refactor material banner and fix focus issue (flutter#152646)

- Refactored material banner setup to make it more simple and stateless
- Added focusNode to where the dismiss button focuses automatically when banner is shown

Before: https://screencast.googleplex.com/cast/NTI4OTMxNjU2MDQwNDQ4MHw2ODdiMmYwOC1hMg
After: https://screencast.googleplex.com/cast/NjAzNTY4NDA2NTI4MDAwMHwyNTRjMWUxMi0zMA

fixes b/346646604

Author: DBowen33

dev/a11y_assessments/lib/use_cases/material_banner.dart

@@ -25,46 +25,52 @@ class MainWidget extends StatefulWidget {
 }
 
 class MainWidgetState extends State<MainWidget> {
-  double currentSliderValue = 20;
-  ScaffoldFeatureController<MaterialBanner, MaterialBannerClosedReason>?
-      controller;
+
+  final FocusNode dismissButtonFocusNode = FocusNode();
+  final FocusNode showButtonFocusNode = FocusNode();
+
+  @override
+  void dispose() {
+    dismissButtonFocusNode.dispose();
+    showButtonFocusNode.dispose();
+    super.dispose();
+  }
+
+  void hideBanner() {
+    ScaffoldMessenger.of(context).hideCurrentMaterialBanner();
+    showButtonFocusNode.requestFocus();
+  }
+
+  void showBanner() {
+    ScaffoldMessenger.of(context).showMaterialBanner(
+      MaterialBanner(
+        padding: const EdgeInsets.all(20),
+        content: const Text('Hello, I am a Material Banner'),
+        leading: const Icon(Icons.agriculture_outlined),
+        backgroundColor: Colors.yellowAccent,
+        actions: <Widget>[
+          TextButton(
+            focusNode: dismissButtonFocusNode,
+            onPressed: hideBanner,
+            child: const Text('DISMISS'),
+          ),
+        ],
+      ),
+    );
+    dismissButtonFocusNode.requestFocus();
+  }
 
   @override
   Widget build(BuildContext context) {
-    VoidCallback? onPress;
-    if (controller == null) {
-      onPress = () {
-        setState(() {
-          controller = ScaffoldMessenger.of(context).showMaterialBanner(
-            MaterialBanner(
-              padding: const EdgeInsets.all(20),
-              content: const Text('Hello, I am a Material Banner'),
-              leading: const Icon(Icons.agriculture_outlined),
-              backgroundColor: Colors.green,
-              actions: <Widget>[
-                TextButton(
-                  onPressed: () {
-                    controller!.close();
-                    setState(() {
-                      controller = null;
-                    });
-                  },
-                  child: const Text('DISMISS'),
-                ),
-              ],
-            ),
-          );
-        });
-      };
-    }
     return Scaffold(
       appBar: AppBar(
         backgroundColor: Theme.of(context).colorScheme.inversePrimary,
         title: Semantics(headingLevel: 1, child: const Text('MaterialBanner Demo')),
       ),
       body: Center(
         child: ElevatedButton(
-          onPressed: onPress,
+          focusNode: showButtonFocusNode,
+          onPressed: showBanner,
           child: const Text('Show a MaterialBanner'),
         ),
       ),

dev/a11y_assessments/test/material_banner_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:a11y_assessments/use_cases/material_banner.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import 'test_utils.dart';
@@ -21,6 +22,26 @@ void main() {
     expect(find.text('Hello, I am a Material Banner'), findsNothing);
   });
 
+  testWidgets('dismiss button focused on banner open', (WidgetTester tester) async {
+    await pumpsUseCase(tester, MaterialBannerUseCase());
+    await tester.tap(find.text('Show a MaterialBanner'));
+    await tester.pumpAndSettle();
+
+    final TextButton dismissButtonFinder = tester.widget<TextButton>(find.byType(TextButton));
+    expect(dismissButtonFinder.focusNode!.hasFocus, isTrue);
+  });
+
+  testWidgets('show button focused on banner close', (WidgetTester tester) async {
+    await pumpsUseCase(tester, MaterialBannerUseCase());
+    await tester.tap(find.text('Show a MaterialBanner'));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byType(TextButton));
+
+    final ElevatedButton showButtonFinder = tester.widget<ElevatedButton>(find.byType(ElevatedButton));
+    expect(showButtonFinder.focusNode!.hasFocus, isTrue);
+  });
+
   testWidgets('material banner has one h1 tag', (WidgetTester tester) async {
     await pumpsUseCase(tester, MaterialBannerUseCase());
     final Finder findHeadingLevelOnes = find.bySemanticsLabel('MaterialBanner Demo');