Add SemanticsNode.isSelected flag (#10610)

* Add SemanticsNode.isSelected flag

* Adds example usage to TabBar

See also https://github.com/flutter/engine/pull/3764

* Review comments

* whitespace fixes

* Fix doc ref and update engine roll

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

@@ -730,9 +730,14 @@ class _TabBarState extends State<TabBar> {
     // then give all of the tabs equal flexibility so that their widths
     // reflect the intrinsic width of their labels.
     for (int index = 0; index < widget.tabs.length; index++) {
-      wrappedTabs[index] = new InkWell(
-        onTap: () { _handleTap(index); },
-        child: wrappedTabs[index],
+      wrappedTabs[index] = new MergeSemantics(
+        child: new Semantics(
+          selected: index == _currentIndex,
+          child: new InkWell(
+            onTap: () { _handleTap(index); },
+            child: wrappedTabs[index],
+          ),
+        ),
       );
       if (!widget.isScrollable)
         wrappedTabs[index] = new Expanded(child: wrappedTabs[index]);

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -2873,10 +2873,12 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     RenderBox child,
     bool container: false,
     bool checked,
-    String label
+    bool selected,
+    String label,
   }) : assert(container != null),
        _container = container,
        _checked = checked,
+       _selected = selected,
        _label = label,
        super(child);
 
@@ -2900,8 +2902,8 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     markNeedsSemanticsUpdate();
   }
 
-  /// If non-null, sets the "hasCheckedState" semantic to true and the
-  /// "isChecked" semantic to the given value.
+  /// If non-null, sets the [SemanticsNode.hasCheckedState] semantic to true and
+  /// the [SemanticsNode.isChecked] semantic to the given value.
   bool get checked => _checked;
   bool _checked;
   set checked(bool value) {
@@ -2912,7 +2914,19 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     markNeedsSemanticsUpdate(onlyChanges: (value != null) == hadValue);
   }
 
-  /// If non-null, sets the "label" semantic to the given value.
+  /// If non-null, sets the [SemanticsNode.isSelected] semantic to the given
+  /// value.
+  bool get selected => _selected;
+  bool _selected;
+  set selected(bool value) {
+    if (selected == value)
+      return;
+    final bool hadValue = selected != null;
+    _selected = value;
+    markNeedsSemanticsUpdate(onlyChanges: (value != null) == hadValue);
+  }
+
+  /// If non-null, sets the [SemanticsNode.label] semantic to the given value.
   String get label => _label;
   String _label;
   set label(String value) {
@@ -2927,7 +2941,7 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
   bool get isSemanticBoundary => container;
 
   @override
-  SemanticsAnnotator get semanticsAnnotator => checked != null || label != null ? _annotate : null;
+  SemanticsAnnotator get semanticsAnnotator => checked != null || selected != null || label != null ? _annotate : null;
 
   void _annotate(SemanticsNode node) {
     if (checked != null) {
@@ -2935,6 +2949,8 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
         ..hasCheckedState = true
         ..isChecked = checked;
     }
+    if (selected != null)
+      node.isSelected = selected;
     if (label != null)
       node.label = label;
   }

packages/flutter/lib/src/rendering/semantics.dart

@@ -292,6 +292,10 @@ class SemanticsNode extends AbstractNode {
   bool get isChecked => (_flags & SemanticsFlags.isChecked.index) != 0;
   set isChecked(bool value) => _setFlag(SemanticsFlags.isChecked, value);
 
+  /// Whether the current node is selected (true) or not (false).
+  bool get isSelected => (_flags & SemanticsFlags.isSelected.index) != 0;
+  set isSelected(bool value) => _setFlag(SemanticsFlags.isSelected, value);
+
   /// A textual description of this node.
   String get label => _label;
   String _label = '';
@@ -595,6 +599,8 @@ class SemanticsNode extends AbstractNode {
       else
         buffer.write('; unchecked');
     }
+    if (isSelected)
+      buffer.write('; selected');
     if (label.isNotEmpty)
       buffer.write('; "$label"');
     buffer.write(')');

packages/flutter/lib/src/widgets/basic.dart

@@ -3635,7 +3635,8 @@ class Semantics extends SingleChildRenderObjectWidget {
     Widget child,
     this.container: false,
     this.checked,
-    this.label
+    this.selected,
+    this.label,
   }) : assert(container != null),
        super(key: key, child: child);
 
