Add test for fade_transition.0.dart API example. (#148178)

This PR contributes to flutter/flutter#130459

### Description
- Adds disposal of the `CurvedAnimation` in `examples/api/test/widgets/transitions/fade_transition.0.dart`
- Adds tests for `examples/api/lib/widgets/transitions/fade_transition.0.dart`

dev/bots/check_code_samples.dart

@@ -404,7 +404,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/transitions/positioned_transition.0_test.dart',
   'examples/api/test/widgets/transitions/sliver_fade_transition.0_test.dart',
   'examples/api/test/widgets/transitions/align_transition.0_test.dart',
-  'examples/api/test/widgets/transitions/fade_transition.0_test.dart',
   'examples/api/test/widgets/transitions/animated_builder.0_test.dart',
   'examples/api/test/widgets/transitions/rotation_transition.0_test.dart',
   'examples/api/test/widgets/transitions/animated_widget.0_test.dart',

examples/api/lib/widgets/transitions/fade_transition.0.dart

@@ -11,35 +11,66 @@ void main() => runApp(const FadeTransitionExampleApp());
 class FadeTransitionExampleApp extends StatelessWidget {
   const FadeTransitionExampleApp({super.key});
 
+  static const Duration duration = Duration(seconds: 2);
+  static const Curve curve = Curves.easeIn;
+
   @override
   Widget build(BuildContext context) {
     return const MaterialApp(
-      home: FadeTransitionExample(),
+      home: FadeTransitionExample(
+        duration: duration,
+        curve: curve,
+      ),
     );
   }
 }
 
 class FadeTransitionExample extends StatefulWidget {
-  const FadeTransitionExample({super.key});
+  const FadeTransitionExample({
+    required this.duration,
+    required this.curve,
+    super.key,
+  });
+
+  final Duration duration;
+
+  final Curve curve;
 
   @override
   State<FadeTransitionExample> createState() => _FadeTransitionExampleState();
 }
 
 /// [AnimationController]s can be created with `vsync: this` because of
 /// [TickerProviderStateMixin].
-class _FadeTransitionExampleState extends State<FadeTransitionExample> with TickerProviderStateMixin {
+class _FadeTransitionExampleState extends State<FadeTransitionExample>
+    with TickerProviderStateMixin {
   late final AnimationController _controller = AnimationController(
-    duration: const Duration(seconds: 2),
+    duration: widget.duration,
     vsync: this,
   )..repeat(reverse: true);
-  late final Animation<double> _animation = CurvedAnimation(
+  late final CurvedAnimation _animation = CurvedAnimation(
     parent: _controller,
-    curve: Curves.easeIn,
+    curve: widget.curve,
   );
 
+  @override
+  void didUpdateWidget(FadeTransitionExample oldWidget) {
+    super.didUpdateWidget(oldWidget);
+
+    if (oldWidget.duration != widget.duration) {
+      _controller
+        ..duration = widget.duration
+        ..repeat(reverse: true);
+    }
+
+    if (oldWidget.curve != widget.curve) {
+      _animation.curve = widget.curve;
+    }
+  }
+
   @override
   void dispose() {
+    _animation.dispose();
     _controller.dispose();
     super.dispose();
   }
@@ -50,7 +81,10 @@ class _FadeTransitionExampleState extends State<FadeTransitionExample> with Tick
       color: Colors.white,
       child: FadeTransition(
         opacity: _animation,
-        child: const Padding(padding: EdgeInsets.all(8), child: FlutterLogo()),
+        child: const Padding(
+          padding: EdgeInsets.all(8),
+          child: FlutterLogo(),
+        ),
       ),
     );
   }

examples/api/test/widgets/transitions/fade_transition.0_test.dart

@@ -0,0 +1,83 @@
+// 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';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/fade_transition.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows FlutterLogo inside a FadeTransition', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FadeTransitionExampleApp(),
+    );
+
+    expect(
+      find.ancestor(
+        of: find.byType(FlutterLogo),
+        matching: find.byType(FadeTransition),
+      ),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('FadeTransition animates', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FadeTransitionExampleApp(),
+    );
+
+    final Finder fadeTransitionFinder = find.ancestor(
+      of: find.byType(FlutterLogo),
+      matching: find.byType(FadeTransition),
+    );
+
+    const double beginOpacity = 0.0;
+    const double endOpacity = 1.0;
+
+    FadeTransition fadeTransition = tester.widget(fadeTransitionFinder);
+    expect(fadeTransition.opacity.value, equals(beginOpacity));
+
+    // Advance animation to the middle.
+    await tester.pump(
+      example.FadeTransitionExampleApp.duration ~/ 2,
+    );
+
+    final double t = example.FadeTransitionExampleApp.curve.transform(0.5);
+
+    fadeTransition = tester.widget(fadeTransitionFinder);
+    expect(
+      fadeTransition.opacity.value,
+      equals(lerpDouble(beginOpacity, endOpacity, t)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(
+      example.FadeTransitionExampleApp.duration ~/ 2,
+    );
+
+    fadeTransition = tester.widget(fadeTransitionFinder);
+    expect(fadeTransition.opacity.value, equals(endOpacity));
+
+    // Advance animation to the middle.
+    await tester.pump(
+      example.FadeTransitionExampleApp.duration ~/ 2,
+    );
+
+    fadeTransition = tester.widget(fadeTransitionFinder);
+    expect(
+      fadeTransition.opacity.value,
+      equals(lerpDouble(beginOpacity, endOpacity, t)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(
+      example.FadeTransitionExampleApp.duration ~/ 2,
+    );
+
+    fadeTransition = tester.widget(fadeTransitionFinder);
+    expect(fadeTransition.opacity.value, equals(beginOpacity));
+  });
+}