Test remaining transitions api examples (#148302)

Adds tests for `relative_positioned_transition`, `positioned_transition`, `sliver_fade_transition`, `align_transition`, `animated_builder`, `rotation_transition`, `animated_widget`, `slide_transition`, `listenable_builder`, `scale_transition`, `default_text_style_transition`, `decorated_box_transition`, `size_transition` api examples. Makes double type in the `align_transition` example explicit.

A test for `fade_transition` is already in currently open #148178.

Part of #130459.

dev/bots/check_code_samples.dart

@@ -380,19 +380,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/media_query/media_query_data.system_gesture_insets.0_test.dart',
   'examples/api/test/widgets/async/future_builder.0_test.dart',
   'examples/api/test/widgets/animated_switcher/animated_switcher.0_test.dart',
-  'examples/api/test/widgets/transitions/relative_positioned_transition.0_test.dart',
-  '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/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/test/widgets/transitions/slide_transition.0_test.dart',
-  'examples/api/test/widgets/transitions/listenable_builder.2_test.dart',
-  'examples/api/test/widgets/transitions/scale_transition.0_test.dart',
-  'examples/api/test/widgets/transitions/default_text_style_transition.0_test.dart',
-  'examples/api/test/widgets/transitions/decorated_box_transition.0_test.dart',
-  'examples/api/test/widgets/transitions/size_transition.0_test.dart',
   'examples/api/test/widgets/animated_list/animated_list.0_test.dart',
   'examples/api/test/widgets/focus_traversal/focus_traversal_group.0_test.dart',
   'examples/api/test/widgets/focus_traversal/ordered_traversal_policy.0_test.dart',

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

@@ -58,7 +58,7 @@ class _AlignTransitionExampleState extends State<AlignTransitionExample> with Ti
       child: AlignTransition(
         alignment: _animation,
         child: const Padding(
-          padding: EdgeInsets.all(8),
+          padding: EdgeInsets.all(8.0),
           child: FlutterLogo(size: 150.0),
         ),
       ),

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

@@ -0,0 +1,49 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/align_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.AlignTransitionExampleApp());
+    expect(find.byType(ColoredBox), findsOneWidget);
+    expect(
+      find.byWidgetPredicate((Widget padding) => padding is Padding
+        && padding.padding == const EdgeInsets.all(8.0)),
+      findsOneWidget,
+    );
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(AlignTransition), findsOneWidget);
+  });
+
+  testWidgets('Animates repeatedly every 2 seconds', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.AlignTransitionExampleApp());
+    final Finder paddingFinder = find.byWidgetPredicate(
+      (Widget padding) => padding is Padding
+        && padding.padding == const EdgeInsets.all(8.0));
+
+    expect(
+      tester.getBottomLeft(paddingFinder),
+      tester.getBottomLeft(find.byType(AlignTransition)),
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(
+      tester.getCenter(paddingFinder),
+      tester.getCenter(find.byType(AlignTransition)),
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(
+      tester.getBottomLeft(paddingFinder),
+      tester.getBottomLeft(find.byType(AlignTransition)),
+    );
+  });
+}

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

@@ -0,0 +1,30 @@
+// 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:math' as math;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/animated_builder.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Rotates text and container', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.AnimatedBuilderExampleApp());
+    expect(find.text('Whee!'), findsOneWidget);
+    expect(find.byType(Container), findsOneWidget);
+    expect(tester.widget(find.byType(Container)), isA<Container>()
+      .having((Container container) => container.color, 'color', Colors.green));
+
+    expect(find.byWidgetPredicate((Widget widget) => widget is Transform
+        && widget.transform == Transform.rotate(angle: 0.0).transform),
+      findsOneWidget);
+
+    await tester.pump(const Duration(seconds: 5));
+    await tester.pump();
+
+    expect(find.byWidgetPredicate((Widget widget) => widget is Transform
+        && widget.transform == Transform.rotate(angle: math.pi).transform),
+      findsOneWidget);
+  });
+}

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

@@ -0,0 +1,29 @@
+// 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:math' as math;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/animated_widget.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Rotates green container', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.AnimatedWidgetExampleApp());
+    expect(find.byType(Container), findsOneWidget);
+    expect(tester.widget(find.byType(Container)), isA<Container>()
+      .having((Container container) => container.color, 'color', Colors.green));
+
+    expect(find.byWidgetPredicate((Widget widget) => widget is Transform
+        && widget.transform == Transform.rotate(angle: 0.0).transform),
+      findsOneWidget);
+
+    await tester.pump(const Duration(seconds: 5));
+    await tester.pump();
+
+    expect(find.byWidgetPredicate((Widget widget) => widget is Transform
+        && widget.transform == Transform.rotate(angle: math.pi).transform),
+      findsOneWidget);
+  });
+}

