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

// Copyright 2014 The Flutter 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:async';
import 'dart:ui' as ui show lerpDouble;

import 'package:flutter/foundation.dart';
import 'package:flutter/physics.dart';
import 'package:flutter/scheduler.dart';
import 'package:flutter/semantics.dart';

import 'animation.dart';
import 'curves.dart';
import 'listener_helpers.dart';

export 'package:flutter/scheduler.dart' show TickerFuture, TickerCanceled;

// Examples can assume:
// AnimationController _controller, fadeAnimationController, sizeAnimationController;
// bool dismissed;
// void setState(VoidCallback fn) { }

/// The direction in which an animation is running.
enum _AnimationDirection {
  /// The animation is running from beginning to end.
  forward,

  /// The animation is running backwards, from end to beginning.
  reverse,
}

final SpringDescription _kFlingSpringDescription = SpringDescription.withDampingRatio(
  mass: 1.0,
  stiffness: 500.0,
  ratio: 1.0,
);

const Tolerance _kFlingTolerance = Tolerance(
  velocity: double.infinity,
  distance: 0.01,
);

/// Configures how an [AnimationController] behaves when animations are
/// disabled.
///
/// When [AccessibilityFeatures.disableAnimations] is true, the device is asking
/// Flutter to reduce or disable animations as much as possible. To honor this,
/// we reduce the duration and the corresponding number of frames for
/// animations. This enum is used to allow certain [AnimationController]s to opt
/// out of this behavior.
///
/// For example, the [AnimationController] which controls the physics simulation
/// for a scrollable list will have [AnimationBehavior.preserve], so that when
/// a user attempts to scroll it does not jump to the end/beginning too quickly.
enum AnimationBehavior {
  /// The [AnimationController] will reduce its duration when
  /// [AccessibilityFeatures.disableAnimations] is true.
  normal,

  /// The [AnimationController] will preserve its behavior.
  ///
  /// This is the default for repeating animations in order to prevent them from
  /// flashing rapidly on the screen if the widget does not take the
  /// [AccessibilityFeatures.disableAnimations] flag into account.
  preserve,
}

