Teach the system to shutdown semantics (#4811)

We now stop updating semantics when there are no remaining clients.

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

@@ -90,18 +90,12 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   PipelineOwner _pipelineOwner;
 
   /// The render tree that's attached to the output surface.
-  RenderView get renderView => _renderView;
-  RenderView _renderView;
+  RenderView get renderView => _pipelineOwner.rootRenderObject;
   /// Sets the given [RenderView] object (which must not be null), and its tree, to
   /// be the new render tree to display. The previous tree, if any, is detached.
   set renderView(RenderView value) {
     assert(value != null);
-    if (_renderView == value)
-      return;
-    if (_renderView != null)
-      _renderView.detach();
-    _renderView = value;
-    _renderView.attach(pipelineOwner);
+    _pipelineOwner.rootRenderObject = value;
   }
 
   /// Called when the system metrics change.
@@ -134,18 +128,15 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   /// Called automatically when the binding is created.
   void initSemantics() {
     shell.provideService(mojom.SemanticsServer.serviceName, (core.MojoMessagePipeEndpoint endpoint) {
-      ensureSemantics();
-      mojom.SemanticsServerStub server = new mojom.SemanticsServerStub.fromEndpoint(endpoint);
-      server.impl = new SemanticsServer(semanticsOwner: pipelineOwner.semanticsOwner);
+      mojom.SemanticsServerStub stub = new mojom.SemanticsServerStub.fromEndpoint(endpoint);
+      SemanticsServer server = new SemanticsServer(pipelineOwner);
+      stub.impl = server;
+      stub.ctrl.onError = (_) {
+        server.dispose();
+      };
     });
   }
 
-  void ensureSemantics() {
-    if (pipelineOwner.semanticsOwner == null)
-      renderView.scheduleInitialSemantics();
-    assert(pipelineOwner.semanticsOwner != null);
-  }
-
   void _handlePersistentFrameCallback(Duration timeStamp) {
     beginFrame();
   }
@@ -165,7 +156,7 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   @override
   void reassembleApplication() {
     super.reassembleApplication();
-    pipelineOwner.reassemble(renderView);
+    pipelineOwner.reassemble();
   }
 
   @override
@@ -202,6 +193,44 @@ void debugDumpSemanticsTree() {
   debugPrint(RendererBinding.instance?.renderView?.debugSemantics?.toStringDeep() ?? 'Semantics not collected.');
 }
 
+/// Exposes the [SemanticsNode] tree to the underlying platform.
+class SemanticsServer extends mojom.SemanticsServer {
+  /// Creates a semantics server that listens to semantic informationa about the
+  /// given [PipelineOwner].
+  ///
+  /// Call [dispose] to stop listening for semantic updates.
+  SemanticsServer(PipelineOwner pipelineOwner) {
+    _semanticsOwner = pipelineOwner.addSemanticsListener(_updateSemanticsTree);
+  }
+
+  SemanticsOwner _semanticsOwner;
+  final List<mojom.SemanticsListenerProxy> _listeners = <mojom.SemanticsListenerProxy>[];
+
+  /// Stops listening for semantic updates and closes all outstanding listeners.
+  void dispose() {
+    for (mojom.SemanticsListenerProxy listener in _listeners)
+      listener.close();
+    _listeners.clear();
+    _semanticsOwner.removeListener(_updateSemanticsTree);
+    _semanticsOwner = null;
+  }
+
+  void _updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
+    for (mojom.SemanticsListenerProxy listener in _listeners)
+      listener.updateSemanticsTree(nodes);
+  }
+
+  @override
+  void addSemanticsListener(mojom.SemanticsListenerProxy listener) {
+    _listeners.add(listener);
+  }
+
+  @override
+  void performAction(int id, mojom.SemanticAction encodedAction) {
+    _semanticsOwner.performAction(id, SemanticAction.values[encodedAction.mojoEnumValue]);
+  }
+}
+
 /// A concrete binding for applications that use the Rendering framework
 /// directly. This is the glue that binds the framework to the Flutter engine.
 ///

