Add SemanticsFlag.isDisabled (flutter/engine#4503)

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

@@ -145,6 +145,7 @@ class SemanticsFlags {
   static const int _kIsButtonIndex = 1 << 3;
   static const int _kIsTextFieldIndex = 1 << 4;
   static const int _kIsFocusedIndex = 1 << 5;
+  static const int _kIsDisabledIndex = 1 << 6;
 
   const SemanticsFlags._(this.index);
 
@@ -193,6 +194,16 @@ class SemanticsFlags {
   /// The focused element is usually the current receiver of keyboard inputs.
   static const SemanticsFlags isFocused = const SemanticsFlags._(_kIsFocusedIndex);
 
+  /// Whether the semantic node is currently disabled.
+  ///
+  /// A disabled element does not respond to user interaction. For example, a
+  /// button that currently does not respond to user interaction should be
+  /// marked as disabled.
+  ///
+  /// Elements, that never respond to user interactions (e.g. static text)
+  /// should not be marked as disabled.
+  static const SemanticsFlags isDisabled = const SemanticsFlags._(_kIsDisabledIndex);
+
   /// The possible semantics flags.
   ///
   /// The map's key is the [index] of the flag and the value is the flag itself.
@@ -203,6 +214,7 @@ class SemanticsFlags {
     _kIsButtonIndex: isButton,
     _kIsTextFieldIndex: isTextField,
     _kIsFocusedIndex: isFocused,
+    _kIsDisabledIndex: isDisabled,
   };
 
   @override
@@ -220,6 +232,8 @@ class SemanticsFlags {
         return 'SemanticsFlags.isTextField';
       case _kIsFocusedIndex:
         return 'SemanticsFlags.isFocused';
+      case _kIsDisabledIndex:
+        return 'SemanticsFlags.isDisabled';
     }
     return null;
   }

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

@@ -41,6 +41,7 @@ enum class SemanticsFlags : int32_t {
   kIsButton = 1 << 3,
   kIsTextField = 1 << 4,
   kIsFocused = 1 << 5,
+  kIsDisabled = 1 << 6,
 };
 
 struct SemanticsNode {

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

@@ -65,7 +65,8 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
         IS_SELECTED(1 << 2),
         IS_BUTTON(1 << 3),
         IS_TEXT_FIELD(1 << 4),
-        IS_FOCUSED(1 << 5);
+        IS_FOCUSED(1 << 5),
+        IS_DISABLED(1 << 6);
 
         Flag(int value) {
             this.value = value;
@@ -157,7 +158,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
         }
         result.setBoundsInScreen(bounds);
         result.setVisibleToUser(true);
-        result.setEnabled(true); // TODO(ianh): Expose disabled subtrees
+        result.setEnabled(!object.hasFlag(Flag.IS_DISABLED));
 
         if (object.hasAction(Action.TAP)) {
             result.addAction(AccessibilityNodeInfo.ACTION_CLICK);

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

@@ -194,6 +194,9 @@ bool GeometryComparator(SemanticsObject* a, SemanticsObject* b) {
   if (_node.HasFlag(blink::SemanticsFlags::kIsButton)) {
     traits |= UIAccessibilityTraitButton;
   }
+  if (_node.HasFlag(blink::SemanticsFlags::kIsDisabled)) {
+    traits |= UIAccessibilityTraitNotEnabled;
+  }
   return traits;
 }