fix semantics to only send relevant node update (#60925)

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

@@ -4,7 +4,7 @@
 
 // @dart = 2.8
 
-import 'dart:ui' as ui show AccessibilityFeatures;
+import 'dart:ui' as ui show AccessibilityFeatures, SemanticsUpdateBuilder;
 
 import 'package:flutter/foundation.dart';
 
@@ -34,6 +34,16 @@ mixin SemanticsBinding on BindingBase {
     _accessibilityFeatures = window.accessibilityFeatures;
   }
 
+  /// Creates an empty semantics update builder.
+  ///
+  /// The caller is responsible for filling out the semantics node updates.
+  ///
+  /// This method is used by the [SemanticsOwner] to create builder for all its
+  /// semantics updates.
+  ui.SemanticsUpdateBuilder createSemanticsUpdateBuilder() {
+    return ui.SemanticsUpdateBuilder();
+  }
+
   /// The currently active set of [AccessibilityFeatures].
   ///
   /// This is initialized the first time [runApp] is called and updated whenever

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

@@ -2624,13 +2624,15 @@ class SemanticsOwner extends ChangeNotifier {
         if (node.isPartOfNodeMerging) {
           assert(node.mergeAllDescendantsIntoThisNode || node.parent != null);
           // if we're merged into our parent, make sure our parent is added to the dirty list
-          if (node.parent != null && node.parent.isPartOfNodeMerging)
+          if (node.parent != null && node.parent.isPartOfNodeMerging) {
             node.parent._markDirty(); // this can add the node to the dirty list
+            node._dirty = false; // We don't want to send update for this node.
+          }
         }
       }
     }
     visitedNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
-    final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
+    final ui.SemanticsUpdateBuilder builder = SemanticsBinding.instance.createSemanticsUpdateBuilder();
     for (final SemanticsNode node in visitedNodes) {
       assert(node.parent?._dirty != true); // could be null (no parent) or false (not dirty)
       // The _serialize() method marks the node as not dirty, and