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

@@ -793,6 +793,41 @@ class PipelineOwner {
       onNeedVisualUpdate();
   }
 
+  /// The unique render object managed by this pipeline that has no parent.
+  RenderObject get rootRenderObject => _rootRenderObject;
+  RenderObject _rootRenderObject;
+  set rootRenderObject(RenderObject value) {
+    if (_rootRenderObject == value)
+      return;
+    _rootRenderObject?.detach();
+    _rootRenderObject = value;
+    _rootRenderObject?.attach(this);
+  }
+
+  /// Calls the given listener whenever the semantics of the render tree change.
+  ///
+  /// Creates [semanticsOwner] if necessary.
+  SemanticsOwner addSemanticsListener(SemanticsListener listener) {
+    if (_semanticsOwner == null) {
+      _semanticsOwner = new SemanticsOwner(
+        initialListener: listener,
+        onLastListenerRemoved: _handleLastSemanticsListenerRemoved
+      );
+      _rootRenderObject.scheduleInitialSemantics();
+    } else {
+      _semanticsOwner.addListener(listener);
+    }
+    assert(_semanticsOwner != null);
+    return _semanticsOwner;
+  }
+
+  void _handleLastSemanticsListenerRemoved() {
+    assert(!_debugDoingSemantics);
+    rootRenderObject._clearSemantics();
+    _semanticsOwner.dispose();
+    _semanticsOwner = null;
+  }
+
   List<RenderObject> _nodesNeedingLayout = <RenderObject>[];
 
   /// Whether this pipeline is currently in the layout phase.
@@ -883,7 +918,7 @@ class PipelineOwner {
   SemanticsOwner get semanticsOwner => _semanticsOwner;
   SemanticsOwner _semanticsOwner;
   bool _debugDoingSemantics = false;
-  List<RenderObject> _nodesNeedingSemantics = <RenderObject>[];
+  final List<RenderObject> _nodesNeedingSemantics = <RenderObject>[];
 
   /// Update the semantics for all render objects.
   ///
@@ -912,17 +947,14 @@ class PipelineOwner {
     _semanticsOwner.sendSemanticsTree();
   }
 
-  /// Cause the entire subtree rooted at the given [RenderObject] to
-  /// be entirely reprocessed. This is used by development tools when
-  /// the application code has changed, to cause the rendering tree to
-  /// pick up any changed implementations.
+  /// Cause the entire render tree rooted at [rootRenderObject] to be entirely
+  /// reprocessed. This is used by development tools when the application code
+  /// has changed, to cause the rendering tree to pick up any changed
+  /// implementations.
   ///
-  /// This is expensive and should not be called except during
-  /// development.
-  void reassemble(RenderObject root) {
-    assert(root.parent is! RenderObject);
-    assert(root.owner == this);
-    root._reassemble();
+  /// This is expensive and should not be called except during development.
+  void reassemble() {
+    _rootRenderObject?._reassemble();
   }
 }
 
@@ -1862,8 +1894,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     assert(!owner._debugDoingSemantics);
     assert(_semantics == null);
     assert(_needsSemanticsUpdate);
-    assert(owner._semanticsOwner == null);
-    owner._semanticsOwner = new SemanticsOwner();
+    assert(owner._semanticsOwner != null);
     owner._nodesNeedingSemantics.add(this);
     owner.requestVisualUpdate();
   }
@@ -1896,6 +1927,16 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     return result;
   }
 
+  /// Removes all semantics from this render object and its descendants.
+  void _clearSemantics() {
+    _needsSemanticsUpdate = true;
+    _needsSemanticsGeometryUpdate = true;
+    _semantics = null;
+    visitChildren((RenderObject child) {
+      child._clearSemantics();
+    });
+  }
+
   /// Mark this node as needing an update to its semantics
   /// description.
   ///

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

@@ -455,32 +455,68 @@ class SemanticsNode extends AbstractNode {
   }
 }
 
+/// Signature for functions that receive updates about render tree semantics.
+typedef void SemanticsListener(List<mojom.SemanticsNode> nodes);
+
+/// Owns [SemanticsNode] objects and notifies listeners of changes to the
+/// render tree semantics.
+///
+/// To listen for semantic updates, call [PipelineOwner.addSemanticsListener],
+/// which will create a [SemanticsOwner] if necessary.
 class SemanticsOwner {
+  /// Creates a [SemanticsOwner].
+  ///
+  /// The `onLastListenerRemoved` argument must not be null and will be called
+  /// when the last listener is removed from this object.
+  SemanticsOwner({
+    @required SemanticsListener initialListener,
+    @required VoidCallback onLastListenerRemoved
+  }) : _onLastListenerRemoved = onLastListenerRemoved {
+    assert(_onLastListenerRemoved != null);
+    addListener(initialListener);
+  }
+
+  final VoidCallback _onLastListenerRemoved;
+
   final List<SemanticsNode> _dirtyNodes = <SemanticsNode>[];
   final Map<int, SemanticsNode> _nodes = <int, SemanticsNode>{};
   final Set<SemanticsNode> _detachedNodes = new Set<SemanticsNode>();
 
