use RadioListTile in expansion panels demo (#20240)

* use RadioListTile in expansion panels demo

Fixes #6048.
- expose `_Location`
- appease analyzer

examples/flutter_gallery/lib/demo/material/expansion_panels_demo.dart

@@ -6,7 +6,8 @@ import 'package:flutter/material.dart';
 
 import '../../gallery/demo.dart';
 
-enum _Location {
+@visibleForTesting
+enum Location {
   Barbados,
   Bahamas,
   Bermuda
@@ -225,12 +226,12 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
           );
         },
       ),
-      DemoItem<_Location>(
+      DemoItem<Location>(
         name: 'Location',
-        value: _Location.Bahamas,
+        value: Location.Bahamas,
         hint: 'Select location',
-        valueToString: (_Location location) => location.toString().split('.')[1],
-        builder: (DemoItem<_Location> item) {
+        valueToString: (Location location) => location.toString().split('.')[1],
+        builder: (DemoItem<Location> item) {
           void close() {
             setState(() {
               item.isExpanded = false;
@@ -242,47 +243,32 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                 return CollapsibleBody(
                   onSave: () { Form.of(context).save(); close(); },
                   onCancel: () { Form.of(context).reset(); close(); },
-                  child: FormField<_Location>(
+                  child: FormField<Location>(
                     initialValue: item.value,
-                    onSaved: (_Location result) { item.value = result; },
-                    builder: (FormFieldState<_Location> field) {
+                    onSaved: (Location result) { item.value = result; },
+                    builder: (FormFieldState<Location> field) {
                       return Column(
                         mainAxisSize: MainAxisSize.min,
                         crossAxisAlignment: CrossAxisAlignment.start,
                         children: <Widget>[
-                          Row(
-                            mainAxisSize: MainAxisSize.min,
-                            children: <Widget>[
-                              Radio<_Location>(
-                                value: _Location.Bahamas,
+                          RadioListTile<Location>(
+                            value: Location.Bahamas,
+                            title: const Text('Bahamas'),
                             groupValue: field.value,
                             onChanged: field.didChange,
                           ),
-                              const Text('Bahamas')
-                            ]
-                          ),
-                          Row(
-                            mainAxisSize: MainAxisSize.min,
-                            children: <Widget>[
-                              Radio<_Location>(
-                                value: _Location.Barbados,
+                          RadioListTile<Location>(
+                            value: Location.Barbados,
+                            title: const Text('Barbados'),
                             groupValue: field.value,
                             onChanged: field.didChange,
                           ),
-                              const Text('Barbados')
-                            ]
-                          ),
-                          Row(
-                            mainAxisSize: MainAxisSize.min,
-                            children: <Widget>[
-                              Radio<_Location>(
-                                value: _Location.Bermuda,
+                          RadioListTile<Location>(
+                            value: Location.Bermuda,
+                            title: const Text('Bermuda'),
                             groupValue: field.value,
                             onChanged: field.didChange,
                           ),
-                              const Text('Bermuda')
-                            ]
-                          )
                         ]
                       );
                     }

examples/flutter_gallery/test/demo/material/expansion_panels_demo_test.dart

+// Copyright 2018 The Chromium 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/material.dart';
+import 'package:flutter_gallery/demo/material/expansion_panels_demo.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+Future<void> main() async {
+  testWidgets('Expansion panel demo: radio tile selection changes on tap',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(home: ExpansionPanelsDemo()));
+
+    expect(_expandIcons, findsNWidgets(3));
+
+    await tester.tap(find.byWidget(_radioPanelExpandIcon));
+    await tester.pumpAndSettle();
+
+    expect(_radioFinder, findsNWidgets(3));
+
+    const int i = 1;
+
+    expect(_isRadioSelected(0), isTrue);
+    expect(_isRadioSelected(i), isFalse);
+
+    await tester.tap(find.byWidget(_radioListTiles[i]));
+    await tester.pumpAndSettle();
+
+    expect(_isRadioSelected(0), isFalse);
+    expect(_isRadioSelected(i), isTrue);
+  });
+}
+
+Finder get _expandIcons => find.byType(ExpandIcon);
+
+Widget get _radioPanelExpandIcon => _expandIcons.evaluate().toList()[1].widget;
+
+bool _isRadioSelected(int index) =>
+    _radios[index].value == _radios[index].groupValue;
+
+List<Radio<Location>> get _radios => List<Radio<Location>>.from(
+    _radioFinder.evaluate().map<Widget>((Element e) => e.widget));
+
+// [find.byType] and [find.widgetWithText] do not match subclasses; `Radio` is not sufficient to find a `Radio<_Location>`.
+// Another approach is to grab the `runtimeType` of a dummy instance; see packages/flutter/test/material/control_list_tile_test.dart.
+Finder get _radioFinder =>
+    find.byWidgetPredicate((Widget w) => w is Radio<Location>);
+
+List<RadioListTile<Location>> get _radioListTiles =>
+    List<RadioListTile<Location>>.from(
+        _radioListTilesFinder.evaluate().map<Widget>((Element e) => e.widget));
+
+Finder get _radioListTilesFinder =>
+    find.byWidgetPredicate((Widget w) => w is RadioListTile<Location>);