Add NavigationBar dartpad example (#97046)

examples/api/lib/material/navigation_bar/navigation_bar.dart

+// 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.
+
+// Flutter code sample for NavigationBar
+
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(home: NavigationExample());
+  }
+}
+
+class NavigationExample extends StatefulWidget {
+  const NavigationExample({Key? key}) : super(key: key);
+
+  @override
+  State<NavigationExample> createState() => _NavigationExampleState();
+}
+
+class _NavigationExampleState extends State<NavigationExample> {
+  int currentPageIndex = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      bottomNavigationBar: NavigationBar(
+        onDestinationSelected: (int index) {
+          setState(() {
+            currentPageIndex = index;
+          });
+        },
+        selectedIndex: currentPageIndex,
+        destinations: const <Widget>[
+          NavigationDestination(
+            icon: Icon(Icons.explore),
+            label: 'Explore',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.commute),
+            label: 'Commute',
+          ),
+          NavigationDestination(
+            selectedIcon: Icon(Icons.bookmark),
+            icon: Icon(Icons.bookmark_border),
+            label: 'Saved',
+          ),
+        ],
+      ),
+      body: <Widget>[
+        Container(
+          color: Colors.red,
+          alignment: Alignment.center,
+          child: const Text('Page 1'),
+        ),
+        Container(
+          color: Colors.green,
+          alignment: Alignment.center,
+          child: const Text('Page 2'),
+        ),
+        Container(
+          color: Colors.blue,
+          alignment: Alignment.center,
+          child: const Text('Page 3'),
+        ),
+      ][currentPageIndex],
+    );
+  }
+}

examples/api/test/material/navigation_bar/navigation_bar_test.dart

+// 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/material/navigation_bar/navigation_bar.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Navigation bar updates destination on tap',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.MyApp(),
+    );
+    final NavigationBar navigationBarWidget =
+        tester.firstWidget(find.byType(NavigationBar));
+
+    /// NavigationDestinations must be rendered
+    expect(find.text('Explore'), findsOneWidget);
+    expect(find.text('Commute'), findsOneWidget);
+    expect(find.text('Saved'), findsOneWidget);
+
+    /// initial index must be zero
+    expect(navigationBarWidget.selectedIndex, 0);
+
+    /// switch to second tab
+    await tester.tap(find.text('Commute'));
+    await tester.pumpAndSettle();
+    expect(find.text('Page 2'), findsOneWidget);
+
+    /// switch to third tab
+    await tester.tap(find.text('Saved'));
+    await tester.pumpAndSettle();
+    expect(find.text('Page 3'), findsOneWidget);
+  });
+}

packages/flutter/lib/src/material/navigation_bar.dart

@@ -33,47 +33,19 @@ import 'tooltip.dart';
 /// This widget holds a collection of destinations (usually
 /// [NavigationDestination]s).
 ///
-/// Usage:
-/// ```dart
-/// Scaffold(
-///   bottomNavigationBar: NavigationBar(
-///     onDestinationSelected: (int index) {
-///       setState(() { _currentPageIndex = index; }),
-///     },
-///     selectedIndex: _currentPageIndex,
-///     destinations: [
-///       NavigationDestination(
-///         icon: Icon(Icons.explore),
-///         label: 'Explore',
-///       ),
-///       NavigationDestination(
-///         icon: Icon(Icons.commute),
-///         label: 'Commute',
-///       ),
-///       NavigationDestination(
-///         selectedIcon: Icon(Icons.bookmark),
-///         icon: Icon(Icons.bookmark_border),
-///         label: 'Saved',
-///       ),
-///     ],
-///   ),
-/// ),
-/// ```
-///
 /// {@tool dartpad}
-/// This example has a [NavigationBar] where each destination has its
-/// own Navigator, Scaffold, and Appbar. That means that each
-/// destination has an independent route history and (app bar) back
-/// button. A [Stack] is used to display one destination at a time and
-/// destination changes are handled by cross fade transitions. Destinations
-/// that have been completely faded out are [Offstage].
-///
-/// One can see that the appearance of each destination's dialogs, bottom sheet,
-/// list scrolling state, and text field state, persist when another destination
-/// is selected.
+/// This example shows a [NavigationBar] as it is used within a [Scaffold]
+/// widget. The [NavigationBar] has three [NavigationDestination] widgets
+/// and the [selectedIndex] is set to index 0. The `onDestinationSelected` callback
+/// changes the selected item's index and displays a corresponding widget in the body of the [Scaffold].
 ///
-/// ** See code in examples/api/lib/material/navigation_bar/navigation_bar.0.dart **
+/// ** See code in examples/api/lib/material/navigation_bar/navigation_bar.dart **
 /// {@end-tool}
+/// See also:
+///
+///  * [NavigationDestination]
+///  * [BottomNavigationBar]
+///  * <https://api.flutter.dev/flutter/material/NavigationDestination-class.html>
 class NavigationBar extends StatelessWidget {
   /// Creates a Material 3 Navigation Bar component.
   ///