-  List<mojom.SemanticsListener> _listeners;
+  final List<SemanticsListener> _listeners = <SemanticsListener>[];
 
-  /// Whether there are currently any consumers of semantic data.
+  /// Releases any resources retained by this object.
   ///
-  /// If there are no consumers of semantic data, there is no need to compile
-  /// semantic data into a [SemanticsNode] tree.
-  bool get hasListeners => _listeners != null && _listeners.length > 0;
+  /// Requires that there are no listeners registered with [addListener].
+  void dispose() {
+    assert(_listeners.isEmpty);
+    _dirtyNodes.clear();
+    _nodes.clear();
+    _detachedNodes.clear();
+  }
 
   /// Add a consumer of semantic data.
   ///
   /// After the [PipelineOwner] updates the semantic data for a given frame, it
   /// calls [sendSemanticsTree], which uploads the data to each listener
   /// registered with this function.
-  void addListener(mojom.SemanticsListener listener) {
-    _listeners ??= <mojom.SemanticsListener>[];
+  ///
+  /// Listeners can be removed with [removeListener].
+  void addListener(SemanticsListener listener) {
     _listeners.add(listener);
   }
 
+  /// Removes a consumer of semantic data.
+  ///
+  /// Listeners can be added with [addListener].
+  void removeListener(SemanticsListener listener) {
+    _listeners.remove(listener);
+    if (_listeners.isEmpty)
+      _onLastListenerRemoved();
+  }
+
   /// Uploads the semantics tree to the listeners registered with [addListener].
   void sendSemanticsTree() {
-    assert(hasListeners);
+    assert(_listeners.isNotEmpty);
     for (SemanticsNode oldNode in _detachedNodes) {
       // The other side will have forgotten this node if we even send
       // it again, so make sure to mark it dirty so that it'll get
@@ -540,8 +576,8 @@ class SemanticsOwner {
       if (node._dirty && node.attached)
         updatedNodes.add(node._serialize());
     }
-    for (mojom.SemanticsListener listener in _listeners)
-      listener.updateSemanticsTree(updatedNodes);
+    for (SemanticsListener listener in new List<SemanticsListener>.from(_listeners))
+      listener(updatedNodes);
     _dirtyNodes.clear();
   }
 
@@ -562,29 +598,12 @@ class SemanticsOwner {
     return result._actionHandler;
   }
 
+  /// Asks the [SemanticsNode] with the given id to perform the given action.
+  ///
+  /// If the [SemanticsNode] has not indicated that it can perform the action,
+  /// this function does nothing.
   void performAction(int id, SemanticAction action) {
     SemanticActionHandler handler = _getSemanticActionHandlerForId(id, action: action);
     handler?.performAction(action);
   }
 }
-
-/// Exposes the [SemanticsNode] tree to the underlying platform.
-class SemanticsServer extends mojom.SemanticsServer {
-  SemanticsServer({ @required this.semanticsOwner }) {
-    assert(semanticsOwner != null);
-  }
-
-  final SemanticsOwner semanticsOwner;
-
-  @override
-  void addSemanticsListener(mojom.SemanticsListenerProxy listener) {
-    // TODO(abarth): We should remove the listener when this pipe closes.
-    // See <https://github.com/flutter/flutter/issues/3342>.
-    semanticsOwner.addListener(listener);
-  }
-
-  @override
-  void performAction(int id, mojom.SemanticAction encodedAction) {
-    semanticsOwner.performAction(id, SemanticAction.values[encodedAction.mojoEnumValue]);
-  }
-}

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

@@ -40,14 +40,15 @@ class _SemanticsDebuggerState extends State<SemanticsDebugger> {
     // static here because we might not be in a tree that's attached to that
     // binding. Instead, we should find a way to get to the PipelineOwner from
     // the BuildContext.
-    WidgetsBinding.instance.ensureSemantics();
-    _client = new _SemanticsClient(WidgetsBinding.instance.pipelineOwner.semanticsOwner)
+    _client = new _SemanticsClient(WidgetsBinding.instance.pipelineOwner)
       ..addListener(_update);
   }
 
   @override
   void dispose() {
-    _client.removeListener(_update);
+    _client
+      ..removeListener(_update)
+      ..dispose();
     super.dispose();
   }
 
