Adds a widgets that blocks all semantics of widgets below it in paint order...

Adds a widgets that blocks all semantics of widgets below it in paint order within the same container (#10425)

packages/flutter/lib/src/material/drawer.dart

@@ -270,10 +270,12 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
         child: new RepaintBoundary(
           child: new Stack(
             children: <Widget>[
-              new GestureDetector(
-                onTap: close,
-                child: new Container(
-                  color: _color.evaluate(_controller)
+              new BlockSemantics(
+                child: new GestureDetector(
+                  onTap: close,
+                  child: new Container(
+                    color: _color.evaluate(_controller)
+                  ),
                 ),
               ),
               new Align(

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

@@ -662,7 +662,8 @@ abstract class _SemanticsFragment {
   _SemanticsFragment({
     @required RenderObject renderObjectOwner,
     this.annotator,
-    List<_SemanticsFragment> children
+    List<_SemanticsFragment> children,
+    this.dropSemanticsOfPreviousSiblings,
   }) {
     assert(renderObjectOwner != null);
     _ancestorChain = <RenderObject>[renderObjectOwner];
@@ -678,6 +679,9 @@ abstract class _SemanticsFragment {
   }
 
   final SemanticsAnnotator annotator;
+  bool dropSemanticsOfPreviousSiblings;
+
+  bool get producesSemanticNodes => true;
 
   List<RenderObject> _ancestorChain;
   void addAncestor(RenderObject ancestor) {
@@ -695,6 +699,20 @@ abstract class _SemanticsFragment {
   String toString() => '$runtimeType#$hashCode';
 }
 
+/// A SemanticsFragment that doesn't produce any [SemanticsNode]s when compiled.
+class _EmptySemanticsFragment extends _SemanticsFragment {
+  _EmptySemanticsFragment({
+    @required RenderObject renderObjectOwner,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
+
+  @override
+  Iterable<SemanticsNode> compile({ _SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics }) sync* { }
+
+  @override
+  bool get producesSemanticNodes => false;
+}
+
 /// Represents a [RenderObject] which is in no way dirty.
 ///
 /// This class has no children and no annotators, and when compiled, it returns
@@ -702,8 +720,9 @@ abstract class _SemanticsFragment {
 /// the matrix, since that comes from the (dirty) ancestors.)
 class _CleanSemanticsFragment extends _SemanticsFragment {
   _CleanSemanticsFragment({
-    @required RenderObject renderObjectOwner
-  }) : super(renderObjectOwner: renderObjectOwner) {
+    @required RenderObject renderObjectOwner,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings) {
     assert(renderObjectOwner != null);
     assert(renderObjectOwner._semantics != null);
   }
@@ -728,8 +747,9 @@ abstract class _InterestingSemanticsFragment extends _SemanticsFragment {
   _InterestingSemanticsFragment({
     RenderObject renderObjectOwner,
     SemanticsAnnotator annotator,
-    Iterable<_SemanticsFragment> children
-  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children);
+    Iterable<_SemanticsFragment> children,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
 
   bool get haveConcreteNode => true;
 
@@ -765,8 +785,9 @@ class _RootSemanticsFragment extends _InterestingSemanticsFragment {
   _RootSemanticsFragment({
     RenderObject renderObjectOwner,
     SemanticsAnnotator annotator,
-    Iterable<_SemanticsFragment> children
-  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children);
+    Iterable<_SemanticsFragment> children,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
 
   @override
   SemanticsNode establishSemanticsNode(_SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics) {
@@ -798,8 +819,9 @@ class _ConcreteSemanticsFragment extends _InterestingSemanticsFragment {
   _ConcreteSemanticsFragment({
     RenderObject renderObjectOwner,
     SemanticsAnnotator annotator,
-    Iterable<_SemanticsFragment> children
-  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children);
+    Iterable<_SemanticsFragment> children,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
 
   @override
   SemanticsNode establishSemanticsNode(_SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics) {
@@ -833,8 +855,9 @@ class _ImplicitSemanticsFragment extends _InterestingSemanticsFragment {
   _ImplicitSemanticsFragment({
     RenderObject renderObjectOwner,
     SemanticsAnnotator annotator,
-    Iterable<_SemanticsFragment> children
-  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children);
+    Iterable<_SemanticsFragment> children,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
 