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

@@ -0,0 +1,66 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/decorated_box_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows container in 3 second loop', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.DecoratedBoxTransitionExampleApp());
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Center), findsOneWidget);
+    expect(find.descendant(
+      of: find.byType(Center),
+      matching: find.byType(FlutterLogo)
+    ), findsOneWidget);
+    expect(find.ancestor(
+      of: find.byType(FlutterLogo),
+      matching: find.byType(Container)
+    ), findsAtLeast(1));
+    expect(find.byType(DecoratedBoxTransition), findsOneWidget);
+
+    expect(
+      tester.widget(find.byType(DecoratedBoxTransition)),
+      isA<DecoratedBoxTransition>()
+        .having(
+          (DecoratedBoxTransition transition) => transition.decoration.value,
+          'decoration',
+          BoxDecoration(
+            color: const Color(0xFFFFFFFF),
+            border: Border.all(style: BorderStyle.none),
+            borderRadius: BorderRadius.circular(60.0),
+            boxShadow: const <BoxShadow>[
+              BoxShadow(
+                color: Color(0x66666666),
+                blurRadius: 10.0,
+                spreadRadius: 3.0,
+                offset: Offset(0, 6.0),
+              ),
+            ],
+          ),
+        ),
+    );
+
+    await tester.pump(const Duration(seconds: 3));
+    await tester.pump();
+
+    expect(
+      tester.widget(find.byType(DecoratedBoxTransition)),
+      isA<DecoratedBoxTransition>()
+        .having(
+          (DecoratedBoxTransition transition) => transition.decoration.value,
+          'decoration',
+          BoxDecoration(
+            color: const Color(0xFFFFFFFF),
+            border: Border.all(
+              style: BorderStyle.none,
+            ),
+            borderRadius: BorderRadius.zero,
+            // No shadow.
+          ),
+        ),
+    );
+  });
+}

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

@@ -0,0 +1,56 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/default_text_style_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Transforms text style periodically', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.DefaultTextStyleTransitionExampleApp());
+    expect(find.byType(Center), findsOneWidget);
+    expect(find.byType(Text), findsOneWidget);
+    expect(find.text('Flutter'), findsOneWidget);
+    expect(
+      find.descendant(
+        of: find.byType(Center),
+        matching: find.byType(Text)
+      ),
+      findsOneWidget,
+    );
+    expect(find.byType(DefaultTextStyleTransition), findsOneWidget);
+    expect(tester.widget(find.byType(DefaultTextStyleTransition)),
+      isA<DefaultTextStyleTransition>()
+        .having(
+          (DefaultTextStyleTransition transition) => transition.style.value,
+          'style',
+          const TextStyle(fontSize: 50, color: Colors.blue, fontWeight: FontWeight.w900),
+        ),
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.widget(find.byType(DefaultTextStyleTransition)),
+      isA<DefaultTextStyleTransition>()
+        .having(
+          (DefaultTextStyleTransition transition) => transition.style.value,
+          'style',
+          const TextStyle(fontSize: 50, color: Colors.red, fontWeight: FontWeight.w100),
+        ),
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.widget(find.byType(DefaultTextStyleTransition)),
+      isA<DefaultTextStyleTransition>()
+        .having(
+          (DefaultTextStyleTransition transition) => transition.style.value,
+          'style',
+          const TextStyle(fontSize: 50, color: Colors.blue, fontWeight: FontWeight.w900),
+        ),
+    );
+  });
+}

examples/api/test/widgets/transitions/listenable_builder.2_test.dart

@@ -0,0 +1,34 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/listenable_builder.2.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Increments counter', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.ListenableBuilderExample());
+    expect(find.text('ListenableBuilder Example'), findsOneWidget);
+    expect(find.text('Current counter value:'), findsOneWidget);
+    expect(find.byIcon(Icons.add), findsOneWidget);
+    expect(find.descendant(
+      of: find.byType(FloatingActionButton),
+      matching: find.byIcon(Icons.add)
+    ), findsOneWidget);
+
+
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsNothing);
+    await tester.tap(find.byIcon(Icons.add));
+    await tester.pump();
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsOneWidget);
+
+    for (int i = 0; i < 4; i++) {
+      await tester.tap(find.byIcon(Icons.add));
+    }
+    await tester.pump();
+    expect(find.text('5'), findsOneWidget);
+  });
+}

