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// 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 'package:flutter/foundation.dart';
import 'tween.dart';
// Examples can assume:
// AnimationController _controller;
/// The status of an animation
enum AnimationStatus {
/// The animation is stopped at the beginning
dismissed,
/// The animation is running from beginning to end
forward,
/// The animation is running backwards, from end to beginning
reverse,
/// The animation is stopped at the end
completed,
}
/// Signature for listeners attached using [Animation.addStatusListener].
typedef AnimationStatusListener = void Function(AnimationStatus status);
/// An animation with a value of type `T`.
///
/// An animation consists of a value (of type `T`) together with a status. The
/// status indicates whether the animation is conceptually running from
/// beginning to end or from the end back to the beginning, although the actual
/// value of the animation might not change monotonically (e.g., if the
/// animation uses a curve that bounces).
///
/// Animations also let other objects listen for changes to either their value
/// or their status. These callbacks are called during the "animation" phase of
/// the pipeline, just prior to rebuilding widgets.
///
/// To create a new animation that you can run forward and backward, consider
/// using [AnimationController].
///
/// See also:
///
/// * [Tween], which can be used to create [Animation] subclasses that
/// convert `Animation<double>`s into other kinds of `Animation`s.
abstract class Animation<T> extends Listenable implements ValueListenable<T> {
/// Abstract const constructor. This constructor enables subclasses to provide
/// const constructors so that they can be used in const expressions.
const Animation();
// keep these next five dartdocs in sync with the dartdocs in AnimationWithParentMixin<T>
/// Calls the listener every time the value of the animation changes.
///
/// Listeners can be removed with [removeListener].
@override
void addListener(VoidCallback listener);
/// Stop calling the listener every time the value of the animation changes.
///
/// If `listener` is not currently registered as a listener, this method does
/// nothing.
///
/// Listeners can be added with [addListener].
@override
void removeListener(VoidCallback listener);
/// Calls listener every time the status of the animation changes.
///
/// Listeners can be removed with [removeStatusListener].
void addStatusListener(AnimationStatusListener listener);
/// Stops calling the listener every time the status of the animation changes.
///
/// If `listener` is not currently registered as a status listener, this
/// method does nothing.
///
/// Listeners can be added with [addStatusListener].
void removeStatusListener(AnimationStatusListener listener);
/// The current status of this animation.
AnimationStatus get status;
/// The current value of the animation.
@override
T get value;
/// Whether this animation is stopped at the beginning.
bool get isDismissed => status == AnimationStatus.dismissed;
/// Whether this animation is stopped at the end.
bool get isCompleted => status == AnimationStatus.completed;
/// Chains a [Tween] (or [CurveTween]) to this [Animation].
///
/// This method is only valid for `Animation<double>` instances (i.e. when `T`
/// is `double`). This means, for instance, that it can be called on
/// [AnimationController] objects, as well as [CurvedAnimation]s,
/// [ProxyAnimation]s, [ReverseAnimation]s, [TrainHoppingAnimation]s, etc.
///
/// It returns an [Animation] specialized to the same type, `U`, as the
/// argument to the method (`child`), whose value is derived by applying the
/// given [Tween] to the value of this [Animation].
///
/// {@tool snippet}
///
/// Given an [AnimationController] `_controller`, the following code creates
/// an `Animation<Alignment>` that swings from top left to top right as the
/// controller goes from 0.0 to 1.0:
///
/// ```dart
/// Animation<Alignment> _alignment1 = _controller.drive(
/// AlignmentTween(
/// begin: Alignment.topLeft,
/// end: Alignment.topRight,
/// ),
/// );
/// ```
/// {@end-tool}
/// {@tool snippet}
///
/// The `_alignment.value` could then be used in a widget's build method, for
/// instance, to position a child using an [Align] widget such that the
/// position of the child shifts over time from the top left to the top right.
///
/// It is common to ease this kind of curve, e.g. making the transition slower
/// at the start and faster at the end. The following snippet shows one way to
/// chain the alignment tween in the previous example to an easing curve (in
/// this case, [Curves.easeIn]). In this example, the tween is created
/// elsewhere as a variable that can be reused, since none of its arguments
/// vary.
///
/// ```dart
/// final Animatable<Alignment> _tween = AlignmentTween(begin: Alignment.topLeft, end: Alignment.topRight)
/// .chain(CurveTween(curve: Curves.easeIn));
/// // ...
/// Animation<Alignment> _alignment2 = _controller.drive(_tween);
/// ```
/// {@end-tool}
/// {@tool snippet}
///
/// The following code is exactly equivalent, and is typically clearer when
/// the tweens are created inline, as might be preferred when the tweens have
/// values that depend on other variables:
///
/// ```dart
/// Animation<Alignment> _alignment3 = _controller
/// .drive(CurveTween(curve: Curves.easeIn))
/// .drive(AlignmentTween(
/// begin: Alignment.topLeft,
/// end: Alignment.topRight,
/// ));
/// ```
/// {@end-tool}
///
/// See also:
///
/// * [Animatable.animate], which does the same thing.
/// * [AnimationController], which is usually used to drive animations.
/// * [CurvedAnimation], an alternative to [CurveTween] for applying easing
/// curves, which supports distinct curves in the forward direction and the
/// reverse direction.
@optionalTypeArgs
Animation<U> drive<U>(Animatable<U> child) {
assert(this is Animation<double>);
return child.animate(this as Animation<double>);
}
@override
String toString() {
return '${describeIdentity(this)}(${toStringDetails()})';
}
/// Provides a string describing the status of this object, but not including
/// information about the object itself.
///
/// This function is used by [Animation.toString] so that [Animation]
/// subclasses can provide additional details while ensuring all [Animation]
/// subclasses have a consistent [toString] style.
///
/// The result of this function includes an icon describing the status of this
/// [Animation] object:
///
/// * "&#x25B6;": [AnimationStatus.forward] ([value] increasing)
/// * "&#x25C0;": [AnimationStatus.reverse] ([value] decreasing)
/// * "&#x23ED;": [AnimationStatus.completed] ([value] == 1.0)
/// * "&#x23EE;": [AnimationStatus.dismissed] ([value] == 0.0)
String toStringDetails() {
assert(status != null);
String icon;
switch (status) {
case AnimationStatus.forward:
icon = '\u25B6'; // >
break;
case AnimationStatus.reverse:
icon = '\u25C0'; // <
break;
case AnimationStatus.completed:
icon = '\u23ED'; // >>|
break;
case AnimationStatus.dismissed:
icon = '\u23EE'; // |<<
break;
}
assert(icon != null);
return icon;
}
}