Add live region semantics flag

lib/ui/semantics.dart

@@ -228,6 +228,7 @@ class SemanticsFlag {
   static const int _kScopesRouteIndex= 1 << 11;
   static const int _kNamesRouteIndex = 1 << 12;
   static const int _kIsHiddenIndex = 1 << 13;
+  static const int _kIsLiveRegionIndex = 1 << 14;
 
   const SemanticsFlag._(this.index);
 
@@ -366,6 +367,21 @@ class SemanticsFlag {
   /// used to implement accessibility scrolling on iOS.
   static const SemanticsFlag isHidden = const SemanticsFlag._(_kIsHiddenIndex);
 
+  /// Whether the semantics node is a live region.
+  /// 
+  /// A live region indicates that this semantics node will update its 
+  /// semantics label. Platforms are free to use this information to
+  /// make polite updates to the user to inform them of this.
+  /// 
+  /// On Android, TalkBack will make a polite announcement of the first and
+  /// subsequent updates to the label of this node. This flag is not currently
+  /// supported on iOS.
+  /// 
+  /// An example of a live region is a [SnackBar] widget. When it appears
+  /// on the screen it may be difficult to focus to read the value. A live
+  /// region causes a polite announcement to be generated automatically.
+  static const SemanticsFlag isLiveRegion = const SemanticsFlag._(_kIsLiveRegionIndex);
+
   /// The possible semantics flags.
   ///
   /// The map's key is the [index] of the flag and the value is the flag itself.
@@ -384,6 +400,7 @@ class SemanticsFlag {
     _kScopesRouteIndex: scopesRoute,
     _kNamesRouteIndex: namesRoute,
     _kIsHiddenIndex: isHidden,
+    _kIsLiveRegionIndex: isLiveRegion,
   };
 
   @override
@@ -417,6 +434,8 @@ class SemanticsFlag {
         return 'SemanticsFlag.namesRoute';
       case _kIsHiddenIndex:
         return 'SemanticsFlag.isHidden';
+      case _kIsLiveRegionIndex:
+        return 'SemanticsFlag.isLiveRegion';
     }
     return null;
   }

lib/ui/semantics/semantics_node.h

@@ -59,6 +59,7 @@ enum class SemanticsFlags : int32_t {
   kScopesRoute = 1 << 11,
   kNamesRoute = 1 << 12,
   kIsHidden = 1 << 13,
+  kIsLiveRegion = 1 << 14,
 };
 
 struct SemanticsNode {

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

@@ -41,6 +41,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
     private static final int ROOT_NODE_ID = 0;
 
     private Map<Integer, SemanticsObject> mObjects;
+    private Map<Integer, String> mLiveRegions;
     private final FlutterView mOwner;
     private boolean mAccessibilityEnabled = false;
     private SemanticsObject mA11yFocusedObject;
@@ -91,7 +92,8 @@ enum Flag {
         IS_OBSCURED(1 << 10),
         SCOPES_ROUTE(1 << 11),
         NAMES_ROUTE(1 << 12),
-        IS_HIDDEN(1 << 13);
+        IS_HIDDEN(1 << 13),
+        IS_LIVE_REGION(1 << 14);
 
         Flag(int value) {
             this.value = value;
@@ -104,6 +106,7 @@ enum Flag {
         assert owner != null;
         mOwner = owner;
         mObjects = new HashMap<Integer, SemanticsObject>();
+        mLiveRegions = new HashMap<Integer, String>();
         previousRoutes = new ArrayList<>();
         mFlutterAccessibilityChannel = new BasicMessageChannel<>(owner, "flutter/accessibility",
             StandardMessageCodec.INSTANCE);
@@ -238,7 +241,20 @@ public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
                 result.addAction(AccessibilityNodeInfo.ACTION_SCROLL_BACKWARD);
             }
         }
-
+        if (object.hasFlag(Flag.IS_LIVE_REGION)) {
+            result.setLiveRegion(View.ACCESSIBILITY_LIVE_REGION_POLITE);
+            String priorLabelHint = mLiveRegions.get(object.id);
+            String labelHint = object.getLabelHint();
+            // It is very important that this is added to mLiveRegions before sending the accessibility event
+            // below.
+            mLiveRegions.put(object.id, labelHint);
+            if (priorLabelHint == null || !priorLabelHint.equals(labelHint)) {
+                sendAccessibilityEvent(object.id, AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED);
+            }
+        } else if (object.hadFlag(Flag.IS_LIVE_REGION)) {
+            mLiveRegions.remove(object.id);
+        }
+
         boolean hasCheckedState = object.hasFlag(Flag.HAS_CHECKED_STATE);
         result.setCheckable(hasCheckedState);
         if (hasCheckedState) {
@@ -499,7 +515,7 @@ void updateSemantics(ByteBuffer buffer, String[] strings) {
           rootObject.updateRecursively(identity, visitedObjects, false);
           rootObject.collectRoutes(newRoutes);
         }
-
+        
         // Dispatch a TYPE_WINDOW_STATE_CHANGED event if the most recent route id changed from the
         // previously cached route id.
         SemanticsObject lastAdded = null;
@@ -705,6 +721,7 @@ private void createWindowChangeEvent(SemanticsObject route) {
     private void willRemoveSemanticsObject(SemanticsObject object) {
         assert mObjects.containsKey(object.id);
         assert mObjects.get(object.id) == object;
+        mLiveRegions.remove(object.id);
         object.parent = null;
         if (mA11yFocusedObject == object) {
             sendAccessibilityEvent(mA11yFocusedObject.id, AccessibilityEvent.TYPE_VIEW_ACCESSIBILITY_FOCUS_CLEARED);
@@ -1061,6 +1078,20 @@ private float max(float a, float b, float c, float d) {
             return Math.max(a, Math.max(b, Math.max(c, d)));
         }
 
+        private String getLabelHint() {
+            StringBuilder sb = new StringBuilder();
+            if (label != null) {
+                sb.append(label);
+            }
+            if (sb.length() > 0) {
+                sb.append(", ");
+            }
+            if (hint != null) {
+                sb.append(hint);
+            }
+            return sb.length() > 0 ? sb.toString() : null;
+        }
+
         private String getValueLabelHint() {
             StringBuilder sb = new StringBuilder();
             String[] array = { value, label, hint };