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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 'dart:ui' show Color, Rect, Size;
import 'package:flutter/foundation.dart';
import 'animations.dart';
export 'dart:ui' show Color, Rect, Size;
export 'animation.dart' show Animation;
export 'curves.dart' show Curve;
// Examples can assume:
// late Animation<Offset> _animation;
// late AnimationController _controller;
/// A typedef used by [Animatable.fromCallback] to create an [Animatable]
/// from a callback.
typedef AnimatableCallback<T> = T Function(double);
/// An object that can produce a value of type `T` given an [Animation<double>]
/// as input.
///
/// Typically, the values of the input animation are nominally in the range 0.0
/// to 1.0. In principle, however, any value could be provided.
///
/// The main subclass of [Animatable] is [Tween].
abstract class Animatable<T> {
/// Abstract const constructor. This constructor enables subclasses to provide
/// const constructors so that they can be used in const expressions.
const Animatable();
/// Create a new [Animatable] from the provided [callback].
///
/// See also:
///
/// * [Animation.drive], which provides an example for how this can be
/// used.
const factory Animatable.fromCallback(AnimatableCallback<T> callback) = _CallbackAnimatable<T>;
/// Returns the value of the object at point `t`.
///
/// The value of `t` is nominally a fraction in the range 0.0 to 1.0, though
/// in practice it may extend outside this range.
///
/// See also:
///
/// * [evaluate], which is a shorthand for applying [transform] to the value
/// of an [Animation].
/// * [Curve.transform], a similar method for easing curves.
T transform(double t);
/// The current value of this object for the given [Animation].
///
/// This function is implemented by deferring to [transform]. Subclasses that
/// want to provide custom behavior should override [transform], not
/// [evaluate].
///
/// See also:
///
/// * [transform], which is similar but takes a `t` value directly instead of
/// an [Animation].
/// * [animate], which creates an [Animation] out of this object, continually
/// applying [evaluate].
T evaluate(Animation<double> animation) => transform(animation.value);
/// Returns a new [Animation] that is driven by the given animation but that
/// takes on values determined by this object.
///
/// Essentially this returns an [Animation] that automatically applies the
/// [evaluate] method to the parent's value.
///
/// See also:
///
/// * [AnimationController.drive], which does the same thing from the
/// opposite starting point.
Animation<T> animate(Animation<double> parent) {
return _AnimatedEvaluation<T>(parent, this);
}
/// Returns a new [Animatable] whose value is determined by first evaluating
/// the given parent and then evaluating this object.
///
/// This allows [Tween]s to be chained before obtaining an [Animation].
Animatable<T> chain(Animatable<double> parent) {
return _ChainedEvaluation<T>(parent, this);
}
}
// A concrete subclass of `Animatable` used by `Animatable.fromCallback`.
class _CallbackAnimatable<T> extends Animatable<T> {
const _CallbackAnimatable(this._callback);
final AnimatableCallback<T> _callback;
@override
T transform(double t) {
return _callback(t);
}
}
class _AnimatedEvaluation<T> extends Animation<T> with AnimationWithParentMixin<double> {
_AnimatedEvaluation(this.parent, this._evaluatable);
@override
final Animation<double> parent;
final Animatable<T> _evaluatable;
@override
T get value => _evaluatable.evaluate(parent);
@override
String toString() {
return '$parent\u27A9$_evaluatable\u27A9$value';
}
@override
String toStringDetails() {
return '${super.toStringDetails()} $_evaluatable';
}
}
class _ChainedEvaluation<T> extends Animatable<T> {
_ChainedEvaluation(this._parent, this._evaluatable);
final Animatable<double> _parent;
final Animatable<T> _evaluatable;
@override
T transform(double t) {
return _evaluatable.transform(_parent.transform(t));
}
@override
String toString() {
return '$_parent\u27A9$_evaluatable';
}
}
/// A linear interpolation between a beginning and ending value.
///
/// [Tween] is useful if you want to interpolate across a range.
///
/// To use a [Tween] object with an animation, call the [Tween] object's
/// [animate] method and pass it the [Animation] object that you want to
/// modify.
///
/// You can chain [Tween] objects together using the [chain] method, so that a
/// single [Animation] object is configured by multiple [Tween] objects called
/// in succession. This is different than calling the [animate] method twice,
/// which results in two separate [Animation] objects, each configured with a
/// single [Tween].
