Add example for CupertinoCheckbox (#151779)

examples/api/lib/cupertino/checkbox/cupertino_checkbox.0.dart

@@ -0,0 +1,55 @@
+// 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/cupertino.dart';
+
+/// Flutter code sample for [CupertinoCheckbox].
+
+void main() => runApp(const CupertinoCheckboxApp());
+
+class CupertinoCheckboxApp extends StatelessWidget {
+  const CupertinoCheckboxApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const CupertinoApp(
+      theme: CupertinoThemeData(brightness: Brightness.light),
+      home: CupertinoPageScaffold(
+        navigationBar: CupertinoNavigationBar(
+          middle: Text('CupertinoCheckbox Example'),
+        ),
+        child: SafeArea(
+          child: CupertinoCheckboxExample(),
+        ),
+      ),
+    );
+  }
+}
+
+class CupertinoCheckboxExample extends StatefulWidget {
+  const CupertinoCheckboxExample({super.key});
+
+  @override
+  State<CupertinoCheckboxExample> createState() => _CupertinoCheckboxExampleState();
+}
+
+class _CupertinoCheckboxExampleState extends State<CupertinoCheckboxExample> {
+  bool? isChecked = true;
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoCheckbox(
+      checkColor: CupertinoColors.white,
+      // Set tristate to true to make the checkbox display a null value
+      // in addition to the default true and false values.
+      tristate: true,
+      value: isChecked,
+      onChanged: (bool? value) {
+        setState(() {
+          isChecked = value;
+        });
+      },
+    );
+  }
+}

examples/api/test/cupertino/checkbox/cupertino_checkbox.0_test.dart

@@ -0,0 +1,41 @@
+// 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/cupertino.dart';
+import 'package:flutter_api_samples/cupertino/checkbox/cupertino_checkbox.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Checkbox can be checked', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.CupertinoCheckboxApp(),
+    );
+
+    CupertinoCheckbox checkbox = tester.widget(find.byType(CupertinoCheckbox));
+
+    // Verify the initial state of the checkbox.
+    expect(checkbox.value, isTrue);
+    expect(checkbox.tristate, isTrue);
+
+    // Tap the checkbox and verify the state change.
+    await tester.tap(find.byType(CupertinoCheckbox));
+    await tester.pump();
+    checkbox = tester.widget(find.byType(CupertinoCheckbox));
+
+    expect(checkbox.value, isNull);
+
+    // Tap the checkbox and verify the state change.
+    await tester.tap(find.byType(CupertinoCheckbox));
+    await tester.pump();
+    checkbox = tester.widget(find.byType(CupertinoCheckbox));
+
+    expect(checkbox.value, isFalse);
+
+    await tester.tap(find.byType(CupertinoCheckbox));
+    await tester.pump();
+    checkbox = tester.widget(find.byType(CupertinoCheckbox));
+
+    expect(checkbox.value, isTrue);
+  });
+}

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

@@ -41,6 +41,12 @@ const double _kCupertinoFocusColorSaturation = 0.835;
 /// ([CupertinoSwitch] in Flutter) instead, or to find a creative custom
 /// solution.
 ///
+/// {@tool dartpad}
+/// This example shows a toggleable [CupertinoCheckbox].
+///
+/// ** See code in examples/api/lib/cupertino/checkbox/cupertino_checkbox.0.dart **
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [Checkbox], the Material Design equivalent.