Semantics for CupertinoTabBar (#19924)

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

@@ -156,9 +156,15 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
     final List<Widget> result = <Widget>[];
 
     for (int index = 0; index < items.length; index += 1) {
+      final bool active = index == currentIndex;
       result.add(
         _wrapActiveItem(
           new Expanded(
+            child: new Semantics(
+              selected: active,
+              // TODO(https://github.com/flutter/flutter/issues/13452):
+              // This needs localization support.
+              hint: 'tab, ${index + 1} of ${items.length}',
               child: new GestureDetector(
                 behavior: HitTestBehavior.opaque,
                 onTap: onTap == null ? null : () { onTap(index); },
@@ -174,7 +180,8 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
                 ),
               ),
             ),
-          active: index == currentIndex,
+          ),
+          active: active,
         ),
       );
     }
@@ -183,7 +190,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   }
 
   /// Change the active tab item's icon and title colors to active.
-  Widget _wrapActiveItem(Widget item, { bool active }) {
+  Widget _wrapActiveItem(Widget item, { @required bool active }) {
     if (!active)
       return item;
 

packages/flutter/test/cupertino/bottom_tab_bar_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter/cupertino.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../painting/mocks_for_image_cache.dart';
+import '../widgets/semantics_tester.dart';
 
 Future<Null> pumpWidgetWithBoilerplate(WidgetTester tester, Widget widget) async {
   await tester.pumpWidget(
@@ -169,4 +170,39 @@ void main() {
     await tester.tap(find.text('Tab 1'));
     expect(callbackTab, 0);
   });
+
+  testWidgets('tabs announce semantics', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    await pumpWidgetWithBoilerplate(tester, new MediaQuery(
+      data: const MediaQueryData(),
+      child: new CupertinoTabBar(
+        items: const <BottomNavigationBarItem>[
+          const BottomNavigationBarItem(
+            icon: const ImageIcon(const TestImageProvider(24, 24)),
+            title: const Text('Tab 1'),
+          ),
+          const BottomNavigationBarItem(
+            icon: const ImageIcon(const TestImageProvider(24, 24)),
+            title: const Text('Tab 2'),
+          ),
+        ],
+      ),
+    ));
+
+    expect(semantics, includesNodeWith(
+      label: 'Tab 1',
+      hint: 'tab, 1 of 2',
+      flags: <SemanticsFlag>[SemanticsFlag.isSelected],
+      textDirection: TextDirection.ltr,
+    ));
+
+    expect(semantics, includesNodeWith(
+      label: 'Tab 2',
+      hint: 'tab, 2 of 2',
+      textDirection: TextDirection.ltr,
+    ));
+
+    semantics.dispose();
+  });
 }

packages/flutter/test/widgets/semantics_tester.dart

@@ -384,6 +384,7 @@ class SemanticsTester {
   Iterable<SemanticsNode> nodesWith({
     String label,
     String value,
+    String hint,
     TextDirection textDirection,
     List<SemanticsAction> actions,
     List<SemanticsFlag> flags,
@@ -397,6 +398,8 @@ class SemanticsTester {
         return false;
       if (value != null && node.value != value)
         return false;
+      if (hint != null && node.hint != hint)
+        return false;
       if (textDirection != null && node.textDirection != textDirection)
         return false;
       if (actions != null) {
@@ -636,6 +639,7 @@ class _IncludesNodeWith extends Matcher {
   const _IncludesNodeWith({
     this.label,
     this.value,
+    this.hint,
     this.textDirection,
     this.actions,
     this.flags,
@@ -646,6 +650,7 @@ class _IncludesNodeWith extends Matcher {
 
   final String label;
   final String value;
+  final String hint;
   final TextDirection textDirection;
   final List<SemanticsAction> actions;
   final List<SemanticsFlag> flags;
@@ -658,6 +663,7 @@ class _IncludesNodeWith extends Matcher {
     return item.nodesWith(
       label: label,
       value: value,
+      hint: hint,
       textDirection: textDirection,
       actions: actions,
       flags: flags,
@@ -683,6 +689,8 @@ class _IncludesNodeWith extends Matcher {
       strings.add('label "$label"');
     if (value != null)
       strings.add('value "$value"');
+    if (hint != null)
+      strings.add('hint "$hint"');
     if (textDirection != null)
       strings.add(' (${describeEnum(textDirection)})');
     if (actions != null)
@@ -706,6 +714,7 @@ class _IncludesNodeWith extends Matcher {
 Matcher includesNodeWith({
   String label,
   String value,
+  String hint,
   TextDirection textDirection,
   List<SemanticsAction> actions,
   List<SemanticsFlag> flags,
@@ -716,6 +725,7 @@ Matcher includesNodeWith({
   return new _IncludesNodeWith(
     label: label,
     value: value,
+    hint: hint,
     textDirection: textDirection,
     actions: actions,
     flags: flags,