///
/// {@tool snippet}
///
/// Suppose `_controller` is an [AnimationController], and we want to create an
/// [Animation<Offset>] that is controlled by that controller, and save it in
/// `_animation`. Here are two possible ways of expressing this:
///
/// ```dart
/// _animation = _controller.drive(
/// Tween<Offset>(
/// begin: const Offset(100.0, 50.0),
/// end: const Offset(200.0, 300.0),
/// ),
/// );
/// ```
/// {@end-tool}
/// {@tool snippet}
///
/// ```dart
/// _animation = Tween<Offset>(
/// begin: const Offset(100.0, 50.0),
/// end: const Offset(200.0, 300.0),
/// ).animate(_controller);
/// ```
/// {@end-tool}
///
/// In both cases, the `_animation` variable holds an object that, over the
/// lifetime of the `_controller`'s animation, returns a value
/// (`_animation.value`) that depicts a point along the line between the two
/// offsets above. If we used a [MaterialPointArcTween] instead of a
/// [Tween<Offset>] in the code above, the points would follow a pleasing curve
/// instead of a straight line, with no other changes necessary.
///
/// ## Performance optimizations
///
/// Tweens are mutable; specifically, their [begin] and [end] values can be
/// changed at runtime. An object created with [Animation.drive] using a [Tween]
/// will immediately honor changes to that underlying [Tween] (though the
/// listeners will only be triggered if the [Animation] is actively animating).
/// This can be used to change an animation on the fly without having to
/// recreate all the objects in the chain from the [AnimationController] to the
/// final [Tween].
///
/// If a [Tween]'s values are never changed, however, a further optimization can
/// be applied: the object can be stored in a `static final` variable, so that
/// the exact same instance is used whenever the [Tween] is needed. This is
/// preferable to creating an identical [Tween] afresh each time a [State.build]
/// method is called, for example.
///
/// ## Types with special considerations
///
/// Classes with [lerp] static methods typically have corresponding dedicated
/// [Tween] subclasses that call that method. For example, [ColorTween] uses
/// [Color.lerp] to implement the [ColorTween.lerp] method.
///
/// Types that define `+` and `-` operators to combine values (`T + T → T` and
/// `T - T → T`) and an `*` operator to scale by multiplying with a double (`T *
/// double → T`) can be directly used with `Tween<T>`.
///
/// This does not extend to any type with `+`, `-`, and `*` operators. In
/// particular, [int] does not satisfy this precise contract (`int * double`
/// actually returns [num], not [int]). There are therefore two specific classes
/// that can be used to interpolate integers:
///
/// * [IntTween], which is an approximation of a linear interpolation (using
/// [double.round]).
/// * [StepTween], which uses [double.floor] to ensure that the result is
/// never greater than it would be using if a `Tween<double>`.
///
/// The relevant operators on [Size] also don't fulfill this contract, so
/// [SizeTween] uses [Size.lerp].
///
/// In addition, some of the types that _do_ have suitable `+`, `-`, and `*`
/// operators still have dedicated [Tween] subclasses that perform the
/// interpolation in a more specialized manner. One such class is
/// [MaterialPointArcTween], which is mentioned above. The [AlignmentTween], and
/// [AlignmentGeometryTween], and [FractionalOffsetTween] are another group of
/// [Tween]s that use dedicated `lerp` methods instead of merely relying on the
/// operators (in particular, this allows them to handle null values in a more
/// useful manner).
///
/// ## Nullability
///
/// The [begin] and [end] fields are nullable; a [Tween] does not have to
/// have non-null values specified when it is created.
///
/// If `T` is nullable, then [lerp] and [transform] may return null.
/// This is typically seen in the case where [begin] is null and `t`
/// is 0.0, or [end] is null and `t` is 1.0, or both are null (at any
/// `t` value).
///
/// If `T` is not nullable, then [begin] and [end] must both be set to
/// non-null values before using [lerp] or [transform], otherwise they
/// will throw.
///
/// ## Implementing a Tween
///
/// To specialize this class for a new type, the subclass should implement
/// the [lerp] method (and a constructor). The other methods of this class
/// are all defined in terms of [lerp].
class Tween<T extends Object?> extends Animatable<T> {
/// Creates a tween.
///
/// The [begin] and [end] properties must be non-null before the tween is
/// first used, but the arguments can be null if the values are going to be
/// filled in later.
Tween({
this.begin,
this.end,
});
/// The value this variable has at the beginning of the animation.
///
/// See the constructor for details about whether this property may be null
/// (it varies from subclass to subclass).
T? begin;
/// The value this variable has at the end of the animation.
///
/// See the constructor for details about whether this property may be null
/// (it varies from subclass to subclass).
T? end;
/// Returns the value this variable has at the given animation clock value.
///
/// The default implementation of this method uses the `+`, `-`, and `*`
/// operators on `T`. The [begin] and [end] properties must therefore be
/// non-null by the time this method is called.
