[cupertino_ui] Migrate a snippet in CupertinoCheckbox's API doc to {@example} and add unit tests

packages/cupertino_ui/example/lib/checkbox/cupertino_checkbox.snippet.0.dart

@@ -0,0 +1,37 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:cupertino_ui/cupertino_ui.dart';
+import 'package:material_ui/material_ui.dart' show Colors;
+
+/// Flutter code sample for [CupertinoCheckbox].
+
+class CupertinoCheckboxExample extends StatelessWidget {
+  const CupertinoCheckboxExample({super.key, required this.onChanged});
+
+  final ValueChanged<bool?>? onChanged;
+
+  ValueChanged<bool?>? get _nullableOnChanged => onChanged;
+  bool get _value => true;
+
+  @override
+  Widget build(BuildContext context) {
+    return
+    // #region body
+    CupertinoCheckbox(
+      value: _value,
+      // If `onChanged` is null, this checkbox is disabled.
+      // If `onChanged` is not null, this checkbox is enabled.
+      onChanged: _nullableOnChanged,
+      fillColor: WidgetStateProperty.resolveWith<Color>((Set<WidgetState> states) {
+        if (states.contains(WidgetState.disabled)) {
+          return Colors.orange.withValues(alpha: .32);
+        }
+        return Colors.orange;
+      }),
+    )
+    // #endregion body
+    ;
+  }
+}

packages/cupertino_ui/example/test/checkbox/cupertino_checkbox.snippet.0_test.dart

@@ -0,0 +1,39 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:cupertino_ui/cupertino_ui.dart';
+import 'package:cupertino_ui_examples/checkbox/cupertino_checkbox.snippet.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:material_ui/material_ui.dart' show Colors;
+
+void main() {
+  testWidgets('Checkbox color is affected by whether it is enabled', (WidgetTester tester) async {
+    RenderBox getCheckboxRenderer() {
+      return tester.renderObject<RenderBox>(find.byType(CupertinoCheckbox));
+    }
+
+    Widget buildApp({required ValueChanged<bool?>? onChanged}) {
+      return CupertinoApp(
+        theme: CupertinoThemeData(brightness: .light),
+        home: CupertinoPageScaffold(
+          navigationBar: CupertinoNavigationBar(
+            middle: Text('CupertinoCheckbox Example'),
+          ),
+          child: SafeArea(child: example.CupertinoCheckboxExample(
+            onChanged: onChanged,
+          )),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp(onChanged: (bool? _) {}));
+    await tester.pumpAndSettle();
+    expect(getCheckboxRenderer(), paints..rrect(color: Colors.orange));
+
+    await tester.pumpWidget(buildApp(onChanged: null));
+    await tester.pumpAndSettle();
+    expect(getCheckboxRenderer(), paints..rrect(color: Colors.orange.withValues(alpha: .32)));
+  });
+}

packages/cupertino_ui/lib/src/checkbox.dart

@@ -210,25 +210,12 @@ class CupertinoCheckbox extends StatefulWidget {
   ///
   // TODO(framework): Replace the following block with a blue example container
   // when it's supported. https://github.com/dart-lang/dartdoc/issues/4243
-  // TODO(framework): Add unit tests to this code snippet.
-  // https://github.com/flutter/flutter/issues/188530
   ///
   /// This example resolves the [fillColor] based on the current [WidgetState]
   /// of the [CupertinoCheckbox], providing a different [Color] when it is
   /// [WidgetState.disabled].
   ///
-  /// ```dart
-  /// CupertinoCheckbox(
-  ///   value: true,
-  ///   onChanged: (_){},
-  ///   fillColor: WidgetStateProperty.resolveWith<Color>((Set<WidgetState> states) {
-  ///     if (states.contains(WidgetState.disabled)) {
-  ///       return Colors.orange.withValues(alpha: .32);
-  ///     }
-  ///     return Colors.orange;
-  ///   })
-  /// )
-  /// ```
+  /// {@example /example/lib/checkbox/cupertino_checkbox.snippet.0.dart#body indent=strip}
   ///
   // TODO(framework): End of the blue example container.
   /// {@endtemplate}