@@ -310,12 +311,19 @@ class _SemanticsDebuggerEntry {
   }
 }
 
-class _SemanticsClient extends ChangeNotifier implements mojom.SemanticsListener {
-  _SemanticsClient(this.semanticsOwner) {
-    semanticsOwner.addListener(this);
+class _SemanticsClient extends ChangeNotifier {
+  _SemanticsClient(PipelineOwner pipelineOwner) {
+    _semanticsOwner = pipelineOwner.addSemanticsListener(_updateSemanticsTree);
   }
 
-  final SemanticsOwner semanticsOwner;
+  SemanticsOwner _semanticsOwner;
+
+  @override
+  void dispose() {
+    _semanticsOwner.removeListener(_updateSemanticsTree);
+    _semanticsOwner = null;
+    super.dispose();
+  }
 
   _SemanticsDebuggerEntry get rootNode => _nodes[0];
   final Map<int, _SemanticsDebuggerEntry> _nodes = <int, _SemanticsDebuggerEntry>{};
@@ -353,8 +361,7 @@ class _SemanticsClient extends ChangeNotifier implements mojom.SemanticsListener
 
   int generation = 0;
 
-  @override
-  void updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
+  void _updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
     generation += 1;
     for (mojom.SemanticsNode node in nodes)
       _updateNode(node);
@@ -368,7 +375,7 @@ class _SemanticsClient extends ChangeNotifier implements mojom.SemanticsListener
 
   void _performAction(Point position, SemanticAction action) {
     _SemanticsDebuggerEntry entry = _hitTest(position, (_SemanticsDebuggerEntry entry) => entry.actions.contains(action));
-    semanticsOwner.performAction(entry?.id ?? 0, action);
+    _semanticsOwner.performAction(entry?.id ?? 0, action);
   }
 
   void handlePanEnd(Point position, Velocity velocity) {

packages/flutter/test/material/tooltip_test.dart

@@ -7,7 +7,7 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import '../widget/test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 // This file uses "as dynamic" in a few places to defeat the static
 // analysis. In general you want to avoid using this style in your
@@ -394,7 +394,7 @@ void main() {
   });
 
   testWidgets('Does tooltip contribute semantics', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
     GlobalKey key = new GlobalKey();
     await tester.pumpWidget(
       new Overlay(
@@ -419,7 +419,7 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -431,14 +431,13 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // before using "as dynamic" in your code, see note top of file
     (key.currentState as dynamic).ensureTooltipVisible(); // this triggers a rebuild of the semantics because the tree changes
 
     await tester.pump(const Duration(seconds: 2)); // faded in, show timer started (and at 0.0)
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -450,7 +449,7 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
+    client.dispose();
   });
 }

packages/flutter/test/rendering/reattach_test.dart

@@ -4,10 +4,10 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
-import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 import 'package:test/test.dart';
 
 import 'rendering_tester.dart';
+import 'test_semantics_client.dart';
 
 class TestTree {
   TestTree() {
@@ -77,15 +77,6 @@ class TestCompositingBitsTree {
   bool painted = false;
 }
 
-class TestSemanticsListener implements mojom.SemanticsListener {
-  final List<mojom.SemanticsNode> updates = <mojom.SemanticsNode>[];
-
-  @override
-  void updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
-    updates.addAll(nodes);
-  }
-}
-
 void main() {
   test('objects can be detached and re-attached: layout', () {
     TestTree testTree = new TestTree();
@@ -135,21 +126,20 @@ void main() {
   });
   test('objects can be detached and re-attached: semantics', () {
     TestTree testTree = new TestTree();
-    TestSemanticsListener listener = new TestSemanticsListener();
-    renderer.ensureSemantics();
-    renderer.pipelineOwner.semanticsOwner.addListener(listener);
+    TestSemanticsClient client = new TestSemanticsClient(renderer.pipelineOwner);
     // Lay out, composite, paint, and update semantics
     layout(testTree.root, phase: EnginePhase.flushSemantics);
-    expect(listener.updates.length, equals(1));
+    expect(client.updates.length, equals(1));
     // Remove testTree from the custom render view
     renderer.renderView.child = null;
     expect(testTree.child.owner, isNull);
     // Dirty one of the elements
-    listener.updates.clear();
+    client.updates.clear();
     testTree.child.markNeedsSemanticsUpdate();
-    expect(listener.updates.length, equals(0));
+    expect(client.updates.length, equals(0));
     // Lay out, composite, paint, and update semantics again
     layout(testTree.root, phase: EnginePhase.flushSemantics);
-    expect(listener.updates.length, equals(1));
+    expect(client.updates.length, equals(1));
+    client.dispose();
   });
 }

packages/flutter/test/widget/test_semantics.dart → packages/flutter/test/rendering/test_semantics_client.dart