/// A controller for an animation.
///
/// This class lets you perform tasks such as:
///
/// * Play an animation [forward] or in [reverse], or [stop] an animation.
/// * Set the animation to a specific [value].
/// * Define the [upperBound] and [lowerBound] values of an animation.
/// * Create a [fling] animation effect using a physics simulation.
///
/// By default, an [AnimationController] linearly produces values that range
/// from 0.0 to 1.0, during a given duration. The animation controller generates
/// a new value whenever the device running your app is ready to display a new
/// frame (typically, this rate is around 60 values per second).
///
/// ## Ticker providers
///
/// An [AnimationController] needs a [TickerProvider], which is configured using
/// the `vsync` argument on the constructor.
///
/// The [TickerProvider] interface describes a factory for [Ticker] objects. A
/// [Ticker] is an object that knows how to register itself with the
/// [SchedulerBinding] and fires a callback every frame. The
/// [AnimationController] class uses a [Ticker] to step through the animation
/// that it controls.
///
/// If an [AnimationController] is being created from a [State], then the State
/// can use the [TickerProviderStateMixin] and [SingleTickerProviderStateMixin]
/// classes to implement the [TickerProvider] interface. The
/// [TickerProviderStateMixin] class always works for this purpose; the
/// [SingleTickerProviderStateMixin] is slightly more efficient in the case of
/// the class only ever needing one [Ticker] (e.g. if the class creates only a
/// single [AnimationController] during its entire lifetime).
///
/// The widget test framework [WidgetTester] object can be used as a ticker
/// provider in the context of tests. In other contexts, you will have to either
/// pass a [TickerProvider] from a higher level (e.g. indirectly from a [State]
/// that mixes in [TickerProviderStateMixin]), or create a custom
/// [TickerProvider] subclass.
///
/// ## Life cycle
///
/// An [AnimationController] should be [dispose]d when it is no longer needed.
/// This reduces the likelihood of leaks. When used with a [StatefulWidget], it
/// is common for an [AnimationController] to be created in the
/// [State.initState] method and then disposed in the [State.dispose] method.
///
/// ## Using [Future]s with [AnimationController]
///
/// The methods that start animations return a [TickerFuture] object which
/// completes when the animation completes successfully, and never throws an
/// error; if the animation is canceled, the future never completes. This object
/// also has a [TickerFuture.orCancel] property which returns a future that
/// completes when the animation completes successfully, and completes with an
/// error when the animation is aborted.
///
/// This can be used to write code such as the `fadeOutAndUpdateState` method
/// below.
///
/// {@tool snippet}
///
/// Here is a stateful `Foo` widget. Its [State] uses the
/// [SingleTickerProviderStateMixin] to implement the necessary
/// [TickerProvider], creating its controller in the [State.initState] method
/// and disposing of it in the [State.dispose] method. The duration of the
/// controller is configured from a property in the `Foo` widget; as that
/// changes, the [State.didUpdateWidget] method is used to update the
/// controller.
///
/// ```dart
/// class Foo extends StatefulWidget {
///   Foo({ Key key, this.duration }) : super(key: key);
///
///   final Duration duration;
///
///   @override
///   _FooState createState() => _FooState();
/// }
///
/// class _FooState extends State<Foo> with SingleTickerProviderStateMixin {
///   AnimationController _controller;
///
///   @override
///   void initState() {
///     super.initState();
///     _controller = AnimationController(
///       vsync: this, // the SingleTickerProviderStateMixin
///       duration: widget.duration,
///     );
///   }
///
///   @override
///   void didUpdateWidget(Foo oldWidget) {
///     super.didUpdateWidget(oldWidget);
///     _controller.duration = widget.duration;
///   }
///
///   @override
///   void dispose() {
///     _controller.dispose();
///     super.dispose();
///   }
///
///   @override
///   Widget build(BuildContext context) {
///     return Container(); // ...
///   }
/// }
/// ```
/// {@end-tool}
/// {@tool snippet}
///
/// The following method (for a [State] subclass) drives two animation
/// controllers using Dart's asynchronous syntax for awaiting [Future] objects:
///
/// ```dart
/// Future<void> fadeOutAndUpdateState() async {
///   try {
///     await fadeAnimationController.forward().orCancel;
///     await sizeAnimationController.forward().orCancel;
///     setState(() {
///       dismissed = true;
///     });
///   } on TickerCanceled {
///     // the animation got canceled, probably because we were disposed
///   }
/// }
/// ```
/// {@end-tool}
///
/// The assumption in the code above is that the animation controllers are being
/// disposed in the [State] subclass' override of the [State.dispose] method.
/// Since disposing the controller cancels the animation (raising a
/// [TickerCanceled] exception), the code here can skip verifying whether
/// [State.mounted] is still true at each step. (Again, this assumes that the
/// controllers are created in [State.initState] and disposed in
/// [State.dispose], as described in the previous section.)
///
/// See also:
///
///  * [Tween], the base class for converting an [AnimationController] to a
///    range of values of other types.
class AnimationController extends Animation<double>
  with AnimationEagerListenerMixin, AnimationLocalListenersMixin, AnimationLocalStatusListenersMixin {
  /// Creates an animation controller.
  ///
  /// * `value` is the initial value of the animation. If defaults to the lower
  ///   bound.
  ///
  /// * [duration] is the length of time this animation should last.
  ///
  /// * [debugLabel] is a string to help identify this animation during
  ///   debugging (used by [toString]).
  ///
  /// * [lowerBound] is the smallest value this animation can obtain and the
  ///   value at which this animation is deemed to be dismissed. It cannot be
  ///   null.
  ///
  /// * [upperBound] is the largest value this animation can obtain and the
  ///   value at which this animation is deemed to be completed. It cannot be
  ///   null.
  ///
  /// * `vsync` is the [TickerProvider] for the current context. It can be
  ///   changed by calling [resync]. It is required and must not be null. See
  ///   [TickerProvider] for advice on obtaining a ticker provider.
  AnimationController({
    double value,
    this.duration,
    this.reverseDuration,
    this.debugLabel,
    this.lowerBound = 0.0,
    this.upperBound = 1.0,
    this.animationBehavior = AnimationBehavior.normal,
    @required TickerProvider vsync,
  }) : assert(lowerBound != null),
       assert(upperBound != null),
       assert(upperBound >= lowerBound),
       assert(vsync != null),
       _direction = _AnimationDirection.forward {
    _ticker = vsync.createTicker(_tick);
    _internalSetValue(value ?? lowerBound);
  }

