Adding SemanticsFlag.isSelected (flutter/engine#3764)

* Adding SemanticsFlag.isSelected

* typo

engine/src/flutter/lib/ui/semantics.dart

@@ -112,6 +112,7 @@ class SemanticsAction {
 class SemanticsFlags {
   static const int _kHasCheckedStateIndex = 1 << 0;
   static const int _kIsCheckedIndex = 1 << 1;
+  static const int _kIsSelectedIndex = 1 << 2;
 
   const SemanticsFlags._(this.index);
 
@@ -133,12 +134,22 @@ class SemanticsFlags {
   /// For example, if a checkbox has a visible checkmark, [isChecked] is true.
   static const SemanticsFlags isChecked = const SemanticsFlags._(_kIsCheckedIndex);
 
+
+  /// Whether a semantics node is selected.
+  ///
+  /// If true, the semantics node is "selected". If false, the semantics node is
+  /// "unselected".
+  ///
+  /// For example, the active tab in a tab bar has [isSelected] set to true.
+  static const SemanticsFlags isSelected = const SemanticsFlags._(_kIsSelectedIndex);
+
   /// The possible semantics flags.
   ///
   /// The map's key is the [index] of the flag and the value is the flag itself.
   static final Map<int, SemanticsFlags> values = const <int, SemanticsFlags>{
     _kHasCheckedStateIndex: hasCheckedState,
     _kIsCheckedIndex: isChecked,
+    _kIsSelectedIndex: isSelected,
   };
 
   @override
@@ -148,6 +159,8 @@ class SemanticsFlags {
         return 'SemanticsFlags.hasCheckedState';
       case _kIsCheckedIndex:
         return 'SemanticsFlags.isChecked';
+      case _kIsSelectedIndex:
+        return 'SemanticsFlags.isSelected';
     }
     return null;
   }

engine/src/flutter/lib/ui/semantics/semantics_node.h

@@ -31,6 +31,7 @@ enum class SemanticsAction : int32_t {
 enum class SemanticsFlags : int32_t {
   kHasCheckedState = 1 << 0,
   kIsChecked = 1 << 1,
+  kIsSelected = 1 << 2,
 };
 
 struct SemanticsNode {

engine/src/flutter/shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -48,6 +48,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider {
 
     private static final int SEMANTICS_FLAG_HAS_CHECKED_STATE = 1 << 0;
     private static final int SEMANTICS_FLAG_IS_CHECKED = 1 << 1;
+    private static final int SEMANTICS_FLAG_IS_SELECTED = 1 << 2;
 
     AccessibilityBridge(FlutterView owner) {
         assert owner != null;
@@ -119,6 +120,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider {
 
         result.setCheckable((object.flags & SEMANTICS_FLAG_HAS_CHECKED_STATE) != 0);
         result.setChecked((object.flags & SEMANTICS_FLAG_IS_CHECKED) != 0);
+        result.setSelected((object.flags & SEMANTICS_FLAG_IS_SELECTED) != 0);
         result.setText(object.label);
 
         // TODO(ianh): use setTraversalBefore/setTraversalAfter to set

engine/src/flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm

@@ -116,6 +116,10 @@ blink::SemanticsAction GetSemanticsActionForScrollDirection(
       _node.HasAction(blink::SemanticsAction::kDecrease)) {
     traits |= UIAccessibilityTraitAdjustable;
   }
+  if (_node.HasFlag(blink::SemanticsFlags::kIsSelected) ||
+      _node.HasFlag(blink::SemanticsFlags::kIsChecked)) {
+    traits |= UIAccessibilityTraitSelected;
+  }
   return traits;
 }