[CupertinoTabBar] Add an official interactive sample (#91763)

examples/api/lib/cupertino/bottom_tab_bar/bottom_tab_bar.0.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 CupertinoTabBar
+
+import 'package:flutter/cupertino.dart';
+
+void main() => runApp(const MyApp());
+
+class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return const CupertinoApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+class MyStatefulWidget extends StatefulWidget {
+  const MyStatefulWidget({Key? key}) : super(key: key);
+
+  @override
+  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
+}
+
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoTabScaffold(
+      tabBar: CupertinoTabBar(
+        items: const <BottomNavigationBarItem>[
+          BottomNavigationBarItem(
+            icon: Icon(CupertinoIcons.circle),
+            label: 'Tab 1',
+          ),
+          BottomNavigationBarItem(
+            icon: Icon(CupertinoIcons.add),
+            label: 'Tab 2',
+          ),
+        ],
+      ),
+      tabBuilder: (BuildContext context, int index) {
+        return CupertinoTabView(
+          builder: (BuildContext context) {
+            return Center(
+              child: Text('Content of tab $index'),
+            );
+          },
+        );
+      },
+    );
+  }
+}

packages/flutter/lib/src/cupertino/bottom_tab_bar.dart

@@ -44,6 +44,12 @@ const Color _kDefaultTabBarInactiveColor = CupertinoColors.inactiveGray;
 /// value from the operating system can be retrieved in many ways, such as querying
 /// [MediaQuery.textScaleFactorOf] against [CupertinoApp]'s [BuildContext].
 ///
+/// {@tool dartpad}
+/// This example shows a [CupertinoTabBar] placed in a [CupertinoPageScaffold].
+///
+/// ** See code in examples/api/lib/cupertino/bottom_tab_bar/bottom_tab_bar.0.dart **
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [CupertinoTabScaffold], which hosts the [CupertinoTabBar] at the bottom.