Update CupertinoTextField example (#93738)

examples/api/lib/cupertino/text_field/cupertino_text_field.0.dart

@@ -0,0 +1,57 @@
+// 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 CupertinoTextField
+
+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> {
+  late TextEditingController _textController;
+
+  @override
+  void initState() {
+    super.initState();
+    _textController = TextEditingController(text: 'initial text');
+  }
+
+  @override
+  void dispose() {
+    _textController.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      child: Center(
+        child: CupertinoTextField(
+          controller: _textController,
+        )
+      ),
+    );
+  }
+}

examples/api/test/cupertino/text_field/cupertino_text_field.0.dart

@@ -0,0 +1,16 @@
+// 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_api_samples/cupertino/text_field/cupertino_text_field.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('CupertinoTextField has initial text', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.MyApp(),
+    );
+
+    expect(find.text('initial text'), findsOneWidget);
+  });
+}

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

@@ -139,35 +139,11 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
 ///
 /// {@macro flutter.widgets.EditableText.onChanged}
 ///
-/// To control the text that is displayed in the text field, use the
-/// [controller]. For example, to set the initial value of the text field, use
-/// a [controller] that already contains some text such as:
+/// {@tool dartpad}
+/// This example shows how to set the initial value of the `CupertinoTextField` using
+/// a [controller] that already contains some text.
 ///
-/// {@tool snippet}
-///
-/// ```dart
-/// class MyPrefilledText extends StatefulWidget {
-///   const MyPrefilledText({Key? key}) : super(key: key);
-///
-///   @override
-///   State<MyPrefilledText> createState() => _MyPrefilledTextState();
-/// }
-///
-/// class _MyPrefilledTextState extends State<MyPrefilledText> {
-///   late TextEditingController _textController;
-///
-///   @override
-///   void initState() {
-///     super.initState();
-///     _textController = TextEditingController(text: 'initial text');
-///   }
-///
-///   @override
-///   Widget build(BuildContext context) {
-///     return CupertinoTextField(controller: _textController);
-///   }
-/// }
-/// ```
+/// ** See code in examples/api/lib/cupertino/text_field/cupertino_text_field.0.dart **
 /// {@end-tool}
 ///
 /// The [controller] can also control the selection and composing region (and to