  /// Creates an animation controller with no upper or lower bound for its
  /// value.
  ///
  /// * [value] is the initial value of the animation.
  ///
  /// * [duration] is the length of time this animation should last.
  ///
  /// * [debugLabel] is a string to help identify this animation during
  ///   debugging (used by [toString]).
  ///
  /// * `vsync` is the [TickerProvider] for the current context. It can be
  ///   changed by calling [resync]. It is required and must not be null. See
  ///   [TickerProvider] for advice on obtaining a ticker provider.
  ///
  /// This constructor is most useful for animations that will be driven using a
  /// physics simulation, especially when the physics simulation has no
  /// pre-determined bounds.
  AnimationController.unbounded({
    double value = 0.0,
    this.duration,
    this.reverseDuration,
    this.debugLabel,
    @required TickerProvider vsync,
    this.animationBehavior = AnimationBehavior.preserve,
  }) : assert(value != null),
       assert(vsync != null),
       lowerBound = double.negativeInfinity,
       upperBound = double.infinity,
       _direction = _AnimationDirection.forward {
    _ticker = vsync.createTicker(_tick);
    _internalSetValue(value);
  }

  /// The value at which this animation is deemed to be dismissed.
  final double lowerBound;

  /// The value at which this animation is deemed to be completed.
  final double upperBound;

  /// A label that is used in the [toString] output. Intended to aid with
  /// identifying animation controller instances in debug output.
  final String debugLabel;

  /// The behavior of the controller when [AccessibilityFeatures.disableAnimations]
  /// is true.
  ///
  /// Defaults to [AnimationBehavior.normal] for the [new AnimationController]
  /// constructor, and [AnimationBehavior.preserve] for the
  /// [new AnimationController.unbounded] constructor.
  final AnimationBehavior animationBehavior;

  /// Returns an [Animation<double>] for this animation controller, so that a
  /// pointer to this object can be passed around without allowing users of that
  /// pointer to mutate the [AnimationController] state.
  Animation<double> get view => this;

  /// The length of time this animation should last.
  ///
  /// If [reverseDuration] is specified, then [duration] is only used when going
  /// [forward]. Otherwise, it specifies the duration going in both directions.
  Duration duration;

  /// The length of time this animation should last when going in [reverse].
  ///
  /// The value of [duration] us used if [reverseDuration] is not specified or
  /// set to null.
  Duration reverseDuration;

  Ticker _ticker;

  /// Recreates the [Ticker] with the new [TickerProvider].
  void resync(TickerProvider vsync) {
    final Ticker oldTicker = _ticker;
    _ticker = vsync.createTicker(_tick);
    _ticker.absorbTicker(oldTicker);
  }

  Simulation _simulation;

  /// The current value of the animation.
  ///
  /// Setting this value notifies all the listeners that the value
  /// changed.
  ///
  /// Setting this value also stops the controller if it is currently
  /// running; if this happens, it also notifies all the status
  /// listeners.
  @override
  double get value => _value;
  double _value;
  /// Stops the animation controller and sets the current value of the
  /// animation.
  ///
  /// The new value is clamped to the range set by [lowerBound] and
  /// [upperBound].
  ///
  /// Value listeners are notified even if this does not change the value.
  /// Status listeners are notified if the animation was previously playing.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// See also:
  ///
  ///  * [reset], which is equivalent to setting [value] to [lowerBound].
  ///  * [stop], which aborts the animation without changing its value or status
  ///    and without dispatching any notifications other than completing or
  ///    canceling the [TickerFuture].
  ///  * [forward], [reverse], [animateTo], [animateWith], [fling], and [repeat],
  ///    which start the animation controller.
  set value(double newValue) {
    assert(newValue != null);
    stop();
    _internalSetValue(newValue);
    notifyListeners();
    _checkStatusChanged();
  }