   @override
   bool get haveConcreteNode => _haveConcreteNode;
@@ -878,8 +901,9 @@ class _ImplicitSemanticsFragment extends _InterestingSemanticsFragment {
 class _ForkingSemanticsFragment extends _SemanticsFragment {
   _ForkingSemanticsFragment({
     RenderObject renderObjectOwner,
-    @required Iterable<_SemanticsFragment> children
-  }) : super(renderObjectOwner: renderObjectOwner, children: children) {
+    @required Iterable<_SemanticsFragment> children,
+    bool dropSemanticsOfPreviousSiblings,
+  }) : super(renderObjectOwner: renderObjectOwner, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings) {
     assert(children != null);
     assert(children.length > 1);
   }
@@ -1414,6 +1438,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     super.adoptChild(child);
     markNeedsLayout();
     markNeedsCompositingBitsUpdate();
+    markNeedsSemanticsUpdate();
   }
 
   /// Called by subclasses when they decide a render object is no longer a child.
@@ -1431,6 +1456,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     super.dropChild(child);
     markNeedsLayout();
     markNeedsCompositingBitsUpdate();
+    markNeedsSemanticsUpdate();
   }
 
   /// Calls visitor for each immediate child of this render object.
@@ -2408,6 +2434,22 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   ///    setting [isSemanticBoundary] to true.
   bool get isSemanticBoundary => false;
 
+  /// Whether this [RenderObject] makes other [RenderObject]s previously painted
+  /// within the same semantic boundary unreachable for accessibility purposes.
+  ///
+  /// If `true` is returned, the [SemanticsNode]s for all siblings and cousins
+  /// of this node, that are earlier in a depth-first pre-order traversal, are
+  /// dropped from the semantics tree up until a semantic boundary (as defined
+  /// by [isSemanticBoundary]) is reached.
+  ///
+  /// If [isSemanticBoundary] and [isBlockingSemanticsOfPreviouslyPaintedNodes]
+  /// is set on the same node, all previously painted siblings and cousins
+  /// up until the next ancestor that is a semantic boundary are dropped.
+  ///
+  /// Paint order as established by [visitChildrenForSemantics] is used to
+  /// determine if a node is previous to this one.
+  bool get isBlockingSemanticsOfPreviouslyPaintedNodes => false;
+
   /// The bounding box, in the local coordinate system, of this
   /// object, for accessibility purposes.
   Rect get semanticBounds;
@@ -2545,9 +2587,10 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     // early-exit if we're not dirty and have our own semantics
     if (!_needsSemanticsUpdate && isSemanticBoundary) {
       assert(_semantics != null);
-      return new _CleanSemanticsFragment(renderObjectOwner: this);
+      return new _CleanSemanticsFragment(renderObjectOwner: this, dropSemanticsOfPreviousSiblings: isBlockingSemanticsOfPreviouslyPaintedNodes);
     }
     List<_SemanticsFragment> children;
+    bool dropSemanticsOfPreviousSiblings = isBlockingSemanticsOfPreviouslyPaintedNodes;
     visitChildrenForSemantics((RenderObject child) {
       if (_needsSemanticsGeometryUpdate) {
         // If our geometry changed, make sure the child also does a
@@ -2557,31 +2600,40 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
         child._needsSemanticsGeometryUpdate = true;
       }
       final _SemanticsFragment fragment = child._getSemanticsFragment();
-      if (fragment != null) {
+      assert(fragment != null);
+      if (fragment.dropSemanticsOfPreviousSiblings) {
+        children = null; // throw away all left siblings of [child].
+        dropSemanticsOfPreviousSiblings = true;
+      }
+      if (fragment.producesSemanticNodes) {
         fragment.addAncestor(this);
         children ??= <_SemanticsFragment>[];
         assert(!children.contains(fragment));
         children.add(fragment);
       }
     });
+    if (isSemanticBoundary && !isBlockingSemanticsOfPreviouslyPaintedNodes) {
+      // Don't propagate [dropSemanticsOfPreviousSiblings] up through a semantic boundary.
+      dropSemanticsOfPreviousSiblings = false;
+    }
     _needsSemanticsUpdate = false;
     _needsSemanticsGeometryUpdate = false;
     final SemanticsAnnotator annotator = semanticsAnnotator;
     if (parent is! RenderObject)
-      return new _RootSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children);
+      return new _RootSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
     if (isSemanticBoundary)
-      return new _ConcreteSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children);
+      return new _ConcreteSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
     if (annotator != null)
-      return new _ImplicitSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children);
+      return new _ImplicitSemanticsFragment(renderObjectOwner: this, annotator: annotator, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
     _semantics = null;
     if (children == null) {
       // Introduces no semantics and has no descendants that introduce semantics.
-      return null;
+      return new _EmptySemanticsFragment(renderObjectOwner: this, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
     }
     if (children.length > 1)
-      return new _ForkingSemanticsFragment(renderObjectOwner: this, children: children);
+      return new _ForkingSemanticsFragment(renderObjectOwner: this, children: children, dropSemanticsOfPreviousSiblings: dropSemanticsOfPreviousSiblings);
     assert(children.length == 1);
-    return children.single;
+    return children.single..dropSemanticsOfPreviousSiblings = dropSemanticsOfPreviousSiblings;
   }
 
