[Material] Allow Appbar to exclude header semantics (#52894)

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

@@ -193,6 +193,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.textTheme,
     this.primary = true,
     this.centerTitle,
+    this.excludeHeaderSemantics = false,
     this.titleSpacing = NavigationToolbar.kMiddleSpacing,
     this.toolbarOpacity = 1.0,
     this.bottomOpacity = 1.0,
@@ -387,6 +388,11 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// Defaults to being adapted to the current [TargetPlatform].
   final bool centerTitle;
 
+  /// Whether the title should be wrapped with header [Semantics].
+  ///
+  /// Defaults to false.
+  final bool excludeHeaderSemantics;
+
   /// The spacing around [title] content on the horizontal axis. This spacing is
   /// applied even if there is no [leading] content or [actions]. If you want
   /// [title] to take all the space available, set this value to 0.0.
@@ -526,15 +532,21 @@ class _AppBarState extends State<AppBar> {
         case TargetPlatform.macOS:
           break;
       }
+
+      title = _AppBarTitleBox(child: title);
+      if (!widget.excludeHeaderSemantics) {
+        title = Semantics(
+          namesRoute: namesRoute,
+          child: title,
+          header: true,
+        );
+      }
+
       title = DefaultTextStyle(
         style: centerStyle,
         softWrap: false,
         overflow: TextOverflow.ellipsis,
-        child: Semantics(
+        child: title,
-          namesRoute: namesRoute,
-          child: _AppBarTitleBox(child: title),
-          header: true,
-        ),
       );
     }
 
@@ -725,6 +737,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
     @required this.textTheme,
     @required this.primary,
     @required this.centerTitle,
+    @required this.excludeHeaderSemantics,
     @required this.titleSpacing,
     @required this.expandedHeight,
     @required this.collapsedHeight,
@@ -752,6 +765,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   final TextTheme textTheme;
   final bool primary;
   final bool centerTitle;
+  final bool excludeHeaderSemantics;
   final double titleSpacing;
   final double expandedHeight;
   final double collapsedHeight;
@@ -803,7 +817,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
         automaticallyImplyLeading: automaticallyImplyLeading,
         title: title,
         actions: actions,
-        flexibleSpace: (title == null && flexibleSpace != null)
+        flexibleSpace: (title == null && flexibleSpace != null && !excludeHeaderSemantics)
           ? Semantics(child: flexibleSpace, header: true)
           : flexibleSpace,
         bottom: bottom,
@@ -815,6 +829,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
         textTheme: textTheme,
         primary: primary,
         centerTitle: centerTitle,
+        excludeHeaderSemantics: excludeHeaderSemantics,
         titleSpacing: titleSpacing,
         shape: shape,
         toolbarOpacity: toolbarOpacity,
@@ -956,6 +971,7 @@ class SliverAppBar extends StatefulWidget {
     this.textTheme,
     this.primary = true,
     this.centerTitle,
+    this.excludeHeaderSemantics = false,
     this.titleSpacing = NavigationToolbar.kMiddleSpacing,
     this.expandedHeight,
     this.floating = false,
@@ -1120,6 +1136,11 @@ class SliverAppBar extends StatefulWidget {
   /// Defaults to being adapted to the current [TargetPlatform].
   final bool centerTitle;
 
+  /// Whether the title should be wrapped with header [Semantics].
+  ///
+  /// Defaults to false.
+  final bool excludeHeaderSemantics;
+
   /// The spacing around [title] content on the horizontal axis. This spacing is
   /// applied even if there is no [leading] content or [actions]. If you want
   /// [title] to take all the space available, set this value to 0.0.
@@ -1309,6 +1330,7 @@ class _SliverAppBarState extends State<SliverAppBar> with TickerProviderStateMix
           textTheme: widget.textTheme,
           primary: widget.primary,
           centerTitle: widget.centerTitle,
+          excludeHeaderSemantics: widget.excludeHeaderSemantics,
           titleSpacing: widget.titleSpacing,
           expandedHeight: widget.expandedHeight,
           collapsedHeight: collapsedHeight,

packages/flutter/test/material/app_bar_test.dart

@@ -1432,6 +1432,120 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('AppBar excludes header semantics correctly', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Center(
+          child: AppBar(
+            leading: const Text('Leading'),
+            title: const ExcludeSemantics(child: Text('Title')),
+            excludeHeaderSemantics: true,
+            actions: const <Widget>[
+              Text('Action 1'),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(
+      TestSemantics.root(
+        children: <TestSemantics>[
+          TestSemantics(
+            children: <TestSemantics>[
+              TestSemantics(
+                flags: <SemanticsFlag>[SemanticsFlag.scopesRoute],
+                children: <TestSemantics>[
+                  TestSemantics(
+                    children: <TestSemantics>[
+                      TestSemantics(
+                        label: 'Leading',
+                        textDirection: TextDirection.ltr,
+                      ),
+                      TestSemantics(
+                        label: 'Action 1',
+                        textDirection: TextDirection.ltr,
+                      ),
+                    ],
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ],
+      ),
+      ignoreRect: true,
+      ignoreTransform: true,
+      ignoreId: true,
+    ));
+
+    semantics.dispose();
+  });
+
+  testWidgets('SliverAppBar excludes header semantics correctly', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: CustomScrollView(
+          slivers: <Widget>[
+            SliverAppBar(
+              leading: Text('Leading'),
+              flexibleSpace: ExcludeSemantics(child: Text('Title')),
+              actions: <Widget>[Text('Action 1')],
+              excludeHeaderSemantics: true,
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(
+      TestSemantics.root(
+        children: <TestSemantics>[
+          TestSemantics(
+            textDirection: TextDirection.ltr,
+            children: <TestSemantics>[
+              TestSemantics(
+                flags: <SemanticsFlag>[SemanticsFlag.scopesRoute],
+                children: <TestSemantics>[
+                  TestSemantics(
+                    children: <TestSemantics>[
+                      TestSemantics(
+                        flags: <SemanticsFlag>[SemanticsFlag.hasImplicitScrolling],
+                        children: <TestSemantics>[
+                          TestSemantics(
+                            children: <TestSemantics>[
+                              TestSemantics(
+                                label: 'Leading',
+                                textDirection: TextDirection.ltr,
+                              ),
+                              TestSemantics(
+                                label: 'Action 1',
+                                textDirection: TextDirection.ltr,
+                              ),
+                            ],
+                          ),
+                        ],
+                      ),
+                    ],
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ],
+      ),
+      ignoreRect: true,
+      ignoreTransform: true,
+      ignoreId: true,
+    ));
+
+    semantics.dispose();
+  });
+
   testWidgets('AppBar draws a light system bar for a dark background', (WidgetTester tester) async {
     final ThemeData darkTheme = ThemeData.dark();
     await tester.pumpWidget(MaterialApp(