Implement simulation groups for kinetic scrolling

packages/newton/lib/newton.dart

@@ -7,9 +7,9 @@ library newton;
 import 'dart:math' as Math;
 
 part 'src/simulation.dart';
+part 'src/simulation_group.dart';
 part 'src/utils.dart';
 
-part 'src/fall.dart';
 part 'src/friction.dart';
 part 'src/gravity.dart';
 part 'src/scroll.dart';

packages/newton/lib/src/fall.dart

@@ -1,8 +0,0 @@
-// Copyright (c) 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.
-
-part of newton;
-
-// TODO(csg): Composite simulation
-class Fall {}

packages/newton/lib/src/scroll.dart

@@ -4,5 +4,48 @@
 
 part of newton;
 
-// TODO(csg): Composite simulation
-class Scroll {}
+/// Simulates kinetic scrolling behavior between a leading and trailing
+/// boundary. Friction is applied within the extends and a spring action applied
+/// at the boundaries. This simulation can only step forward.
+class Scroll extends SimulationGroup {
+  final double _leadingExtent;
+  final double _trailingExtent;
+  final SpringDesc _springDesc;
+  final double _drag;
+
+  bool _isSpringing = false;
+  Simulation _currentSimulation;
+
+  Scroll(double position, double velocity, double leading, double trailing,
+      SpringDesc spring, double drag)
+      : _leadingExtent = leading,
+        _trailingExtent = trailing,
+        _springDesc = spring,
+        _drag = drag {
+    _chooseSimulation(position, velocity);
+  }
+
+  @override
+  void step(double time) => _chooseSimulation(
+      _currentSimulation.x(time), _currentSimulation.dx(time));
+
+  @override
+  Simulation get currentSimulation => _currentSimulation;
+
+  void _chooseSimulation(double position, double velocity) {
+    /// This simulation can only step forward
+    if (!_isSpringing) {
+      if (position > _trailingExtent) {
+        _isSpringing = true;
+        _currentSimulation =
+            new Spring(_springDesc, position, _trailingExtent, velocity);
+      } else if (position < _leadingExtent) {
+        _isSpringing = true;
+        _currentSimulation =
+            new Spring(_springDesc, position, _leadingExtent, velocity);
+      }
+    } else if (_currentSimulation == null) {
+      _currentSimulation = new Friction(_drag, position, velocity);
+    }
+  }
+}

packages/newton/lib/src/simulation.dart

@@ -16,9 +16,7 @@ abstract class Simulatable {
 /// instance of a simulation and query the same for the position and velocity
 /// of the body at a given interval.
 ///
-/// Note: All operation on subclasses of Simulation are idempotent. Composite
-/// simulations are not guaranteed to be idempotent however. FIXME(csg): How do
-/// I make this apparent?
+/// Note: All operations on subclasses of Simulation are idempotent.
 abstract class Simulation implements Simulatable {
 
   /// Returns if the simulation is done at a given time

packages/newton/lib/src/simulation_group.dart

@@ -0,0 +1,42 @@
+// Copyright (c) 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.
+
+part of newton;
+
+/// The abstract base class of all composite simulations. Concrete subclasses
+/// must implement the appropriate methods to select the appropriate simulation
+/// at a given time interval. The simulation group takes care to call the `step`
+/// method at appropriate intervals. If more fine grained control over the the
+/// step is necessary, subclasses may override the `Simulatable` methods.
+abstract class SimulationGroup extends Simulation {
+  Simulation get currentSimulation;
+
+  void step(double time);
+
+  double x(double time) {
+    _stepIfNecessary(time);
+    return currentSimulation.x(time);
+  }
+
+  double dx(double time) {
+    _stepIfNecessary(time);
+    return currentSimulation.dx(time);
+  }
+
+  @override
+  bool isDone(double time) {
+    _stepIfNecessary(time);
+    return currentSimulation.isDone(time);
+  }
+
+  double _lastStep = -1.0;
+  void _stepIfNecessary(double time) {
+    if (_nearEqual(_lastStep, time)) {
+      return;
+    }
+
+    _lastStep = time;
+    step(time);
+  }
+}

packages/newton/lib/src/spring_solution.dart

@@ -23,63 +23,87 @@ abstract class _SpringSolution implements Simulatable {
 }
 
 class _CriticalSolution implements _SpringSolution {
-  double r, c1, c2;
+  final double _r, _c1, _c2;
 
-  _CriticalSolution(SpringDesc desc, double distance, double velocity) {
-    r = -desc.damping / (2.0 * desc.mass);
-    c1 = distance;
-    c2 = velocity / (r * distance);
+  factory _CriticalSolution(SpringDesc desc, double distance, double velocity) {
+    final double r = -desc.damping / (2.0 * desc.mass);
+    final double c1 = distance;
+    final double c2 = velocity / (r * distance);
+    return new _CriticalSolution.withArgs(r, c1, c2);
   }
 