   /// Called when collecting the semantics of this node.
@@ -2727,6 +2779,10 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       description.add('layer: $_layer');
     if (_semantics != null)
       description.add('semantics: $_semantics');
+    if (isBlockingSemanticsOfPreviouslyPaintedNodes)
+      description.add('blocks semantics of earlier render objects below the common boundary');
+    if (isSemanticBoundary)
+      description.add('semantic boundary');
   }
 
   /// Returns a string describing the current node's descendants. Each line of

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

@@ -2891,6 +2891,18 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
   }
 }
 
+/// Causes the semantics of all siblings and cousins painted before it in the
+/// same semantic container to be dropped.
+///
+/// This is useful in a stack where an overlay should prevent interactions
+/// with the underlying layers.
+class RenderBlockSemantics extends RenderProxyBox {
+  RenderBlockSemantics({ RenderBox child }) : super(child);
+
+  @override
+  bool get isBlockingSemanticsOfPreviouslyPaintedNodes => true;
+}
+
 /// Causes the semantics of all descendants to be merged into this
 /// node such that the entire subtree becomes a single leaf in the
 /// semantics tree.

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

@@ -3613,6 +3613,27 @@ class MergeSemantics extends SingleChildRenderObjectWidget {
   RenderMergeSemantics createRenderObject(BuildContext context) => new RenderMergeSemantics();
 }
 
+/// A widget that drops the semantics of all widget that were painted before it
+/// in the same semantic container.
+///
+/// This is useful to hide widgets from accessibility tools that are painted
+/// behind a certain widget, e.g. an alert should usually disallow interaction
+/// with any widget located "behind" the alert (even when they are still
+/// partially visible). Similarly, an open [Drawer] blocks interactions with
+/// any widget outside the drawer.
+///
+/// See also:
+///
+/// * [ExcludeSemantics] which drops all semantics of its descendants.
+class BlockSemantics extends SingleChildRenderObjectWidget {
+  /// Creates a widget that excludes the semantics of all widgets painted before
+  /// it in the same semantic container.
+  const BlockSemantics({ Key key, Widget child }) : super(key: key, child: child);
+
+  @override
+  RenderBlockSemantics createRenderObject(BuildContext context) => new RenderBlockSemantics();
+}
+
 /// A widget that drops all the semantics of its descendants.
 ///
 /// When [excluding] is true, this widget (and its subtree) is excluded from
@@ -3622,6 +3643,10 @@ class MergeSemantics extends SingleChildRenderObjectWidget {
 /// reported but that would only be confusing. For example, the
 /// material library's [Chip] widget hides the avatar since it is
 /// redundant with the chip label.
+///
+/// See also:
+///
+/// * [BlockSemantics] which drops semantics of widgets earlier in the tree.
 class ExcludeSemantics extends SingleChildRenderObjectWidget {
   /// Creates a widget that drops all the semantics of its descendants.
   const ExcludeSemantics({