  /// Sets the controller's value to [lowerBound], stopping the animation (if
  /// in progress), and resetting to its beginning point, or dismissed state.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// See also:
  ///
  ///  * [value], which can be explicitly set to a specific value as desired.
  ///  * [forward], which starts the animation in the forward direction.
  ///  * [stop], which aborts the animation without changing its value or status
  ///    and without dispatching any notifications other than completing or
  ///    canceling the [TickerFuture].
  void reset() {
    value = lowerBound;
  }

  /// The rate of change of [value] per second.
  ///
  /// If [isAnimating] is false, then [value] is not changing and the rate of
  /// change is zero.
  double get velocity {
    if (!isAnimating)
      return 0.0;
    return _simulation.dx(lastElapsedDuration.inMicroseconds.toDouble() / Duration.microsecondsPerSecond);
  }

  void _internalSetValue(double newValue) {
    _value = newValue.clamp(lowerBound, upperBound) as double;
    if (_value == lowerBound) {
      _status = AnimationStatus.dismissed;
    } else if (_value == upperBound) {
      _status = AnimationStatus.completed;
    } else {
      _status = (_direction == _AnimationDirection.forward) ?
        AnimationStatus.forward :
        AnimationStatus.reverse;
    }
  }

  /// The amount of time that has passed between the time the animation started
  /// and the most recent tick of the animation.
  ///
  /// If the controller is not animating, the last elapsed duration is null.
  Duration get lastElapsedDuration => _lastElapsedDuration;
  Duration _lastElapsedDuration;

  /// Whether this animation is currently animating in either the forward or reverse direction.
  ///
  /// This is separate from whether it is actively ticking. An animation
  /// controller's ticker might get muted, in which case the animation
  /// controller's callbacks will no longer fire even though time is continuing
  /// to pass. See [Ticker.muted] and [TickerMode].
  bool get isAnimating => _ticker != null && _ticker.isActive;

  _AnimationDirection _direction;

  @override
  AnimationStatus get status => _status;
  AnimationStatus _status;

  /// Starts running this animation forwards (towards the end).
  ///
  /// Returns a [TickerFuture] that completes when the animation is complete.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// During the animation, [status] is reported as [AnimationStatus.forward],
  /// which switches to [AnimationStatus.completed] when [upperBound] is
  /// reached at the end of the animation.
  TickerFuture forward({ double from }) {
    assert(() {
      if (duration == null) {
        throw FlutterError(
          'AnimationController.forward() called with no default duration.\n'
          'The "duration" property should be set, either in the constructor or later, before '
          'calling the forward() function.'
        );
      }
      return true;
    }());
    assert(
      _ticker != null,
      'AnimationController.forward() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    _direction = _AnimationDirection.forward;
    if (from != null)
      value = from;
    return _animateToInternal(upperBound);
  }

  /// Starts running this animation in reverse (towards the beginning).
  ///
  /// Returns a [TickerFuture] that completes when the animation is dismissed.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// During the animation, [status] is reported as [AnimationStatus.reverse],
  /// which switches to [AnimationStatus.dismissed] when [lowerBound] is
  /// reached at the end of the animation.
  TickerFuture reverse({ double from }) {
    assert(() {
      if (duration == null && reverseDuration == null) {
        throw FlutterError(
          'AnimationController.reverse() called with no default duration or reverseDuration.\n'
          'The "duration" or "reverseDuration" property should be set, either in the constructor or later, before '
          'calling the reverse() function.'
        );
      }
      return true;
    }());
    assert(
      _ticker != null,
      'AnimationController.reverse() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    _direction = _AnimationDirection.reverse;
    if (from != null)
      value = from;
    return _animateToInternal(lowerBound);
  }

  /// Drives the animation from its current value to target.
  ///
  /// Returns a [TickerFuture] that completes when the animation is complete.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// During the animation, [status] is reported as [AnimationStatus.forward]
  /// regardless of whether `target` > [value] or not. At the end of the
  /// animation, when `target` is reached, [status] is reported as
  /// [AnimationStatus.completed].
  TickerFuture animateTo(double target, { Duration duration, Curve curve = Curves.linear }) {
    assert(
      _ticker != null,
      'AnimationController.animateTo() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    _direction = _AnimationDirection.forward;
    return _animateToInternal(target, duration: duration, curve: curve);
  }