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

@@ -0,0 +1,42 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/positioned_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.PositionedTransitionExampleApp());
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Padding), findsAtLeast(1));
+    expect(find.byType(PositionedTransition), findsOneWidget);
+  });
+
+  testWidgets('Animates repeatedly every 2 seconds', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.PositionedTransitionExampleApp());
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(100.0 - 2.0 * 8.0, 100.0 - 2.0 * 8.0));
+    expect(tester.getTopLeft(find.byType(FlutterLogo)), const Offset(8.0, 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    final Size canvasSize = tester.getSize(find.byType(LayoutBuilder));
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(200.0 - 2.0 * 8.0, 200.0 - 2.0 * 8.0));
+    expect(tester.getBottomRight(find.byType(FlutterLogo)), Offset(canvasSize.width - 8.0, canvasSize.height - 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(100.0 - 2.0 * 8.0, 100.0 - 2.0 * 8.0));
+    expect(tester.getTopLeft(find.byType(FlutterLogo)), const Offset(8.0, 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(200.0 - 2.0 * 8.0, 200.0 - 2.0 * 8.0));
+    expect(tester.getBottomRight(find.byType(FlutterLogo)), Offset(canvasSize.width - 8.0, canvasSize.height - 8.0));
+  });
+}

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

@@ -0,0 +1,42 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/relative_positioned_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.RelativePositionedTransitionExampleApp());
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Padding), findsAtLeast(1));
+    expect(find.byType(RelativePositionedTransition), findsOneWidget);
+  });
+
+  testWidgets('Animates repeatedly every 2 seconds', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.RelativePositionedTransitionExampleApp());
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(200.0 - 2.0 * 8.0, 200.0 - 2.0 * 8.0));
+    expect(tester.getTopLeft(find.byType(FlutterLogo)), const Offset(8.0, 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    final Size canvasSize = tester.getSize(find.byType(LayoutBuilder));
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(100.0 - 2.0 * 8.0, 100.0 - 2.0 * 8.0));
+    expect(tester.getBottomRight(find.byType(FlutterLogo)), Offset(canvasSize.width - 8.0, canvasSize.height - 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(200.0 - 2.0 * 8.0, 200.0 - 2.0 * 8.0));
+    expect(tester.getTopLeft(find.byType(FlutterLogo)), const Offset(8.0, 8.0));
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(tester.getSize(find.byType(FlutterLogo)), const Size(100.0 - 2.0 * 8.0, 100.0 - 2.0 * 8.0));
+    expect(tester.getBottomRight(find.byType(FlutterLogo)), Offset(canvasSize.width - 8.0, canvasSize.height - 8.0));
+  });
+}

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

@@ -0,0 +1,46 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/rotation_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in rotation transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.RotationTransitionExampleApp());
+    expect(
+      find.byWidgetPredicate((Widget widget) => widget is RotationTransition
+        && widget.turns is CurvedAnimation
+        && (widget.turns as CurvedAnimation).curve == Curves.elasticOut,
+      ), findsOneWidget);
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Padding), findsAtLeast(1));
+    expect(
+      find.descendant(
+        of: find.byType(Center),
+        matching: find.byType(FlutterLogo)
+      ),
+      findsOneWidget,
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) => widget is RotationTransition
+        && widget.turns is CurvedAnimation
+        && widget.turns.value == 0.0
+        && widget.turns.status == AnimationStatus.forward
+      ), findsOneWidget);
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 3));
+    await tester.pump();
+
+    expect(
+      find.byWidgetPredicate((Widget widget) => widget is RotationTransition
+        && widget.turns is CurvedAnimation
+        && (widget.turns as CurvedAnimation).parent is AnimationController
+        && ((widget.turns as CurvedAnimation).parent as AnimationController).value == 0.5
+        && widget.turns.status == AnimationStatus.reverse
+      ), findsOneWidget);
+  });
+}

