Adds selected semantics flag to radio button for Apple devices (#125499)

fixes https://github.com/flutter/flutter/issues/125495

The iOS will announce 'Selected, \<label\>, button' for selected radio button after this change

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
@@ -232,9 +233,24 @@ class _CupertinoRadioState<T> extends State<CupertinoRadio<T>> with TickerProvid
 
     final Color effectiveFillColor = widget.fillColor ?? CupertinoColors.white;
 
+    final bool? accessibilitySelected;
+    // Apple devices also use `selected` to annotate radio button's semantics
+    // state.
+    switch(defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        accessibilitySelected = null;
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        accessibilitySelected = widget._selected;
+    }
+
     return Semantics(
       inMutuallyExclusiveGroup: true,
       checked: widget._selected,
+      selected: accessibilitySelected,
       child: buildToggleable(
         focusNode: widget.focusNode,
         autofocus: widget.autofocus,

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

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 
 import 'color_scheme.dart';
 import 'colors.dart';
@@ -498,10 +499,24 @@ class _RadioState<T> extends State<Radio<T>> with TickerProviderStateMixin, Togg
         ? effectiveActivePressedOverlayColor
         : effectiveInactivePressedOverlayColor;
     }
+    final bool? accessibilitySelected;
+    // Apple devices also use `selected` to annotate radio button's semantics
+    // state.
+    switch(defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        accessibilitySelected = null;
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        accessibilitySelected = widget._selected;
+    }
 
     return Semantics(
       inMutuallyExclusiveGroup: true,
       checked: widget._selected,
+      selected: accessibilitySelected,
       child: buildToggleable(
         focusNode: widget.focusNode,
         autofocus: widget.autofocus,

packages/flutter/test/cupertino/radio_test.dart

@@ -118,6 +118,41 @@ void main() {
     expect(log, equals(<int>[1]));
   });
 
+  testWidgets('Radio selected semantics - platform adaptive', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(CupertinoApp(
+      home: Center(
+        child: CupertinoRadio<int>(
+          value: 1,
+          groupValue: 1,
+          onChanged: (int? i) { },
+        ),
+      ),
+    ));
+
+    final bool isApple = defaultTargetPlatform == TargetPlatform.iOS ||
+        defaultTargetPlatform == TargetPlatform.macOS;
+    expect(
+      semantics,
+      includesNodeWith(
+        flags: <SemanticsFlag>[
+          SemanticsFlag.isInMutuallyExclusiveGroup,
+          SemanticsFlag.hasCheckedState,
+          SemanticsFlag.hasEnabledState,
+          SemanticsFlag.isEnabled,
+          SemanticsFlag.isFocusable,
+          SemanticsFlag.isChecked,
+          if (isApple) SemanticsFlag.isSelected,
+        ],
+        actions: <SemanticsAction>[
+          SemanticsAction.tap,
+        ],
+      ),
+    );
+    semantics.dispose();
+  }, variant: TargetPlatformVariant.all());
+
   testWidgets('Radio semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);
 

packages/flutter/test/material/radio_test.dart

@@ -194,6 +194,40 @@ void main() {
     expect(tester.getSize(find.byKey(key2)), const Size(40.0, 40.0));
   });
 
+  testWidgets('Radio selected semantics - platform adaptive', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(Theme(
+      data: theme,
+      child: Material(
+        child: Radio<int>(
+          value: 1,
+          groupValue: 1,
+          onChanged: (int? i) {},
+        ),
+      ),
+    ));
+    final bool isApple = defaultTargetPlatform == TargetPlatform.iOS ||
+                         defaultTargetPlatform == TargetPlatform.macOS;
+    expect(
+      semantics,
+      includesNodeWith(
+        flags: <SemanticsFlag>[
+          SemanticsFlag.isInMutuallyExclusiveGroup,
+          SemanticsFlag.hasCheckedState,
+          SemanticsFlag.hasEnabledState,
+          SemanticsFlag.isEnabled,
+          SemanticsFlag.isFocusable,
+          SemanticsFlag.isChecked,
+          if (isApple) SemanticsFlag.isSelected,
+        ],
+        actions: <SemanticsAction>[
+          SemanticsAction.tap,
+        ],
+      ),
+    );
+    semantics.dispose();
+  }, variant: TargetPlatformVariant.all());
 
   testWidgets('Radio semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);