  /// Drives the animation from its current value to target.
  ///
  /// Returns a [TickerFuture] that completes when the animation is complete.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// During the animation, [status] is reported as [AnimationStatus.reverse]
  /// regardless of whether `target` < [value] or not. At the end of the
  /// animation, when `target` is reached, [status] is reported as
  /// [AnimationStatus.dismissed].
  TickerFuture animateBack(double target, { Duration duration, Curve curve = Curves.linear }) {
    assert(
      _ticker != null,
      'AnimationController.animateBack() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    _direction = _AnimationDirection.reverse;
    return _animateToInternal(target, duration: duration, curve: curve);
  }

  TickerFuture _animateToInternal(double target, { Duration duration, Curve curve = Curves.linear }) {
    double scale = 1.0;
    if (SemanticsBinding.instance.disableAnimations) {
      switch (animationBehavior) {
        case AnimationBehavior.normal:
          // Since the framework cannot handle zero duration animations, we run it at 5% of the normal
          // duration to limit most animations to a single frame.
          // TODO(jonahwilliams): determine a better process for setting duration.
          scale = 0.05;
          break;
        case AnimationBehavior.preserve:
          break;
      }
    }
    Duration simulationDuration = duration;
    if (simulationDuration == null) {
      assert(() {
        if ((this.duration == null && _direction == _AnimationDirection.reverse && reverseDuration == null) || this.duration == null) {
          throw FlutterError(
            'AnimationController.animateTo() called with no explicit duration and no default duration or reverseDuration.\n'
            'Either the "duration" argument to the animateTo() method should be provided, or the '
            '"duration" and/or "reverseDuration" property should be set, either in the constructor or later, before '
            'calling the animateTo() function.'
          );
        }
        return true;
      }());
      final double range = upperBound - lowerBound;
      final double remainingFraction = range.isFinite ? (target - _value).abs() / range : 1.0;
      final Duration directionDuration =
        (_direction == _AnimationDirection.reverse && reverseDuration != null)
        ? reverseDuration
        : this.duration;
      simulationDuration = directionDuration * remainingFraction;
    } else if (target == value) {
      // Already at target, don't animate.
      simulationDuration = Duration.zero;
    }
    stop();
    if (simulationDuration == Duration.zero) {
      if (value != target) {
        _value = target.clamp(lowerBound, upperBound) as double;
        notifyListeners();
      }
      _status = (_direction == _AnimationDirection.forward) ?
        AnimationStatus.completed :
        AnimationStatus.dismissed;
      _checkStatusChanged();
      return TickerFuture.complete();
    }
    assert(simulationDuration > Duration.zero);
    assert(!isAnimating);
    return _startSimulation(_InterpolationSimulation(_value, target, simulationDuration, curve, scale));
  }

  /// Starts running this animation in the forward direction, and
  /// restarts the animation when it completes.
  ///
  /// Defaults to repeating between the [lowerBound] and [upperBound] of the
  /// [AnimationController] when no explicit value is set for [min] and [max].
  ///
  /// With [reverse] set to true, instead of always starting over at [min]
  /// the starting value will alternate between [min] and [max] values on each
  /// repeat. The [status] will be reported as [AnimationStatus.reverse] when
  /// the animation runs from [max] to [min].
  ///
  /// Returns a [TickerFuture] that never completes. The [TickerFuture.orCancel] future
  /// completes with an error when the animation is stopped (e.g. with [stop]).
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  TickerFuture repeat({ double min, double max, bool reverse = false, Duration period }) {
    min ??= lowerBound;
    max ??= upperBound;
    period ??= duration;
    assert(() {
      if (period == null) {
        throw FlutterError(
          'AnimationController.repeat() called without an explicit period and with no default Duration.\n'
          'Either the "period" argument to the repeat() method should be provided, or the '
          '"duration" property should be set, either in the constructor or later, before '
          'calling the repeat() function.'
        );
      }
      return true;
    }());
    assert(max >= min);
    assert(max <= upperBound && min >= lowerBound);
    assert(reverse != null);
    stop();
    return _startSimulation(_RepeatingSimulation(_value, min, max, reverse, period, _directionSetter));
  }

  void _directionSetter(_AnimationDirection direction) {
    _direction = direction;
    _status = (_direction == _AnimationDirection.forward) ?
      AnimationStatus.forward :
      AnimationStatus.reverse;
    _checkStatusChanged();
  }