packages/flutter/test/semantics/semantics_update_test.dart

@@ -0,0 +1,218 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.8
+
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  SemanticsUpdateTestBinding();
+
+  testWidgets('Semantics update does not send update for merged nodes.', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    // Pumps a placeholder to trigger the warm up frame.
+    await tester.pumpWidget(
+      const Placeholder(),
+      // Stops right after the warm up frame.
+      null,
+      EnginePhase.build
+    );
+    // The warm up frame will send update for an empty semantics tree. We
+    // ignore this one time update.
+    SemanticsUpdateBuilderSpy.observations.clear();
+
+    // Builds the real widget tree.
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MergeSemantics(
+          child: Semantics(
+            label: 'outer',
+            // This semantics node should not be part of the semantics update
+            // because it is under another semantics container.
+            child: Semantics(
+              label: 'inner',
+              container: true,
+              child: const Text('text'),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(SemanticsUpdateBuilderSpy.observations.length, 2);
+
+    expect(SemanticsUpdateBuilderSpy.observations.containsKey(0), isTrue);
+    expect(SemanticsUpdateBuilderSpy.observations[0].childrenInTraversalOrder.length, 1);
+    expect(SemanticsUpdateBuilderSpy.observations[0].childrenInTraversalOrder[0], 1);
+
+    expect(SemanticsUpdateBuilderSpy.observations.containsKey(1), isTrue);
+    expect(SemanticsUpdateBuilderSpy.observations[1].childrenInTraversalOrder.length, 0);
+    expect(SemanticsUpdateBuilderSpy.observations[1].label, 'outer\ninner\ntext');
+
+    SemanticsUpdateBuilderSpy.observations.clear();
+
+    // Updates the inner semantics label and verifies it only sends update for
+    // the merged parent.
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MergeSemantics(
+          child: Semantics(
+            label: 'outer',
+            // This semantics node should not be part of the semantics update
+            // because it is under another semantics container.
+            child: Semantics(
+              label: 'inner-updated',
+              container: true,
+              child: const Text('text'),
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(SemanticsUpdateBuilderSpy.observations.length, 1);
+
+    expect(SemanticsUpdateBuilderSpy.observations.containsKey(1), isTrue);
+    expect(SemanticsUpdateBuilderSpy.observations[1].childrenInTraversalOrder.length, 0);
+    expect(SemanticsUpdateBuilderSpy.observations[1].label, 'outer\ninner-updated\ntext');
+
+    SemanticsUpdateBuilderSpy.observations.clear();
+    handle.dispose();
+  });
+}
+
+class SemanticsUpdateTestBinding extends AutomatedTestWidgetsFlutterBinding {
+  @override
+  ui.SemanticsUpdateBuilder createSemanticsUpdateBuilder() {
+    return SemanticsUpdateBuilderSpy();
+  }
+}
+
+class SemanticsUpdateBuilderSpy extends ui.SemanticsUpdateBuilder {
+  static Map<int, SemanticsNodeUpdateObservation> observations = <int, SemanticsNodeUpdateObservation>{};
+
+  @override
+  void updateNode({
+    @required int id,
+    @required int flags,
+    @required int actions,
+    @required int maxValueLength,
+    @required int currentValueLength,
+    @required int textSelectionBase,
+    @required int textSelectionExtent,
+    @required int platformViewId,
+    @required int scrollChildren,
+    @required int scrollIndex,
+    @required double scrollPosition,
+    @required double scrollExtentMax,
+    @required double scrollExtentMin,
+    @required double elevation,
+    @required double thickness,
+    @required Rect rect,
+    @required String label,
+    @required String hint,
+    @required String value,
+    @required String increasedValue,
+    @required String decreasedValue,
+    TextDirection textDirection,
+    @required Float64List transform,
+    @required Int32List childrenInTraversalOrder,
+    @required Int32List childrenInHitTestOrder,
+    @required Int32List additionalActions,
+  }) {
+    // Makes sure we don't send the same id twice.
+    assert(!observations.containsKey(id));
+    observations[id] = SemanticsNodeUpdateObservation(
+      id: id,
+      flags: flags,
+      actions: actions,
+      maxValueLength: maxValueLength,
+      currentValueLength: currentValueLength,
+      textSelectionBase: textSelectionBase,
+      textSelectionExtent: textSelectionExtent,
+      platformViewId: platformViewId,
+      scrollChildren: scrollChildren,
+      scrollIndex: scrollIndex,
+      scrollPosition: scrollPosition,
+      scrollExtentMax: scrollExtentMax,
+      scrollExtentMin: scrollExtentMin,
+      elevation: elevation,
+      thickness: thickness,
+      rect: rect,
+      label: label,
+      hint: hint,
+      value: value,
+      increasedValue: increasedValue,
+      decreasedValue: decreasedValue,
+      textDirection: textDirection,
+      transform: transform,
+      childrenInTraversalOrder: childrenInTraversalOrder,
+      childrenInHitTestOrder: childrenInHitTestOrder,
+      additionalActions: additionalActions,
+    );
+  }
+}
+
+class SemanticsNodeUpdateObservation {
+  const SemanticsNodeUpdateObservation({
+    @required this.id,
+    @required this.flags,
+    @required this.actions,
+    @required this.maxValueLength,
+    @required this.currentValueLength,
+    @required this.textSelectionBase,
+    @required this.textSelectionExtent,
+    @required this.platformViewId,
+    @required this.scrollChildren,
+    @required this.scrollIndex,
+    @required this.scrollPosition,
+    @required this.scrollExtentMax,
+    @required this.scrollExtentMin,
+    @required this.elevation,
+    @required this.thickness,
+    @required this.rect,
+    @required this.label,
+    @required this.hint,
+    @required this.value,
+    @required this.increasedValue,
+    @required this.decreasedValue,
+    this.textDirection,
+    @required this.transform,
+    @required this.childrenInTraversalOrder,
+    @required this.childrenInHitTestOrder,
+    @required this.additionalActions,
+  });
+
+  final int id;
+  final int flags;
+  final int actions;
+  final int maxValueLength;
+  final int currentValueLength;
+  final int textSelectionBase;
+  final int textSelectionExtent;
+  final int platformViewId;
+  final int scrollChildren;
+  final int scrollIndex;
+  final double scrollPosition;
+  final double scrollExtentMax;
+  final double scrollExtentMin;
+  final double elevation;
+  final double thickness;
+  final Rect rect;
+  final String label;
+  final String hint;
+  final String value;
+  final String increasedValue;
+  final String decreasedValue;
+  final TextDirection textDirection;
+  final Float64List transform;
+  final Int32List childrenInTraversalOrder;
+  final Int32List childrenInHitTestOrder;
+  final Int32List additionalActions;
+}