@@ -3656,6 +3657,13 @@ class Semantics extends SingleChildRenderObjectWidget {
   /// state is.
   final bool checked;
 
+  /// If non-null indicates that this subtree represents something that can be
+  /// in a selected or unselected state, and what its current state is.
+  ///
+  /// The active tab in a tab bar for example is considered "selected", whereas
+  /// all other tabs are unselected.
+  final bool selected;
+
   /// Provides a textual description of the widget.
   final String label;
 
@@ -3663,7 +3671,8 @@ class Semantics extends SingleChildRenderObjectWidget {
   RenderSemanticsAnnotations createRenderObject(BuildContext context) => new RenderSemanticsAnnotations(
     container: container,
     checked: checked,
-    label: label
+    selected: selected,
+    label: label,
   );
 
   @override
@@ -3671,6 +3680,7 @@ class Semantics extends SingleChildRenderObjectWidget {
     renderObject
       ..container = container
       ..checked = checked
+      ..selected = selected
       ..label = label;
   }
 
@@ -3680,6 +3690,8 @@ class Semantics extends SingleChildRenderObjectWidget {
     description.add('container: $container');
     if (checked != null)
       description.add('checked: $checked');
+    if (selected != null)
+      description.add('selected: $selected');
     if (label != null)
       description.add('label: "$label"');
   }

packages/flutter/test/material/tabs_test.dart

@@ -2,12 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' show SemanticsFlags, SemanticsAction;
+
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 
 import '../rendering/mock_canvas.dart';
 import '../rendering/recording_canvas.dart';
+import '../widgets/semantics_tester.dart';
 
 class StateMarker extends StatefulWidget {
   const StateMarker({ Key key, this.child }) : super(key: key);
@@ -900,6 +903,62 @@ void main() {
       rect: new Rect.fromLTRB(tabLeft + padLeft, height, tabRight - padRight, height + weight)
     ));
   });
+  
+  testWidgets('correct semantics', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    final List<Tab> tabs = new List<Tab>.generate(2, (int index) {
+      return new Tab(text: 'TAB #$index');
+    });
+
+    final TabController controller = new TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+      initialIndex: 0,
+    );
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Semantics(
+          container: true,
+          child: new TabBar(
+            isScrollable: true,
+            controller: controller,
+            tabs: tabs,
+          ),
+        ),
+      ),
+    );
+
+    final TestSemantics expectedSemantics = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          id: 1,
+          rect: TestSemantics.fullScreen,
+          children: <TestSemantics>[
+            new TestSemantics(
+              id: 2,
+              actions: SemanticsAction.tap.index,
+              flags: SemanticsFlags.isSelected.index,
+              label: 'TAB #0',
+              rect: new Rect.fromLTRB(0.0, 0.0, 108.0, 46.0),
+              transform: new Matrix4.translationValues(0.0, 276.0, 0.0),
+            ),
+            new TestSemantics(
+              id: 4,
+              actions: SemanticsAction.tap.index,
+              label: 'TAB #1',
+              rect: new Rect.fromLTRB(0.0, 0.0, 108.0, 46.0),
+              transform: new Matrix4.translationValues(108.0, 276.0, 0.0),
+            ),
+          ]),
+      ],
+    );
+
+    expect(semantics, hasSemantics(expectedSemantics));
+
+    semantics.dispose();
+  });
 
   testWidgets('TabBar etc with zero tabs', (WidgetTester tester) async {
     final TabController controller = new TabController(