Add test for tab_controller.1.dart API example. (#148189)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Updates `examples/api/lib/material/tab_controller/tab_controller.1.dart` to properly remove the listener from the `TabController`
- Adds tests for `examples/api/lib/material/tab_controller/tab_controller.1.dart`

dev/bots/check_code_samples.dart

@@ -346,7 +346,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.2_test.dart',
   'examples/api/test/material/about/about_list_tile.0_test.dart',
-  'examples/api/test/material/tab_controller/tab_controller.1_test.dart',
   'examples/api/test/material/selection_area/selection_area.0_test.dart',
   'examples/api/test/material/scaffold/scaffold.end_drawer.0_test.dart',
   'examples/api/test/material/scaffold/scaffold.drawer.0_test.dart',

examples/api/lib/material/tab_controller/tab_controller.1.dart

@@ -11,42 +11,39 @@ void main() => runApp(const TabControllerExampleApp());
 class TabControllerExampleApp extends StatelessWidget {
   const TabControllerExampleApp({super.key});
 
-  @override
+  static const List<Tab> tabs = <Tab>[
-  Widget build(BuildContext context) {
-    return const MaterialApp(
-      home: TabControllerExample(),
-    );
-  }
-}
-
-const List<Tab> tabs = <Tab>[
     Tab(text: 'Zeroth'),
     Tab(text: 'First'),
     Tab(text: 'Second'),
   ];
 
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: TabControllerExample(tabs: tabs),
+    );
+  }
+}
+
 class TabControllerExample extends StatelessWidget {
-  const TabControllerExample({super.key});
+  const TabControllerExample({
+    required this.tabs,
+    super.key,
+  });
+
+  final List<Tab> tabs;
 
   @override
   Widget build(BuildContext context) {
     return DefaultTabController(
       length: tabs.length,
-      // The Builder widget is used to have a different BuildContext to access
+      child: DefaultTabControllerListener(
-      // closest DefaultTabController.
+        onTabChanged: (int index) {
-      child: Builder(builder: (BuildContext context) {
+          debugPrint('tab changed: $index');
-        final TabController tabController = DefaultTabController.of(context);
+        },
-        tabController.addListener(() {
+        child: Scaffold(
-          if (!tabController.indexIsChanging) {
-            // Your code goes here.
-            // To get index of current tab use tabController.index
-          }
-        });
-        return Scaffold(
           appBar: AppBar(
-            bottom: const TabBar(
+            bottom: TabBar(tabs: tabs),
-              tabs: tabs,
-            ),
           ),
           body: TabBarView(
             children: tabs.map((Tab tab) {
@@ -58,8 +55,75 @@ class TabControllerExample extends StatelessWidget {
               );
             }).toList(),
           ),
-        );
+        ),
-      }),
+      ),
     );
   }
 }
+
+class DefaultTabControllerListener extends StatefulWidget {
+  const DefaultTabControllerListener({
+    required this.onTabChanged,
+    required this.child,
+    super.key,
+  });
+
+  final ValueChanged<int> onTabChanged;
+
+  final Widget child;
+
+  @override
+  State<DefaultTabControllerListener> createState() =>
+      _DefaultTabControllerListenerState();
+}
+
+class _DefaultTabControllerListenerState
+    extends State<DefaultTabControllerListener> {
+  TabController? _controller;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    final TabController? defaultTabController =
+        DefaultTabController.maybeOf(context);
+
+    assert(() {
+      if (defaultTabController == null) {
+        throw FlutterError(
+          'No DefaultTabController for ${widget.runtimeType}.\n'
+          'When creating a ${widget.runtimeType}, you must ensure that there '
+          'is a DefaultTabController above the ${widget.runtimeType}.',
+        );
+      }
+      return true;
+    }());
+
+    if (defaultTabController != _controller) {
+      _controller?.removeListener(_listener);
+      _controller = defaultTabController;
+      _controller?.addListener(_listener);
+    }
+  }
+
+  void _listener() {
+    final TabController? controller = _controller;
+
+    if (controller == null || controller.indexIsChanging) {
+      return;
+    }
+
+    widget.onTabChanged(controller.index);
+  }
+
+  @override
+  void dispose() {
+    _controller?.removeListener(_listener);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.child;
+  }
+}

examples/api/test/material/tab_controller/tab_controller.1_test.dart

@@ -0,0 +1,96 @@
+// 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/tab_controller/tab_controller.1.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Verify first tab is selected by default', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TabControllerExampleApp(),
+    );
+
+    final Tab firstTab = example.TabControllerExampleApp.tabs.first;
+
+    expect(
+      find.descendant(
+        of: find.byType(TabBarView),
+        matching: find.text('${firstTab.text} Tab'),
+      ),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('Verify tabs can be changed', (WidgetTester tester) async {
+    final List<String?> log = <String?>[];
+
+    final DebugPrintCallback originalDebugPrint = debugPrint;
+    debugPrint = (String? message, {int? wrapWidth}) {
+      log.add(message);
+    };
+
+    await tester.pumpWidget(
+      const example.TabControllerExampleApp(),
+    );
+
+    const List<Tab> tabs = example.TabControllerExampleApp.tabs;
+    final List<Tab> tabsTraversalOrder = <Tab>[];
+
+    // The traverse order is from the second tab from the start to the last,
+    // and then from the second tab from the end to the first.
+    tabsTraversalOrder.addAll(tabs.skip(1));
+    tabsTraversalOrder.addAll(tabs.reversed.skip(1));
+
+    for (final Tab tab in tabsTraversalOrder) {
+      // Tap on the TabBar's tab to select it.
+      await tester.tap(find.descendant(
+        of: find.byType(TabBar),
+        matching: find.text(tab.text!),
+      ));
+      await tester.pumpAndSettle();
+
+      expect(
+        find.descendant(
+          of: find.byType(TabBarView),
+          matching: find.text('${tab.text} Tab'),
+        ),
+        findsOneWidget,
+      );
+
+      expect(log.length, equals(1));
+      expect(log.last, equals('tab changed: ${tabs.indexOf(tab)}'));
+
+      log.clear();
+    }
+
+    debugPrint = originalDebugPrint;
+  });
+
+  testWidgets('DefaultTabControllerListener throws when no DefaultTabController above', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      example.DefaultTabControllerListener(
+        onTabChanged: (_) {},
+        child: const SizedBox.shrink(),
+      ),
+    );
+
+    final dynamic exception = tester.takeException();
+    expect(exception, isFlutterError);
+
+    final FlutterError error = exception as FlutterError;
+    expect(
+      error.toStringDeep(),
+      equalsIgnoringHashCodes(
+        'FlutterError\n'
+        '   No DefaultTabController for DefaultTabControllerListener.\n'
+        '   When creating a DefaultTabControllerListener, you must ensure\n'
+        '   that there is a DefaultTabController above the\n'
+        '   DefaultTabControllerListener.\n',
+      ),
+    );
+  });
+}