import 'package:newton/newton.dart';

// Base class for creating Simulations for the animation Timeline.
abstract class Force {
  Simulation release(double position, double velocity);
}

class SpringForce extends Force {
  SpringForce(this.spring, {this.left: 0.0, this.right: 1.0});

  // We overshoot the target by this distance, but stop the simulation when
  // the spring gets within this distance (regardless of how fast it's moving).
  // This causes the spring to settle a bit faster than it otherwise would.
  static final Tolerance tolerance =
      new Tolerance(velocity: double.INFINITY, distance: 0.01);

  final SpringDescription spring;
  // Where to put the spring's resting point when releasing left or right,
  // respectively.
  final double left, right;

  Simulation release(double position, double velocity) {
    double target = velocity < 0.0 ?
        this.left - tolerance.distance : this.right + tolerance.distance;
    return new SpringSimulation(spring, position, target, velocity)
      ..tolerance = tolerance;
  }
}

final SpringDescription _kDefaultSpringDesc =
    new SpringDescription.withDampingRatio(mass: 1.0, springConstant: 500.0, ratio: 1.0);
final SpringForce kDefaultSpringForce = new SpringForce(_kDefaultSpringDesc);