@@ -2,21 +2,24 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/rendering.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
-class TestSemanticsListener implements mojom.SemanticsListener {
-  TestSemanticsListener(WidgetTester tester) {
-    tester.binding.ensureSemantics();
-    tester.binding.pipelineOwner.semanticsOwner.addListener(this);
+class TestSemanticsClient {
+  TestSemanticsClient(PipelineOwner pipelineOwner) {
+    _semanticsOwner = pipelineOwner.addSemanticsListener(_updateSemanticsTree);
+  }
+
+  SemanticsOwner _semanticsOwner;
+
+  void dispose() {
+    _semanticsOwner.removeListener(_updateSemanticsTree);
+    _semanticsOwner = null;
   }
 
   final List<mojom.SemanticsNode> updates = <mojom.SemanticsNode>[];
 
-  @override
-  void updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
-    assert(!nodes.any((mojom.SemanticsNode node) => node == null));
+  void _updateSemanticsTree(List<mojom.SemanticsNode> nodes) {
     updates.addAll(nodes);
-    updates.add(null);
   }
 }

packages/flutter/test/widget/buttons_test.dart

@@ -7,11 +7,11 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Does FlatButton contribute semantics', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
     await tester.pumpWidget(
       new Material(
         child: new Center(
@@ -22,7 +22,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -40,7 +40,7 @@ void main() {
     expect(client.updates[0].children[0].flags.isChecked, isFalse);
     expect(client.updates[0].children[0].strings.label, equals('Hello'));
     expect(client.updates[0].children[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_1_test.dart

@@ -7,11 +7,11 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 1', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     // smoketest
     await tester.pumpWidget(
@@ -22,7 +22,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -34,7 +34,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // control for forking
@@ -56,7 +55,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -68,7 +67,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // forking semantics
@@ -90,7 +88,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -124,7 +122,6 @@ void main() {
     expect(client.updates[0].children[1].geometry.width, equals(800.0));
     expect(client.updates[0].children[1].geometry.height, equals(10.0));
     expect(client.updates[0].children[1].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // toggle a branch off
@@ -146,7 +143,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -158,7 +155,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // toggle a branch back on
@@ -180,7 +176,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -214,7 +210,7 @@ void main() {
     expect(client.updates[0].children[1].geometry.width, equals(800.0));
     expect(client.updates[0].children[1].geometry.height, equals(10.0));
     expect(client.updates[0].children[1].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_2_test.dart

@@ -7,11 +7,11 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 2', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     // this test is the same as the test in Semantics 1, but
     // starting with the second branch being ignored and then
@@ -36,7 +36,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -70,7 +70,6 @@ void main() {
     expect(client.updates[0].children[1].geometry.width, equals(800.0));
     expect(client.updates[0].children[1].geometry.height, equals(10.0));
     expect(client.updates[0].children[1].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // toggle a branch off
@@ -92,7 +91,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -104,7 +103,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // toggle a branch back on
@@ -126,7 +124,7 @@ void main() {
         crossAxisAlignment: CrossAxisAlignment.stretch
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -160,7 +158,7 @@ void main() {
     expect(client.updates[0].children[1].geometry.width, equals(800.0));
     expect(client.updates[0].children[1].geometry.height, equals(10.0));
     expect(client.updates[0].children[1].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_3_test.dart

@@ -6,11 +6,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 3', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     // implicit annotators
     await tester.pumpWidget(
@@ -25,7 +25,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isTrue);
@@ -37,7 +37,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // remove one
@@ -50,7 +49,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isTrue);
@@ -62,7 +61,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // change what it says
@@ -75,7 +73,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -87,7 +85,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // add a node
@@ -103,7 +100,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isTrue);
@@ -115,7 +112,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // make no changes
@@ -132,6 +128,6 @@ void main() {
       )
     );
     expect(client.updates.length, equals(0));
-
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_4_test.dart

@@ -6,11 +6,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 4', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     //    O
     //   / \       O=root
@@ -40,7 +40,7 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].children.length, equals(2));
     expect(client.updates[0].children[0].id, equals(1));
@@ -51,7 +51,6 @@ void main() {
     expect(client.updates[0].children[1].children[0].children.length, equals(0));
     expect(client.updates[0].children[1].children[1].id, equals(4));
     expect(client.updates[0].children[1].children[1].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     //    O        O=root
@@ -79,10 +78,9 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(2));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     //             O=root
@@ -107,11 +105,10 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
-
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_5_test.dart