  /// 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.
  ///
  /// Returns a [TickerFuture] that completes when the animation is complete.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  TickerFuture fling({ double velocity = 1.0, AnimationBehavior animationBehavior }) {
    _direction = velocity < 0.0 ? _AnimationDirection.reverse : _AnimationDirection.forward;
    final double target = velocity < 0.0 ? lowerBound - _kFlingTolerance.distance
                                         : upperBound + _kFlingTolerance.distance;
    double scale = 1.0;
    final AnimationBehavior behavior = animationBehavior ?? this.animationBehavior;
    if (SemanticsBinding.instance.disableAnimations) {
      switch (behavior) {
        case AnimationBehavior.normal:
          // TODO(jonahwilliams): determine a better process for setting velocity.
          // the value below was arbitrarily chosen because it worked for the drawer widget.
          scale = 200.0;
          break;
        case AnimationBehavior.preserve:
          break;
      }
    }
    final Simulation simulation = SpringSimulation(_kFlingSpringDescription, value, target, velocity * scale)
      ..tolerance = _kFlingTolerance;
    stop();
    return _startSimulation(simulation);
  }

  /// Drives the animation according to the given simulation.
  ///
  /// The values from the simulation are clamped to the [lowerBound] and
  /// [upperBound]. To avoid this, consider creating the [AnimationController]
  /// using the [new AnimationController.unbounded] constructor.
  ///
  /// Returns a [TickerFuture] that completes when the animation is complete.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  ///
  /// The [status] is always [AnimationStatus.forward] for the entire duration
  /// of the simulation.
  TickerFuture animateWith(Simulation simulation) {
    assert(
      _ticker != null,
      'AnimationController.animateWith() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    stop();
    _direction = _AnimationDirection.forward;
    return _startSimulation(simulation);
  }

  TickerFuture _startSimulation(Simulation simulation) {
    assert(simulation != null);
    assert(!isAnimating);
    _simulation = simulation;
    _lastElapsedDuration = Duration.zero;
    _value = simulation.x(0.0).clamp(lowerBound, upperBound) as double;
    final TickerFuture result = _ticker.start();
    _status = (_direction == _AnimationDirection.forward) ?
      AnimationStatus.forward :
      AnimationStatus.reverse;
    _checkStatusChanged();
    return result;
  }

  /// Stops running this animation.
  ///
  /// This does not trigger any notifications. The animation stops in its
  /// current state.
  ///
  /// By default, the most recently returned [TickerFuture] is marked as having
  /// been canceled, meaning the future never completes and its
  /// [TickerFuture.orCancel] derivative future completes with a [TickerCanceled]
  /// error. By passing the `canceled` argument with the value false, this is
  /// reversed, and the futures complete successfully.
  ///
  /// See also:
  ///
  ///  * [reset], which stops the animation and resets it to the [lowerBound],
  ///    and which does send notifications.
  ///  * [forward], [reverse], [animateTo], [animateWith], [fling], and [repeat],
  ///    which restart the animation controller.
  void stop({ bool canceled = true }) {
    assert(
      _ticker != null,
      'AnimationController.stop() called after AnimationController.dispose()\n'
      'AnimationController methods should not be used after calling dispose.'
    );
    _simulation = null;
    _lastElapsedDuration = null;
    _ticker.stop(canceled: canceled);
  }

  /// Release the resources used by this object. The object is no longer usable
  /// after this method is called.
  ///
  /// The most recently returned [TickerFuture], if any, is marked as having been
  /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
  /// derivative future completes with a [TickerCanceled] error.
  @override
  void dispose() {
    assert(() {
      if (_ticker == null) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('AnimationController.dispose() called more than once.'),
          ErrorDescription('A given $runtimeType cannot be disposed more than once.\n'),
          DiagnosticsProperty<AnimationController>(
            'The following $runtimeType object was disposed multiple times',
            this,
            style: DiagnosticsTreeStyle.errorProperty,
          ),
        ]);
      }
      return true;
    }());
    _ticker.dispose();
    _ticker = null;
    super.dispose();
  }