///
/// In general, however, it is possible for this to return null, especially
/// when `t`=0.0 and [begin] is null, or `t`=1.0 and [end] is null.
@protected
T lerp(double t) {
assert(begin != null);
assert(end != null);
assert(() {
// Assertions that attempt to catch common cases of tweening types
// that do not conform to the Tween requirements.
dynamic result;
try {
// ignore: avoid_dynamic_calls
result = (begin as dynamic) + ((end as dynamic) - (begin as dynamic)) * t;
result as T;
return true;
} on NoSuchMethodError {
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('Cannot lerp between "$begin" and "$end".'),
ErrorDescription(
'The type ${begin.runtimeType} might not fully implement `+`, `-`, and/or `*`. '
'See "Types with special considerations" at https://api.flutter.dev/flutter/animation/Tween-class.html '
'for more information.',
),
if (begin is Color || end is Color)
ErrorHint('To lerp colors, consider ColorTween instead.')
else if (begin is Rect || end is Rect)
ErrorHint('To lerp rects, consider RectTween instead.')
else
ErrorHint(
'There may be a dedicated "${begin.runtimeType}Tween" for this type, '
'or you may need to create one.',
),
]);
} on TypeError {
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('Cannot lerp between "$begin" and "$end".'),
ErrorDescription(
'The type ${begin.runtimeType} returned a ${result.runtimeType} after '
'multiplication with a double value. '
'See "Types with special considerations" at https://api.flutter.dev/flutter/animation/Tween-class.html '
'for more information.',
),
if (begin is int || end is int)
ErrorHint('To lerp int values, consider IntTween or StepTween instead.')
else
ErrorHint(
'There may be a dedicated "${begin.runtimeType}Tween" for this type, '
'or you may need to create one.',
),
]);
}
}());
// ignore: avoid_dynamic_calls
return (begin as dynamic) + ((end as dynamic) - (begin as dynamic)) * t as T;
}
/// Returns the interpolated value for the current value of the given animation.
///
/// This method returns `begin` and `end` when the animation values are 0.0 or
/// 1.0, respectively.
///
/// This function is implemented by deferring to [lerp]. Subclasses that want
/// to provide custom behavior should override [lerp], not [transform] (nor
/// [evaluate]).
///
/// See the constructor for details about whether the [begin] and [end]
/// properties may be null when this is called. It varies from subclass to
/// subclass.
@override
T transform(double t) {
if (t == 0.0) {
return begin as T;
}
if (t == 1.0) {
return end as T;
}
return lerp(t);
}
@override
String toString() => '${objectRuntimeType(this, 'Animatable')}($begin \u2192 $end)';
}
/// A [Tween] that evaluates its [parent] in reverse.
class ReverseTween<T extends Object?> extends Tween<T> {
/// Construct a [Tween] that evaluates its [parent] in reverse.
ReverseTween(this.parent)
: super(begin: parent.end, end: parent.begin);
/// This tween's value is the same as the parent's value evaluated in reverse.
///
/// This tween's [begin] is the parent's [end] and its [end] is the parent's
/// [begin]. The [lerp] method returns `parent.lerp(1.0 - t)` and its
/// [evaluate] method is similar.
final Tween<T> parent;
@override
T lerp(double t) => parent.lerp(1.0 - t);
}
/// An interpolation between two colors.
///
/// This class specializes the interpolation of [Tween<Color>] to use
/// [Color.lerp].
///
/// The values can be null, representing no color (which is distinct to
/// transparent black, as represented by [Colors.transparent]).
///
/// See [Tween] for a discussion on how to use interpolation objects.
class ColorTween extends Tween<Color?> {
/// Creates a [Color] tween.
///
/// The [begin] and [end] properties may be null; the null value
/// is treated as transparent.
///
/// We recommend that you do not pass [Colors.transparent] as [begin]
/// or [end] if you want the effect of fading in or out of transparent.
/// Instead prefer null. [Colors.transparent] refers to black transparent and
/// thus will fade out of or into black which is likely unwanted.
ColorTween({ super.begin, super.end });
/// Returns the value this variable has at the given animation clock value.
@override
Color? lerp(double t) => Color.lerp(begin, end, t);
}
/// An interpolation between two sizes.
///
/// This class specializes the interpolation of [Tween<Size>] to use
/// [Size.lerp].
///
/// The values can be null, representing [Size.zero].
///
/// See [Tween] for a discussion on how to use interpolation objects.
class SizeTween extends Tween<Size?> {
/// Creates a [Size] tween.
///
/// The [begin] and [end] properties may be null; the null value
/// is treated as an empty size.
