Add forces.dart

packages/flutter/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});
+
+  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) {
+    // Target just past the endpoint, because the animation will stop once the
+    // Spring gets within the epsilon, and we want to stop at the endpoint.
+    double target = velocity < 0.0 ?
+        this.left - _kEpsilon : this.right + _kEpsilon;
+    return new SpringSimulation(spring, position, target, velocity);
+  }
+}
+
+final SpringDescription _kDefaultSpringDesc =
+    new SpringDescription.withDampingRatio(
+        mass: 1.0, springConstant: 500.0, ratio: 1.0);
+final SpringForce kDefaultSpringForce = new SpringForce(_kDefaultSpringDesc);
+
+const double _kEpsilon = 0.001;