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

@@ -26,21 +26,23 @@ class ModalBarrier extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new ExcludeSemantics(
-      excluding: !dismissible,
-      child: new Semantics(
-        container: true,
-        child: new GestureDetector(
-          onTapDown: (TapDownDetails details) {
-            if (dismissible)
-              Navigator.pop(context);
-          },
-          behavior: HitTestBehavior.opaque,
-          child: new ConstrainedBox(
-            constraints: const BoxConstraints.expand(),
-            child: color == null ? null : new DecoratedBox(
-              decoration: new BoxDecoration(
-                color: color
+    return new BlockSemantics(
+      child: new ExcludeSemantics(
+        excluding: !dismissible,
+        child: new Semantics(
+          container: true,
+          child: new GestureDetector(
+            onTapDown: (TapDownDetails details) {
+              if (dismissible)
+                Navigator.pop(context);
+            },
+            behavior: HitTestBehavior.opaque,
+            child: new ConstrainedBox(
+              constraints: const BoxConstraints.expand(),
+              child: color == null ? null : new DecoratedBox(
+                decoration: new BoxDecoration(
+                  color: color
+                )
               )
             )
           )

packages/flutter/test/material/dialog_test.dart

@@ -4,6 +4,9 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:matcher/matcher.dart';
+
+import '../widgets/semantics_tester.dart';
 
 void main() {
   testWidgets('Dialog is scrollable', (WidgetTester tester) async {
@@ -192,4 +195,38 @@ void main() {
     expect(find.text('Dialog2'), findsOneWidget);
 
   });
+
+  testWidgets('Dialog hides underlying semantics tree', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    const String buttonText = 'A button covered by dialog overlay';
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Material(
+          child: const Center(
+            child: const RaisedButton(
+              onPressed: null,
+              child: const Text(buttonText),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, includesNodeWithLabel(buttonText));
+
+    final BuildContext context = tester.element(find.text(buttonText));
+
+    const String alertText = 'A button in an overlay alert';
+    showDialog<Null>(
+      context: context,
+      child: const AlertDialog(title: const Text(alertText)),
+    );
+
+    await tester.pumpAndSettle(const Duration(seconds: 1));
+
+    expect(semantics, includesNodeWithLabel(alertText));
+    expect(semantics, isNot(includesNodeWithLabel(buttonText)));
+
+    semantics.dispose();
+  });
 }

packages/flutter/test/material/scaffold_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 
+import '../widgets/semantics_tester.dart';
+
 void main() {
   testWidgets('Scaffold control test', (WidgetTester tester) async {
     final Key bodyKey = new UniqueKey();
@@ -440,4 +442,40 @@ void main() {
       expect(tester.renderObject<RenderBox>(find.byKey(testKey)).localToGlobal(Offset.zero), const Offset(0.0, 0.0));
     });
   });
+
+  testWidgets('Open drawer hides underlying semantics tree', (WidgetTester tester) async {
+    const String bodyLabel = 'I am the body';
+    const String persistentFooterButtonLabel = 'a button on the bottom';
+    const String bottomNavigationBarLabel = 'a bar in an app';
+    const String floatingActionButtonLabel = 'I float in space';
+    const String drawerLabel = 'I am the reason for this test';
+
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(new MaterialApp(home: new Scaffold(
+      body: new Semantics(label: bodyLabel, child: new Container()),
+      persistentFooterButtons: <Widget>[new Semantics(label: persistentFooterButtonLabel, child: new Container())],
+      bottomNavigationBar: new Semantics(label: bottomNavigationBarLabel, child: new Container()),
+      floatingActionButton: new Semantics(label: floatingActionButtonLabel, child: new Container()),
+      drawer: new Drawer(child:new Semantics(label: drawerLabel, child: new Container())),
+    )));
+
+    expect(semantics, includesNodeWithLabel(bodyLabel));
+    expect(semantics, includesNodeWithLabel(persistentFooterButtonLabel));
+    expect(semantics, includesNodeWithLabel(bottomNavigationBarLabel));
+    expect(semantics, includesNodeWithLabel(floatingActionButtonLabel));
+    expect(semantics, isNot(includesNodeWithLabel(drawerLabel)));
+
+    final ScaffoldState state = tester.firstState(find.byType(Scaffold));
+    state.openDrawer();
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(semantics, isNot(includesNodeWithLabel(bodyLabel)));
+    expect(semantics, isNot(includesNodeWithLabel(persistentFooterButtonLabel)));
+    expect(semantics, isNot(includesNodeWithLabel(bottomNavigationBarLabel)));
+    expect(semantics, isNot(includesNodeWithLabel(floatingActionButtonLabel)));
+    expect(semantics, includesNodeWithLabel(drawerLabel));
+
+    semantics.dispose();
+  });
 }