@@ -6,11 +6,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 5', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     await tester.pumpWidget(
       new Stack(
@@ -28,7 +28,7 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].flags.hasCheckedState, isFalse);
     expect(client.updates[0].strings.label, equals(''));
@@ -39,8 +39,7 @@ void main() {
     expect(client.updates[0].children[1].id, equals(2));
     expect(client.updates[0].children[1].flags.hasCheckedState, isFalse);
     expect(client.updates[0].children[1].strings.label, equals('label'));
-    expect(client.updates[1], isNull);
     client.updates.clear();
-
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_7_test.dart

@@ -5,13 +5,13 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
-
-import 'test_semantics.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
+import '../rendering/test_semantics_client.dart';
+
 void main() {
   testWidgets('Semantics 7 - Merging', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     String label;
 
@@ -44,7 +44,7 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isFalse);
@@ -80,7 +80,6 @@ void main() {
     expect(client.updates[0].children[1].geometry.height, equals(600.0));
     expect(client.updates[0].children[1].children.length, equals(0));
     // IDs 5 and 6 are used up by the nodes that get merged in
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     label = '2';
@@ -112,8 +111,7 @@ void main() {
         ]
       )
     );
-    expect(client.updates.length, equals(3));
-    expect(client.updates[2], isNull);
+    expect(client.updates.length, equals(2));
 
     // The order of the nodes is undefined, so allow both orders.
     mojom.SemanticsNode a, b;
@@ -150,6 +148,6 @@ void main() {
     expect(b.children.length, equals(0));
 
     client.updates.clear();
-
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_8_test.dart

@@ -6,11 +6,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'test_semantics.dart';
+import '../rendering/test_semantics_client.dart';
 
 void main() {
   testWidgets('Semantics 8 - Merging with reset', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener(tester);
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
 
     await tester.pumpWidget(
       new MergeSemantics(
@@ -32,7 +32,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isTrue);
@@ -44,7 +44,6 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
 
     // switch the order of the inner Semantics node to trigger a reset
@@ -68,7 +67,7 @@ void main() {
         )
       )
     );
-    expect(client.updates.length, equals(2));
+    expect(client.updates.length, equals(1));
     expect(client.updates[0].id, equals(0));
     expect(client.updates[0].actions, isEmpty);
     expect(client.updates[0].flags.hasCheckedState, isTrue);
@@ -80,8 +79,7 @@ void main() {
     expect(client.updates[0].geometry.width, equals(800.0));
     expect(client.updates[0].geometry.height, equals(600.0));
     expect(client.updates[0].children.length, equals(0));
-    expect(client.updates[1], isNull);
     client.updates.clear();
-
+    client.dispose();
   });
 }

packages/flutter/test/widget/semantics_test.dart

+// Copyright 2015 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_test/flutter_test.dart';
+import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
+
+import '../rendering/test_semantics_client.dart';
+
+void main() {
+  testWidgets('Semantics shutdown and restart', (WidgetTester tester) async {
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
+
+    await tester.pumpWidget(
+      new Container(
+        child: new Semantics(
+          label: 'test1',
+          child: new Container()
+        )
+      )
+    );
+
+    void checkUpdates(List<mojom.SemanticsNode> updates) {
+      expect(updates.length, equals(1));
+      expect(updates[0].id, equals(0));
+      expect(updates[0].actions, isEmpty);
+      expect(updates[0].flags.hasCheckedState, isFalse);
+      expect(updates[0].flags.isChecked, isFalse);
+      expect(updates[0].strings.label, equals('test1'));
+      expect(updates[0].geometry.transform, isNull);
+      expect(updates[0].geometry.left, equals(0.0));
+      expect(updates[0].geometry.top, equals(0.0));
+      expect(updates[0].geometry.width, equals(800.0));
+      expect(updates[0].geometry.height, equals(600.0));
+      expect(updates[0].children.length, equals(0));
+    }
+
+    checkUpdates(client.updates);
+    client.updates.clear();
+    client.dispose();
+
+    expect(tester.binding.hasScheduledFrame, isFalse);
+    client = new TestSemanticsClient(tester.binding.pipelineOwner);
+    expect(tester.binding.hasScheduledFrame, isTrue);
+    await tester.pump();
+
+    checkUpdates(client.updates);
+    client.updates.clear();
+    client.dispose();
+  });
+}