-  double x(double time) => (c1 + c2 * time) * Math.pow(Math.E, r * time);
+  _CriticalSolution.withArgs(double r, double c1, double c2)
+      : _r = r,
+        _c1 = c1,
+        _c2 = c2;
+
+  double x(double time) => (_c1 + _c2 * time) * Math.pow(Math.E, _r * time);
 
   double dx(double time) {
-    final double power = Math.pow(Math.E, r * time);
-    return r * (c1 + c2 * time) * power + c2 * power;
+    final double power = Math.pow(Math.E, _r * time);
+    return _r * (_c1 + _c2 * time) * power + _c2 * power;
   }
 }
 
 class _OverdampedSolution implements _SpringSolution {
-  double r1, r2, c1, c2;
+  final double _r1, _r2, _c1, _c2;
 
-  _OverdampedSolution(SpringDesc desc, double distance, double velocity) {
-    double cmk =
+  factory _OverdampedSolution(
+      SpringDesc desc, double distance, double velocity) {
+    final double cmk =
         desc.damping * desc.damping - 4 * desc.mass * desc.springConstant;
 
-    r1 = (-desc.damping - Math.sqrt(cmk)) / (2.0 * desc.mass);
-    r2 = (-desc.damping + Math.sqrt(cmk)) / (2.0 * desc.mass);
-    c2 = (velocity - r1 * distance) / (r2 - r1);
-    c1 = distance - c2;
+    final double r1 = (-desc.damping - Math.sqrt(cmk)) / (2.0 * desc.mass);
+    final double r2 = (-desc.damping + Math.sqrt(cmk)) / (2.0 * desc.mass);
+    final double c2 = (velocity - r1 * distance) / (r2 - r1);
+    final double c1 = distance - c2;
+
+    return new _OverdampedSolution.withArgs(r1, r2, c1, c2);
   }
 
-  double x(double time) =>
-      (c1 * Math.pow(Math.E, r1 * time) + c2 * Math.pow(Math.E, r2 * time));
+  _OverdampedSolution.withArgs(double r1, double r2, double c1, double c2)
+      : _r1 = r1,
+        _r2 = r2,
+        _c1 = c1,
+        _c2 = c2;
 
-  double dx(double time) => (c1 * r1 * Math.pow(Math.E, r1 * time) +
-      c2 * r2 * Math.pow(Math.E, r2 * time));
+  double x(double time) =>
+      (_c1 * Math.pow(Math.E, _r1 * time) + _c2 * Math.pow(Math.E, _r2 * time));
+
+  double dx(double time) => (_c1 * _r1 * Math.pow(Math.E, _r1 * time) +
+      _c2 * _r2 * Math.pow(Math.E, _r2 * time));
 }
 
 class _UnderdampedSolution implements _SpringSolution {
-  double w, r, c1, c2;
+  final double _w, _r, _c1, _c2;
 
-  _UnderdampedSolution(SpringDesc desc, double distance, double velocity) {
-    w = Math.sqrt(4.0 * desc.mass * desc.springConstant -
+  factory _UnderdampedSolution(
+      SpringDesc desc, double distance, double velocity) {
+    final double w = Math.sqrt(4.0 * desc.mass * desc.springConstant -
             desc.damping * desc.damping) /
         (2.0 * desc.mass);
-    r = -(desc.damping / 2.0 * desc.mass);
-    c1 = distance;
-    c2 = (velocity - r * distance) / w;
+    final double r = -(desc.damping / 2.0 * desc.mass);
+    final double c1 = distance;
+    final double c2 = (velocity - r * distance) / w;
+
+    return new _UnderdampedSolution.withArgs(w, r, c1, c2);
   }
 
-  double x(double time) => Math.pow(Math.E, r * time) *
-      (c1 * Math.cos(w * time) + c2 * Math.sin(w * time));
+  _UnderdampedSolution.withArgs(double w, double r, double c1, double c2)
+      : _w = w,
+        _r = r,
+        _c1 = c1,
+        _c2 = c2;
+
+  double x(double time) => Math.pow(Math.E, _r * time) *
+      (_c1 * Math.cos(_w * time) + _c2 * Math.sin(_w * time));
 
   double dx(double time) {
-    final double power = Math.pow(Math.E, r * time);
-    final double cosine = Math.cos(w * time);
-    final double sine = Math.sin(w * time);
+    final double power = Math.pow(Math.E, _r * time);
+    final double cosine = Math.cos(_w * time);
+    final double sine = Math.sin(_w * time);
 
-    return power * (c2 * w * cosine - c1 * w * sine) +
-        r * power * (c2 * sine + c1 * cosine);
+    return power * (_c2 * _w * cosine - _c1 * _w * sine) +
+        _r * power * (_c2 * sine + _c1 * cosine);
   }
 }