packages/flutter/test/widgets/semantics_9_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'semantics_tester.dart';
+
+void main() {
+  group('BlockSemantics', () {
+    testWidgets('hides semantic nodes of siblings', (WidgetTester tester) async {
+      final SemanticsTester semantics = new SemanticsTester(tester);
+
+      await tester.pumpWidget(new Stack(
+        children: <Widget>[
+          new Semantics(
+            label: 'layer#1',
+            child: new Container(),
+          ),
+          const BlockSemantics(),
+          new Semantics(
+            label: 'layer#2',
+            child: new Container(),
+          ),
+        ],
+      ));
+
+      expect(semantics, isNot(includesNodeWithLabel('layer#1')));
+
+      await tester.pumpWidget(new Stack(
+        children: <Widget>[
+          new Semantics(
+            label: 'layer#1',
+            child: new Container(),
+          ),
+        ],
+      ));
+
+      expect(semantics, includesNodeWithLabel('layer#1'));
+
+      semantics.dispose();
+    });
+
+    testWidgets('does not hides semantic nodes of siblings outside the current semantic boundary', (WidgetTester tester) async {
+      final SemanticsTester semantics = new SemanticsTester(tester);
+
+      await tester.pumpWidget(new Stack(
+        children: <Widget>[
+          new Semantics(
+            label: '#1',
+            child: new Container(),
+          ),
+          new Semantics(
+            label: '#2',
+            container: true,
+            child: new Stack(
+              children: <Widget>[
+                new Semantics(
+                  label: 'NOT#2.1',
+                  child: new Container(),
+                ),
+                new Semantics(
+                  label: '#2.2',
+                  child: new BlockSemantics(
+                    child: new Semantics(
+                      container: true,
+                      label: '#2.2.1',
+                      child: new Container(),
+                    ),
+                  ),
+                ),
+                new Semantics(
+                  label: '#2.3',
+                  child: new Container(),
+                ),
+              ],
+            ),
+          ),
+          new Semantics(
+            label: '#3',
+            child: new Container(),
+          ),
+        ],
+      ));
+
+      expect(semantics, includesNodeWithLabel('#1'));
+      expect(semantics, includesNodeWithLabel('#2'));
+      expect(semantics, isNot(includesNodeWithLabel('NOT#2.1')));
+      expect(semantics, includesNodeWithLabel('#2.2'));
+      expect(semantics, includesNodeWithLabel('#2.2.1'));
+      expect(semantics, includesNodeWithLabel('#2.3'));
+      expect(semantics, includesNodeWithLabel('#3'));
+
+      semantics.dispose();
+    });
+
+    testWidgets('node is semantic boundary and blocking previously painted nodes', (WidgetTester tester) async {
+      final SemanticsTester semantics = new SemanticsTester(tester);
+      final GlobalKey stackKey = new GlobalKey();
+
+      await tester.pumpWidget(new Stack(
+        key: stackKey,
+        children: <Widget>[
+          new Semantics(
+            label: 'NOT#1',
+            child: new Container(),
+          ),
+          new BoundaryBlockSemantics(
+            child: new Semantics(
+              label: '#2.1',
+              child: new Container(),
+            )
+          ),
+          new Semantics(
+            label: '#3',
+            child: new Container(),
+          ),
+        ],
+      ));
+
+      expect(semantics, isNot(includesNodeWithLabel('NOT#1')));
+      expect(semantics, includesNodeWithLabel('#2.1'));
+      expect(semantics, includesNodeWithLabel('#3'));
+
+      semantics.dispose();
+    });
+  });
+}
+
+class BoundaryBlockSemantics extends SingleChildRenderObjectWidget {
+  const BoundaryBlockSemantics({ Key key, Widget child }) : super(key: key, child: child);
+
+  @override
+  RenderBoundaryBlockSemantics createRenderObject(BuildContext context) => new RenderBoundaryBlockSemantics();
+}
+
+class RenderBoundaryBlockSemantics extends RenderProxyBox {
+  RenderBoundaryBlockSemantics({ RenderBox child }) : super(child);
+
+  @override
+  bool get isBlockingSemanticsOfPreviouslyPaintedNodes => true;
+
+  @override
+  bool get isSemanticBoundary => true;
+}
+

packages/flutter/test/widgets/semantics_tester.dart

@@ -194,6 +194,8 @@ class SemanticsTester {
   String toString() => 'SemanticsTester';
 }
 
