Remove Force and SpringForce (#7532)

The only client of this machinery was AnimationController.fling, but
it's easier for that function to just create the simulation it needs
directly.

Fixes #7216

packages/flutter/lib/animation.dart

@@ -15,6 +15,5 @@ export 'src/animation/animation.dart';
 export 'src/animation/animation_controller.dart';
 export 'src/animation/animations.dart';
 export 'src/animation/curves.dart';
-export 'src/animation/forces.dart';
 export 'src/animation/listener_helpers.dart';
 export 'src/animation/tween.dart';

packages/flutter/lib/src/animation/animation_controller.dart

@@ -11,7 +11,6 @@ import 'package:flutter/scheduler.dart';
 
 import 'animation.dart';
 import 'curves.dart';
-import 'forces.dart';
 import 'listener_helpers.dart';
 
 /// The direction in which an animation is running.
@@ -23,6 +22,17 @@ enum _AnimationDirection {
   reverse,
 }
 
+final SpringDescription _kFlingSpringDescription = new SpringDescription.withDampingRatio(
+  mass: 1.0,
+  springConstant: 500.0,
+  ratio: 1.0,
+);
+
+const Tolerance _kFlingTolerance = const Tolerance(
+  velocity: double.INFINITY,
+  distance: 0.01,
+);
+
 /// A controller for an animation.
 ///
 /// This class lets you perform tasks such as:
@@ -305,13 +315,16 @@ class AnimationController extends Animation<double>
     return animateWith(new _RepeatingSimulation(min, max, period));
   }
 
-  /// Flings the timeline with an optional force (defaults to a critically
-  /// damped spring within [lowerBound] and [upperBound]) and initial velocity.
+  /// Drives the animation with a critically damped spring (within [lowerBound] and [upperBound]) and initial velocity.
+  ///
   /// If velocity is positive, the animation will complete, otherwise it will dismiss.
-  Future<Null> fling({ double velocity: 1.0, Force force }) {
-    force ??= kDefaultSpringForce.copyWith(left: lowerBound, right: upperBound);
+  Future<Null> fling({ double velocity: 1.0 }) {
     _direction = velocity < 0.0 ? _AnimationDirection.reverse : _AnimationDirection.forward;
-    return animateWith(force.release(value, velocity));
+    final double target = velocity < 0.0 ? lowerBound - _kFlingTolerance.distance
+                                         : upperBound + _kFlingTolerance.distance;
+    Simulation simulation = new SpringSimulation(_kFlingSpringDescription, value, target, velocity)
+      ..tolerance = _kFlingTolerance;
+    return animateWith(simulation);
   }
 
   /// Drives the animation according to the given simulation.

packages/flutter/lib/src/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:flutter/physics.dart';
-
-export 'package:flutter/physics.dart' show SpringDescription;
-
-/// A factory for simulations.
-abstract class Force {
-  /// Abstract const constructor. This constructor enables subclasses to provide
-  /// const constructors so that they can be used in const expressions.
-  const Force();
-
-  /// Creates a new physics simulation with the given initial conditions.
-  Simulation release(double position, double velocity);
-}
-
-/// A factory for spring-based physics simulations.
-class SpringForce extends Force {
-  /// Creates a spring force.
-  ///
-  /// The [spring], [left], and [right] arguments must not be null. The [left]
-  /// argument defaults to 0.0 and the [right] argument defaults to 1.0.
-  const SpringForce(this.spring, { this.left: 0.0, this.right: 1.0 });
-
-  /// The description of the spring to be used in the created simulations.
-  final SpringDescription spring;
-
-  /// Where to put the spring's resting point when releasing left.
-  final double left;
-
-  /// Where to put the spring's resting point when releasing right.
-  final double right;
-
-  /// Creates a copy of this spring force but with the given fields replaced with the new values.
-  SpringForce copyWith({
-    SpringDescription spring,
-    double left,
-    double right
-  }) {
-    return new SpringForce(
-      spring ?? this.spring,
-      left: left ?? this.left,
-      right: right ?? this.right
-    );
-  }
-
-  /// How pricely to terminate the simulation.
-  ///
-  /// 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 const Tolerance tolerance = const Tolerance(
-    velocity: double.INFINITY,
-    distance: 0.01
-  );
-
-  @override
-  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
-);
-
-/// A spring force with reasonable default values.
-final SpringForce kDefaultSpringForce = new SpringForce(_kDefaultSpringDesc);

packages/flutter/test/animation/forces_test.dart

-// Copyright 2016 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:flutter_test/flutter_test.dart';
-import 'package:flutter/animation.dart';
-import 'package:flutter/widgets.dart';
-
-void main() {
-  test('copyWith defaults to unchanged values', () {
-    SpringForce force = kDefaultSpringForce.copyWith();
-    expect(force.spring, kDefaultSpringForce.spring);
-    expect(force.left, kDefaultSpringForce.left);
-    expect(force.right, kDefaultSpringForce.right);
-  });
-}