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

@@ -0,0 +1,47 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/scale_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.ScaleTransitionExampleApp());
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Center), findsOneWidget);
+    expect(find.descendant(
+      of: find.byType(Center),
+      matching: find.byType(ScaleTransition)
+    ), findsOneWidget);
+    expect(
+      find.byWidgetPredicate((Widget widget) => widget is ScaleTransition
+        && widget.scale is CurvedAnimation
+        && (widget.scale as CurvedAnimation).curve == Curves.fastOutSlowIn,
+      ), findsOneWidget);
+  });
+
+  testWidgets('Scales every 2 seconds', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.ScaleTransitionExampleApp());
+    final Finder transformFinder = find.descendant(
+      of: find.byType(Center),
+      matching: find.byType(Transform)
+    );
+
+    Transform transform = tester.widget(transformFinder);
+    expect(transform.transform[0], 0);
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    transform = tester.widget(transformFinder);
+    expect(transform.transform[0], 1.0);
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    transform = tester.widget(transformFinder);
+    expect(transform.transform[0], 0);
+  });
+}

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

@@ -0,0 +1,32 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/size_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.SizeTransitionExampleApp());
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Center), findsOneWidget);
+    expect(find.descendant(
+      of: find.byType(Center),
+      matching: find.byType(FlutterLogo)
+    ), findsOneWidget);
+    expect(find.byType(SizeTransition), findsOneWidget);
+
+    expect(
+      tester.widget(find.byType(SizeTransition)),
+      isA<SizeTransition>()
+        .having((SizeTransition transition) => transition.axis, 'axis', Axis.horizontal)
+        .having((SizeTransition transition) => transition.axisAlignment, 'axis alignment', -1)
+        .having((SizeTransition transition) => transition.sizeFactor, 'factor', isA<CurvedAnimation>()
+        .having((CurvedAnimation animation) => animation.curve, 'curve', Curves.fastOutSlowIn)
+        .having((CurvedAnimation animation) => animation.parent, 'paren', isA<AnimationController>()
+        .having((AnimationController controller) => controller.duration, 'duration', const Duration(seconds: 3)))
+      )
+    );
+  });
+}

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

@@ -0,0 +1,44 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/transitions/slide_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows flutter logo in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.SlideTransitionExampleApp());
+    expect(find.text('SlideTransition Sample'), findsOneWidget);
+    expect(find.byType(Center), findsOneWidget);
+    expect(find.byType(FlutterLogo), findsOneWidget);
+    expect(find.byType(Padding), findsAtLeast(1));
+    expect(find.byType(SlideTransition), findsOneWidget);
+  });
+
+  testWidgets('Animates repeatedly every 2 seconds', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.SlideTransitionExampleApp());
+
+    expect(
+      tester.getCenter(find.byType(FlutterLogo)),
+      tester.getCenter(find.byType(Center)),
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    final double width = tester.getSize(find.byType(FlutterLogo)).width + 2 * 8.0;
+    expect(
+      tester.getCenter(find.byType(FlutterLogo)).dx,
+      tester.getCenter(find.byType(Center)).dx + 1.5 * width,
+    );
+
+    await tester.pump(const Duration(seconds: 2));
+    await tester.pump();
+
+    expect(
+      tester.getCenter(find.byType(FlutterLogo)),
+      tester.getCenter(find.byType(Center)),
+    );
+  });
+}

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

@@ -0,0 +1,44 @@
+// 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/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_api_samples/widgets/transitions/sliver_fade_transition.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Shows color list in transition', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.SliverFadeTransitionExampleApp());
+    expect(find.text('SliverFadeTransition Sample'), findsOneWidget);
+    expect(find.byType(SliverFadeTransition), findsOneWidget);
+    expect(find.byType(CustomScrollView), findsOneWidget);
+    expect(find.byWidgetPredicate(
+      (Widget widget) => widget is Container &&
+        widget.color == Colors.indigo[200]
+    ), findsNWidgets(3));
+    expect(find.byWidgetPredicate(
+      (Widget widget) => widget is Container &&
+        widget.color == Colors.orange[200]
+    ), findsNWidgets(2));
+  });
+
+  testWidgets('Animates repeatedly every second', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.SliverFadeTransitionExampleApp());
+
+    expect(
+      tester.renderObject(find.byType(SliverFadeTransition)),
+      isA<RenderSliverAnimatedOpacity>()
+        .having((RenderSliverAnimatedOpacity obj) => obj.opacity.value, 'opacity', 0.0)
+    );
+
+    await tester.pump(const Duration(seconds: 1));
+    await tester.pump();
+
+    expect(
+      tester.renderObject(find.byType(SliverFadeTransition)),
+      isA<RenderSliverAnimatedOpacity>()
+        .having((RenderSliverAnimatedOpacity obj) => obj.opacity.value, 'opacity', 1.0)
+    );
+  });
+}