+const String _matcherHelp = 'Try dumping the semantics with debugDumpSemanticsTree() from the rendering library to see what the semantics tree looks like.';
+
 class _HasSemantics extends Matcher {
   const _HasSemantics(this._semantics) : assert(_semantics != null);
 
@@ -211,30 +213,65 @@ class _HasSemantics extends Matcher {
 
   @override
   Description describeMismatch(dynamic item, Description mismatchDescription, Map<dynamic, dynamic> matchState, bool verbose) {
-    const String help = 'Try dumping the semantics with debugDumpSemanticsTree() from the rendering library to see what the semantics tree looks like.';
     final TestSemantics testNode = matchState[TestSemantics];
     final SemanticsNode node = matchState[SemanticsNode];
     if (node == null)
-      return mismatchDescription.add('could not find node with id ${testNode.id}.\n$help');
+      return mismatchDescription.add('could not find node with id ${testNode.id}.\n$_matcherHelp');
     if (testNode.id != node.id)
-      return mismatchDescription.add('expected node id ${testNode.id} but found id ${node.id}.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} but found id ${node.id}.\n$_matcherHelp');
     final SemanticsData data = node.getSemanticsData();
     if (testNode.flags != data.flags)
-      return mismatchDescription.add('expected node id ${testNode.id} to have flags ${testNode.flags} but found flags ${data.flags}.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have flags ${testNode.flags} but found flags ${data.flags}.\n$_matcherHelp');
     if (testNode.actions != data.actions)
-      return mismatchDescription.add('expected node id ${testNode.id} to have actions ${testNode.actions} but found actions ${data.actions}.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have actions ${testNode.actions} but found actions ${data.actions}.\n$_matcherHelp');
     if (testNode.label != data.label)
-      return mismatchDescription.add('expected node id ${testNode.id} to have label "${testNode.label}" but found label "${data.label}".\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have label "${testNode.label}" but found label "${data.label}".\n$_matcherHelp');
     if (testNode.rect != data.rect)
-      return mismatchDescription.add('expected node id ${testNode.id} to have rect ${testNode.rect} but found rect ${data.rect}.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have rect ${testNode.rect} but found rect ${data.rect}.\n$_matcherHelp');
     if (testNode.transform != data.transform)
-      return mismatchDescription.add('expected node id ${testNode.id} to have transform ${testNode.transform} but found transform:.\n${data.transform}.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have transform ${testNode.transform} but found transform:.\n${data.transform}.\n$_matcherHelp');
     final int childrenCount = node.mergeAllDescendantsIntoThisNode ? 0 : node.childrenCount;
     if (testNode.children.length != childrenCount)
-      return mismatchDescription.add('expected node id ${testNode.id} to have ${testNode.children.length} children but found $childrenCount.\n$help');
+      return mismatchDescription.add('expected node id ${testNode.id} to have ${testNode.children.length} children but found $childrenCount.\n$_matcherHelp');
     return mismatchDescription;
   }
 }
 
 /// Asserts that a [SemanticsTester] has a semantics tree that exactly matches the given semantics.
 Matcher hasSemantics(TestSemantics semantics) => new _HasSemantics(semantics);
+
+class _IncludesNodeWithLabel extends Matcher {
+  const _IncludesNodeWithLabel(this._label) : assert(_label != null);
+
+  final String _label;
+
+  @override
+  bool matches(covariant SemanticsTester item, Map<dynamic, dynamic> matchState) {
+    bool result = false;
+    SemanticsNodeVisitor visitor;
+    visitor = (SemanticsNode node) {
+      if (node.label == _label) {
+        result = true;
+      } else {
+        node.visitChildren(visitor);
+      }
+      return !result;
+    };
+    final SemanticsNode root = item.tester.binding.pipelineOwner.semanticsOwner.rootSemanticsNode;
+    visitor(root);
+    return result;
+  }
+
+  @override
+  Description describe(Description description) {
+    return description.add('includes node with label "$_label"');
+  }
+
+  @override
+  Description describeMismatch(dynamic item, Description mismatchDescription, Map<dynamic, dynamic> matchState, bool verbose) {
+    return mismatchDescription.add('could not find node with label "$_label".\n$_matcherHelp');
+  }
+}
+
+/// Asserts that a node in the semantics tree of [SemanticsTester] has [label].
+Matcher includesNodeWithLabel(String label) => new _IncludesNodeWithLabel(label);