[framework] create animation from value listenable (#108644)

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

@@ -13,6 +13,7 @@ export 'tween.dart' show Animatable;
 
 // Examples can assume:
 // late AnimationController _controller;
+// late ValueNotifier<double> _scrollPosition;
 
 /// The status of an animation.
 enum AnimationStatus {
@@ -32,6 +33,9 @@ enum AnimationStatus {
 /// Signature for listeners attached using [Animation.addStatusListener].
 typedef AnimationStatusListener = void Function(AnimationStatus status);
 
+/// Signature for method used to transform values in [Animation.fromValueListenable].
+typedef ValueListenableTransformer<T> = T Function(T);
+
 /// An animation with a value of type `T`.
 ///
 /// An animation consists of a value (of type `T`) together with a status. The
@@ -56,6 +60,58 @@ abstract class Animation<T> extends Listenable implements ValueListenable<T> {
   /// const constructors so that they can be used in const expressions.
   const Animation();
 
+  /// Create a new animation from a [ValueListenable].
+  ///
+  /// The returned animation will always have an animations status of
+  /// [AnimationStatus.forward]. The value of the provided listenable can
+  /// be optionally transformed using the [transformer] function.
+  ///
+  /// {@tool snippet}
+  ///
+  /// This constructor can be used to replace instances of [ValueListenableBuilder]
+  /// widgets with a corresponding animated widget, like a [FadeTransition].
+  ///
+  /// Before:
+  ///
+  /// ```dart
+  /// Widget build(BuildContext context) {
+  ///   return ValueListenableBuilder<double>(
+  ///     valueListenable: _scrollPosition,
+  ///     builder: (BuildContext context, double value, Widget? child) {
+  ///       final double opacity = (value / 1000).clamp(0, 1);
+  ///       return Opacity(opacity: opacity, child: child);
+  ///     },
+  ///     child: Container(
+  ///       color: Colors.red,
+  ///       child: const Text('Hello, Animation'),
+  ///     ),
+  ///   );
+  /// }
+  /// ```
+  ///
+  /// {@end-tool}
+  /// {@tool snippet}
+  ///
+  /// After:
+  ///
+  /// ```dart
+  /// Widget build2(BuildContext context) {
+  ///   return FadeTransition(
+  ///     opacity: Animation<double>.fromValueListenable(_scrollPosition, transformer: (double value) {
+  ///       return (value / 1000).clamp(0, 1);
+  ///     }),
+  ///     child: Container(
+  ///       color: Colors.red,
+  ///       child: const Text('Hello, Animation'),
+  ///     ),
+  ///   );
+  /// }
+  /// ```
+  /// {@end-tool}
+  factory Animation.fromValueListenable(ValueListenable<T> listenable, {
+    ValueListenableTransformer<T>? transformer,
+  }) = _ValueListenableDelegateAnimation<T>;
+
   // keep these next five dartdocs in sync with the dartdocs in AnimationWithParentMixin<T>
 
   /// Calls the listener every time the value of the animation changes.
@@ -207,3 +263,38 @@ abstract class Animation<T> extends Listenable implements ValueListenable<T> {
     }
   }
 }
+
+// An implementation of an animation that delegates to a value listenable with a fixed direction.
+class _ValueListenableDelegateAnimation<T> extends Animation<T> {
+  _ValueListenableDelegateAnimation(this._listenable, { ValueListenableTransformer<T>? transformer })
+    : _transformer = transformer;
+
+  final ValueListenable<T> _listenable;
+  final ValueListenableTransformer<T>? _transformer;
+
+  @override
+  void addListener(VoidCallback listener) {
+    _listenable.addListener(listener);
+  }
+
+  @override
+  void addStatusListener(AnimationStatusListener listener) {
+    // status will never change.
+  }
+
+  @override
+  void removeListener(VoidCallback listener) {
+    _listenable.removeListener(listener);
+  }
+
+  @override
+  void removeStatusListener(AnimationStatusListener listener) {
+    // status will never change.
+  }
+
+  @override
+  AnimationStatus get status => AnimationStatus.forward;
+
+  @override
+  T get value => _transformer?.call(_listenable.value) ?? _listenable.value;
+}

packages/flutter/test/animation/animation_from_listener_test.dart

@@ -0,0 +1,51 @@
+// 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/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('Animation created from ValueListenable', () {
+    final ValueNotifier<double> listenable = ValueNotifier<double>(0.0);
+    final Animation<double> animation = Animation<double>.fromValueListenable(listenable);
+
+    expect(animation.status, AnimationStatus.forward);
+    expect(animation.value, 0.0);
+
+    listenable.value = 1.0;
+
+    expect(animation.value, 1.0);
+
+    bool listenerCalled = false;
+    void listener() {
+      listenerCalled = true;
+    }
+
+    animation.addListener(listener);
+
+    listenable.value = 0.5;
+
+    expect(listenerCalled, true);
+    listenerCalled = false;
+
+    animation.removeListener(listener);
+
+    listenable.value = 0.2;
+    expect(listenerCalled, false);
+  });
+
+  test('Animation created from ValueListenable can transform value', () {
+    final ValueNotifier<double> listenable = ValueNotifier<double>(0.0);
+    final Animation<double> animation = Animation<double>.fromValueListenable(listenable, transformer: (double input) {
+      return input / 10;
+    });
+
+    expect(animation.status, AnimationStatus.forward);
+    expect(animation.value, 0.0);
+
+    listenable.value = 10.0;
+
+    expect(animation.value, 1.0);
+  });
+}