SizeTween({ super.begin, super.end });
/// Returns the value this variable has at the given animation clock value.
@override
Size? lerp(double t) => Size.lerp(begin, end, t);
}
/// An interpolation between two rectangles.
///
/// This class specializes the interpolation of [Tween<Rect>] to use
/// [Rect.lerp].
///
/// The values can be null, representing a zero-sized rectangle at the
/// origin ([Rect.zero]).
///
/// See [Tween] for a discussion on how to use interpolation objects.
class RectTween extends Tween<Rect?> {
/// Creates a [Rect] tween.
///
/// The [begin] and [end] properties may be null; the null value
/// is treated as an empty rect at the top left corner.
RectTween({ super.begin, super.end });
/// Returns the value this variable has at the given animation clock value.
@override
Rect? lerp(double t) => Rect.lerp(begin, end, t);
}
/// An interpolation between two integers that rounds.
///
/// This class specializes the interpolation of [Tween<int>] to be
/// appropriate for integers by interpolating between the given begin
/// and end values and then rounding the result to the nearest
/// integer.
///
/// This is the closest approximation to a linear tween that is possible with an
/// integer. Compare to [StepTween] and [Tween<double>].
///
/// The [begin] and [end] values must be set to non-null values before
/// calling [lerp] or [transform].
///
/// See [Tween] for a discussion on how to use interpolation objects.
class IntTween extends Tween<int> {
/// Creates an int tween.
///
/// The [begin] and [end] properties must be non-null before the tween is
/// first used, but the arguments can be null if the values are going to be
/// filled in later.
IntTween({ super.begin, super.end });
// The inherited lerp() function doesn't work with ints because it multiplies
// the begin and end types by a double, and int * double returns a double.
@override
int lerp(double t) => (begin! + (end! - begin!) * t).round();
}
/// An interpolation between two integers that floors.
///
/// This class specializes the interpolation of [Tween<int>] to be
/// appropriate for integers by interpolating between the given begin
/// and end values and then using [double.floor] to return the current
/// integer component, dropping the fractional component.
///
/// This results in a value that is never greater than the equivalent
/// value from a linear double interpolation. Compare to [IntTween].
///
/// The [begin] and [end] values must be set to non-null values before
/// calling [lerp] or [transform].
///
/// See [Tween] for a discussion on how to use interpolation objects.
class StepTween extends Tween<int> {
/// Creates an [int] tween that floors.
///
/// The [begin] and [end] properties must be non-null before the tween is
/// first used, but the arguments can be null if the values are going to be
/// filled in later.
StepTween({ super.begin, super.end });
// The inherited lerp() function doesn't work with ints because it multiplies
// the begin and end types by a double, and int * double returns a double.
@override
int lerp(double t) => (begin! + (end! - begin!) * t).floor();
}
/// A tween with a constant value.
class ConstantTween<T> extends Tween<T> {
/// Create a tween whose [begin] and [end] values equal [value].
ConstantTween(T value) : super(begin: value, end: value);
/// This tween doesn't interpolate, it always returns the same value.
@override
T lerp(double t) => begin as T;
@override
String toString() => '${objectRuntimeType(this, 'ConstantTween')}(value: $begin)';
}
/// Transforms the value of the given animation by the given curve.
///
/// This class differs from [CurvedAnimation] in that [CurvedAnimation] applies
/// a curve to an existing [Animation] object whereas [CurveTween] can be
/// chained with another [Tween] prior to receiving the underlying [Animation].
/// ([CurvedAnimation] also has the additional ability of having different
/// curves when the animation is going forward vs when it is going backward,
/// which can be useful in some scenarios.)
///
/// {@tool snippet}
///
/// The following code snippet shows how you can apply a curve to a linear
/// animation produced by an [AnimationController] `controller`:
///
/// ```dart
/// final Animation<double> animation = _controller.drive(
/// CurveTween(curve: Curves.ease),
/// );
/// ```
/// {@end-tool}
///
/// See also:
///
/// * [CurvedAnimation], for an alternative way of expressing the sample above.
/// * [AnimationController], for examples of creating and disposing of an
/// [AnimationController].
class CurveTween extends Animatable<double> {
/// Creates a curve tween.
///
/// The [curve] argument must not be null.
CurveTween({ required this.curve });
/// The curve to use when transforming the value of the animation.
Curve curve;
@override
double transform(double t) {
if (t == 0.0 || t == 1.0) {
assert(curve.transform(t).round() == t);
return t;
}
return curve.transform(t);
}
@override
String toString() => '${objectRuntimeType(this, 'CurveTween')}(curve: $curve)';
}