  AnimationStatus _lastReportedStatus = AnimationStatus.dismissed;
  void _checkStatusChanged() {
    final AnimationStatus newStatus = status;
    if (_lastReportedStatus != newStatus) {
      _lastReportedStatus = newStatus;
      notifyStatusListeners(newStatus);
    }
  }

  void _tick(Duration elapsed) {
    _lastElapsedDuration = elapsed;
    final double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.microsecondsPerSecond;
    assert(elapsedInSeconds >= 0.0);
    _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound) as double;
    if (_simulation.isDone(elapsedInSeconds)) {
      _status = (_direction == _AnimationDirection.forward) ?
        AnimationStatus.completed :
        AnimationStatus.dismissed;
      stop(canceled: false);
    }
    notifyListeners();
    _checkStatusChanged();
  }

  @override
  String toStringDetails() {
    final String paused = isAnimating ? '' : '; paused';
    final String ticker = _ticker == null ? '; DISPOSED' : (_ticker.muted ? '; silenced' : '');
    final String label = debugLabel == null ? '' : '; for $debugLabel';
    final String more = '${super.toStringDetails()} ${value.toStringAsFixed(3)}';
    return '$more$paused$ticker$label';
  }
}

class _InterpolationSimulation extends Simulation {
  _InterpolationSimulation(this._begin, this._end, Duration duration, this._curve, double scale)
    : assert(_begin != null),
      assert(_end != null),
      assert(duration != null && duration.inMicroseconds > 0),
      _durationInSeconds = (duration.inMicroseconds * scale) / Duration.microsecondsPerSecond;

  final double _durationInSeconds;
  final double _begin;
  final double _end;
  final Curve _curve;

  @override
  double x(double timeInSeconds) {
    final double t = (timeInSeconds / _durationInSeconds).clamp(0.0, 1.0) as double;
    if (t == 0.0)
      return _begin;
    else if (t == 1.0)
      return _end;
    else
      return _begin + (_end - _begin) * _curve.transform(t);
  }

  @override
  double dx(double timeInSeconds) {
    final double epsilon = tolerance.time;
    return (x(timeInSeconds + epsilon) - x(timeInSeconds - epsilon)) / (2 * epsilon);
  }

  @override
  bool isDone(double timeInSeconds) => timeInSeconds > _durationInSeconds;
}

typedef _DirectionSetter = void Function(_AnimationDirection direction);

class _RepeatingSimulation extends Simulation {
  _RepeatingSimulation(double initialValue, this.min, this.max, this.reverse, Duration period, this.directionSetter)
      : _periodInSeconds = period.inMicroseconds / Duration.microsecondsPerSecond,
        _initialT = (max == min) ? 0.0 : (initialValue / (max - min)) * (period.inMicroseconds / Duration.microsecondsPerSecond) {
    assert(_periodInSeconds > 0.0);
    assert(_initialT >= 0.0);
  }

  final double min;
  final double max;
  final bool reverse;
  final _DirectionSetter directionSetter;

  final double _periodInSeconds;
  final double _initialT;

  @override
  double x(double timeInSeconds) {
    assert(timeInSeconds >= 0.0);

    final double totalTimeInSeconds = timeInSeconds + _initialT;
    final double t = (totalTimeInSeconds / _periodInSeconds) % 1.0;
    final bool _isPlayingReverse = (totalTimeInSeconds ~/ _periodInSeconds) % 2 == 1;

    if (reverse && _isPlayingReverse) {
      directionSetter(_AnimationDirection.reverse);
      return ui.lerpDouble(max, min, t);
    } else {
      directionSetter(_AnimationDirection.forward);
      return ui.lerpDouble(min, max, t);
    }
  }

  @override
  double dx(double timeInSeconds) => (max - min) / _periodInSeconds;

  @override
  bool isDone(double timeInSeconds) => false;
}