A Flutter Animation implementation that wraps a mutable value.

value_animation.dart

import 'package:flutter/widgets.dart';

abstract class ValueAnimation<T> extends Animation<T>
    with
        AnimationEagerListenerMixin,
        AnimationLocalListenersMixin,
        AnimationLocalStatusListenersMixin {
  T _value;

  @override
  T get value => _value;

  set value(T value) {
    if (value == _value) return;
    final status = calculateStatus(_value, value);
    _value = value;
    notifyListeners();

    if (status == _status) return;
    _status = status;
    notifyStatusListeners(status);
  }

  AnimationStatus _status;

  @override
  AnimationStatus get status => _status;

  ValueAnimation(this._value, this._status);

  @protected
  AnimationStatus calculateStatus(T oldValue, T newValue);
}

class DoubleValueAnimation extends ValueAnimation<double> {
  DoubleValueAnimation(double value, AnimationStatus status)
      : assert(value >= 0 && value <= 1),
        assert(status != AnimationStatus.dismissed || value == 0),
        assert(status != AnimationStatus.completed || value == 1),
        super(value, status);

  DoubleValueAnimation.beginning() : this(0, AnimationStatus.dismissed);

  DoubleValueAnimation.end() : this(1, AnimationStatus.completed);

  @override
  set value(double value) {
    assert(value >= 0 && value <= 1);
    super.value = value;
  }

  @override
  AnimationStatus calculateStatus(double oldValue, double newValue) {
    assert(oldValue != newValue,
        'The status should not be calculated for equal values!');
    if (newValue == 0) {
      return AnimationStatus.dismissed;
    }
    if (newValue == 1) {
      return AnimationStatus.completed;
    }
    if (newValue > oldValue) {
      return AnimationStatus.forward;
    } else {
      return AnimationStatus.reverse;
    }
  }
}