Rationalize Dart mojo and sky package structure

sky/framework/animation/mechanics.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 'dart:math' as math;
-
-const double kGravity = -0.980; // m^s-2
-
-abstract class System {
-  void update(double deltaT);
-}
-
-class Particle extends System {
-  final double mass;
-  double velocity;
-  double position;
-
-  Particle({this.mass: 1.0, this.velocity: 0.0, this.position: 0.0});
-
-  void applyImpulse(double impulse) {
-    velocity += impulse / mass;
-  }
-
-  void update(double deltaT) {
-    position += velocity * deltaT;
-  }
-
-  void setVelocityFromEnergy({double energy, double direction}) {
-    assert(direction == -1.0 || direction == 1.0);
-    assert(energy >= 0.0);
-    velocity = math.sqrt(2.0 * energy / mass) * direction;
-  }
-}
-
-abstract class Box {
-  void confine(Particle p);
-}
-
-class ClosedBox extends Box {
-  final double min; // m
-  final double max; // m
-
-  ClosedBox({this.min, this.max}) {
-    assert(min == null || max == null || min <= max);
-  }
-
-  void confine(Particle p) {
-    if (min != null) {
-      p.position = math.max(min, p.position);
-      if (p.position == min)
-        p.velocity = math.max(0.0, p.velocity);
-    }
-    if (max != null) {
-      p.position = math.min(max, p.position);
-      if (p.position == max)
-        p.velocity = math.min(0.0, p.velocity);
-    }
-  }
-}
-
-class GeofenceBox extends Box {
-  final double min; // m
-  final double max; // m
-
-  final Function onEscape;
-
-  GeofenceBox({this.min, this.max, this.onEscape}) {
-    assert(min == null || max == null || min <= max);
-    assert(onEscape != null);
-  }
-
-  void confine(Particle p) {
-    if (((min != null) && (p.position < min)) ||
-        ((max != null) && (p.position > max)))
-      onEscape();
-  }
-}
-
-class ParticleInBox extends System {
-  final Particle particle;
-  final Box box;
-
-  ParticleInBox({this.particle, this.box}) {
-    box.confine(particle);
-  }
-
-  void update(double deltaT) {
-    particle.update(deltaT);
-    box.confine(particle);
-  }
-}
-
-class ParticleInBoxWithFriction extends ParticleInBox {
-  final double friction; // unitless
-  final double _sign;
-
-  final Function onStop;
-
-  ParticleInBoxWithFriction({Particle particle, Box box, this.friction, this.onStop})
-      : super(particle: particle, box: box),
-        _sign = particle.velocity.sign;
-
-  void update(double deltaT) {
-    double force = -_sign * friction * particle.mass * -kGravity;
-    particle.applyImpulse(force * deltaT);
-    if (particle.velocity.sign != _sign) {
-      particle.velocity = 0.0;
-    }
-    super.update(deltaT);
-    if ((particle.velocity == 0.0) && (onStop != null))
-      onStop();
-  }
-}
-
-class Spring {
-  final double k;
-  double displacement;
-
-  Spring(this.k, {this.displacement: 0.0});
-
-  double get force => -k * displacement;
-}
-
-class ParticleAndSpringInBox extends System {
-  final Particle particle;
-  final Spring spring;
-  final Box box;
-
-  ParticleAndSpringInBox({this.particle, this.spring, this.box}) {
-    _applyInvariants();
-  }
-
-  void update(double deltaT) {
-    particle.applyImpulse(spring.force * deltaT);
-    particle.update(deltaT);
-    _applyInvariants();
-  }
-
-  void _applyInvariants() {
-    box.confine(particle);
-    spring.displacement = particle.position;
-  }
-}
-
-class ParticleClimbingRamp extends System {
-
-  // This is technically the same as ParticleInBoxWithFriction. The
-  // difference is in how the system is set up. Here, we configure the
-  // system so as to stop by a certain distance after having been
-  // given an initial impulse from rest, whereas
-  // ParticleInBoxWithFriction is set up to stop with a consistent
-  // decelerating force assuming an initial velocity. The angle theta
-  // (0 < theta < π/2) is used to configure how much energy the
-  // particle is to start with; lower angles result in a gentler kick
-  // while higher angles result in a faster conclusion.
-
-  final Particle particle;
-  final Box box;
-  final double theta;
-  final double _sinTheta;
-
-  ParticleClimbingRamp({
-      this.particle,
-      this.box,
-      double theta, // in radians
-      double targetPosition}) : this.theta = theta, this._sinTheta = math.sin(theta) {
-    assert(theta > 0.0);
-    assert(theta < math.PI / 2.0);
-    double deltaPosition = targetPosition - particle.position;
-    double tanTheta = math.tan(theta);
-    // We need to give the particle exactly as much (kinetic) energy
-    // as it needs to get to the top of the slope and stop with
-    // energy=0. This is exactly the same amount of energy as the
-    // potential energy at the top of the slope, which is g*h*m.
-    // If the slope's horizontal component is delta P long, then
-    // the height is delta P times tan theta.
-    particle.setVelocityFromEnergy(
-      energy: (kGravity * (deltaPosition * tanTheta) * particle.mass).abs(),
-      direction: deltaPosition > 0.0 ? 1.0 : -1.0
-    );
-    box.confine(particle);
-  }
-
-  void update(double deltaT) {
-    particle.update(deltaT);
-    // Note that we apply the impulse from gravity after updating the particle's
-    // position so that we overestimate the distance traveled by the particle.
-    // That ensures that we actually hit the edge of the box and don't wind up
-    // reversing course.
-    particle.applyImpulse(particle.mass * kGravity * _sinTheta * deltaT);
-    box.confine(particle);
-  }
-}
-
-class Multisystem extends System {
-  final Particle particle;
-
-  System _currentSystem;
-
-  Multisystem({ this.particle, System system }) {
-    assert(system != null);
-    _currentSystem = system;
-  }
-
-  void update(double deltaT) {
-    _currentSystem.update(deltaT);
-  }
-
-  void transitionToSystem(System system) {
-    assert(system != null);
-    _currentSystem = system;
-  }
-}