Add excludeSemantics flag to semantics widget (#19650)

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

@@ -3166,6 +3166,7 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     RenderBox child,
     bool container = false,
     bool explicitChildNodes,
+    bool excludeSemantics = false,
     bool enabled,
     bool checked,
     bool toggled,
@@ -3210,6 +3211,7 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
   }) : assert(container != null),
        _container = container,
        _explicitChildNodes = explicitChildNodes,
+       _excludeSemantics = excludeSemantics,
        _enabled = enabled,
        _checked = checked,
        _toggled = toggled,
@@ -3291,6 +3293,22 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     markNeedsSemanticsUpdate();
   }
 
+  /// Whether decendants of this [RenderObject] should have their semantic
+  /// information ignored.
+  ///
+  /// When this flag is set to true, all child semantics nodes are ignored.
+  /// This can be used as a convenience for cases where a child is wrapped in
+  /// an [ExcludeSemantics] widget and then another [Semantics] widget.
+  bool get excludeSemantics => _excludeSemantics;
+  bool _excludeSemantics;
+  set excludeSemantics(bool value) {
+    assert(value != null);
+    if (_excludeSemantics == value)
+      return;
+    _excludeSemantics = value;
+    markNeedsSemanticsUpdate();
+  }
+
   /// If non-null, sets the [SemanticsNode.hasCheckedState] semantic to true and
   /// the [SemanticsNode.isChecked] semantic to the given value.
   bool get checked => _checked;
@@ -3900,6 +3918,14 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     markNeedsSemanticsUpdate();
   }
 
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (excludeSemantics)
+      return;
+    super.visitChildrenForSemantics(visitor);
+  }
+
+
   @override
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);

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

@@ -5077,6 +5077,7 @@ class Semantics extends SingleChildRenderObjectWidget {
     Widget child,
     bool container = false,
     bool explicitChildNodes = false,
+    bool excludeSemantics = false,
     bool enabled,
     bool checked,
     bool selected,
@@ -5122,6 +5123,7 @@ class Semantics extends SingleChildRenderObjectWidget {
     child: child,
     container: container,
     explicitChildNodes: explicitChildNodes,
+    excludeSemantics: excludeSemantics,
     properties: new SemanticsProperties(
       enabled: enabled,
       checked: checked,
@@ -5174,6 +5176,7 @@ class Semantics extends SingleChildRenderObjectWidget {
     Widget child,
     this.container = false,
     this.explicitChildNodes = false,
+    this.excludeSemantics = false,
     @required this.properties,
   }) : assert(container != null),
        assert(properties != null),
@@ -5210,11 +5213,21 @@ class Semantics extends SingleChildRenderObjectWidget {
   /// to create semantic boundaries that are either writable or not for children.
   final bool explicitChildNodes;
 
+  /// Whether to replace all child semantics with this node.
+  ///
+  /// Defaults to false.
+  ///
+  /// When this flag is set to true, all child semantics nodes are ignored.
+  /// This can be used as a convenience for cases where a child is wrapped in
+  /// an [ExcludeSemantics] widget and then another [Semantics] widget.
+  final bool excludeSemantics;
+
   @override
   RenderSemanticsAnnotations createRenderObject(BuildContext context) {
     return new RenderSemanticsAnnotations(
       container: container,
       explicitChildNodes: explicitChildNodes,
+      excludeSemantics: excludeSemantics,
       enabled: properties.enabled,
       checked: properties.checked,
       toggled: properties.toggled,
@@ -5275,6 +5288,7 @@ class Semantics extends SingleChildRenderObjectWidget {
     renderObject
       ..container = container
       ..explicitChildNodes = explicitChildNodes
+      ..excludeSemantics = excludeSemantics
       ..scopesRoute = properties.scopesRoute
       ..enabled = properties.enabled
       ..checked = properties.checked

packages/flutter/test/widgets/semantics_test.dart

@@ -1020,6 +1020,31 @@ void main() {
     handle.dispose();
     semantics.dispose();
   });
+
+  testWidgets('Semantics excludeSemantics ignores children', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(new Semantics(
+      label: 'label',
+      excludeSemantics: true,
+      textDirection: TextDirection.ltr,
+      child: new Semantics(
+        label: 'other label',
+        textDirection: TextDirection.ltr,
+      ),
+    ));
+
+    expect(semantics, hasSemantics(
+      new TestSemantics(
+        children: <TestSemantics>[
+          new TestSemantics(
+            label: 'label',
+            textDirection: TextDirection.ltr,
+          ),
+        ],
+      ), ignoreId: true, ignoreRect: true, ignoreTransform: true)
+    );
+    semantics.dispose();
+  });
 }
 
 class CustomSortKey extends OrdinalSortKey {