Add semantics for Sliders (#4808)

Also, make SemanticsOwner into a real class and use it instead of a static in
several places.

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

@@ -180,6 +180,8 @@ final Tween<double> _kLabelBalloonRadiusTween = new Tween<double>(begin: _kThumb
 final Tween<double> _kLabelBalloonTipTween = new Tween<double>(begin: 0.0, end: -8.0);
 final double _kLabelBalloonTipAttachmentRatio = math.sin(math.PI / 4.0);
 
+const double _kAdjustmentUnit = 0.1; // Matches iOS implementation of material slider.
+
 double _getAdditionalHeightForLabel(String label) {
   return label == null ? 0.0 : _kLabelBalloonRadius * 2.0;
 }
@@ -191,7 +193,7 @@ BoxConstraints _getAdditionalConstraints(String label) {
   );
 }
 
-class _RenderSlider extends RenderConstrainedBox {
+class _RenderSlider extends RenderConstrainedBox implements SemanticActionHandler {
   _RenderSlider({
     double value,
     int divisions,
@@ -291,8 +293,10 @@ class _RenderSlider extends RenderConstrainedBox {
     return dragValue;
   }
 
+  bool get isInteractive => onChanged != null;
+
   void _handleDragStart(DragStartDetails details) {
-    if (onChanged != null) {
+    if (isInteractive) {
       _active = true;
       _currentDragValue = (globalToLocal(details.globalPosition).x - _kReactionRadius) / _trackLength;
       onChanged(_discretizedCurrentDragValue);
@@ -302,7 +306,7 @@ class _RenderSlider extends RenderConstrainedBox {
   }
 
   void _handleDragUpdate(DragUpdateDetails details) {
-    if (onChanged != null) {
+    if (isInteractive) {
       _currentDragValue += details.primaryDelta / _trackLength;
       onChanged(_discretizedCurrentDragValue);
     }
@@ -322,7 +326,7 @@ class _RenderSlider extends RenderConstrainedBox {
 
   @override
   void handleEvent(PointerEvent event, BoxHitTestEntry entry) {
-    if (event is PointerDownEvent && onChanged != null)
+    if (event is PointerDownEvent && isInteractive)
       _drag.addPointer(event);
   }
 
@@ -331,7 +335,7 @@ class _RenderSlider extends RenderConstrainedBox {
     final Canvas canvas = context.canvas;
 
     final double trackLength = _trackLength;
-    final bool enabled = onChanged != null;
+    final bool enabled = isInteractive;
     final double value = _position.value;
 
     final double additionalHeightForLabel = _getAdditionalHeightForLabel(label);
@@ -417,4 +421,32 @@ class _RenderSlider extends RenderConstrainedBox {
     }
     canvas.drawCircle(thumbCenter, thumbRadius + thumbRadiusDelta, thumbPaint);
   }
+
+  @override
+  bool get hasSemantics => isInteractive;
+
+  @override
+  Iterable<SemanticAnnotator> getSemanticAnnotators() sync* {
+    yield (SemanticsNode semantics) {
+      if (isInteractive)
+        semantics.addAdjustmentActions();
+    };
+  }
+
+  @override
+  void performAction(SemanticAction action) {
+    switch (action) {
+      case SemanticAction.increase:
+        if (isInteractive)
+          onChanged((value + _kAdjustmentUnit).clamp(0.0, 1.0));
+        break;
+      case SemanticAction.decrease:
+        if (isInteractive)
+          onChanged((value - _kAdjustmentUnit).clamp(0.0, 1.0));
+        break;
+      default:
+        assert(false);
+        break;
+    }
+  }
 }

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

@@ -133,13 +133,19 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   ///
   /// Called automatically when the binding is created.
   void initSemantics() {
-    SemanticsNode.onSemanticsEnabled = renderView.scheduleInitialSemantics;
     shell.provideService(mojom.SemanticsServer.serviceName, (core.MojoMessagePipeEndpoint endpoint) {
+      ensureSemantics();
       mojom.SemanticsServerStub server = new mojom.SemanticsServerStub.fromEndpoint(endpoint);
-      server.impl = new SemanticsServer();
+      server.impl = new SemanticsServer(semanticsOwner: pipelineOwner.semanticsOwner);
     });
   }
 
+  void ensureSemantics() {
+    if (pipelineOwner.semanticsOwner == null)
+      renderView.scheduleInitialSemantics();
+    assert(pipelineOwner.semanticsOwner != null);
+  }
+
   void _handlePersistentFrameCallback(Duration timeStamp) {
     beginFrame();
   }
@@ -153,10 +159,7 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
     pipelineOwner.flushCompositingBits();
     pipelineOwner.flushPaint();
     renderView.compositeFrame(); // this sends the bits to the GPU
-    if (SemanticsNode.hasListeners) {
-      pipelineOwner.flushSemantics();
-      SemanticsNode.sendSemanticsTree();
-    }
+    pipelineOwner.flushSemantics();
   }
 
   @override

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

@@ -507,12 +507,12 @@ class _SemanticsGeometry {
 
 abstract class _SemanticsFragment {
   _SemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<SemanticAnnotator> annotators,
     List<_SemanticsFragment> children
   }) {
-    assert(owner != null);
-    _ancestorChain = <RenderObject>[owner];
+    assert(renderObjectOwner != null);
+    _ancestorChain = <RenderObject>[renderObjectOwner];
     if (annotators != null)
       addAnnotators(annotators);
     assert(() {
@@ -531,7 +531,7 @@ abstract class _SemanticsFragment {
     _ancestorChain.add(ancestor);
   }
 
-  RenderObject get owner => _ancestorChain.first;
+  RenderObject get renderObjectOwner => _ancestorChain.first;
 
   List<SemanticAnnotator> _annotators;
   void addAnnotators(Iterable<SemanticAnnotator> moreAnnotators) {
@@ -555,20 +555,20 @@ abstract class _SemanticsFragment {
 /// that comes from the (dirty) ancestors.)
 class _CleanSemanticsFragment extends _SemanticsFragment {
   _CleanSemanticsFragment({
-    RenderObject owner
-  }) : super(owner: owner) {
-    assert(owner._semantics != null);
+    RenderObject renderObjectOwner
+  }) : super(renderObjectOwner: renderObjectOwner) {
+    assert(renderObjectOwner._semantics != null);
   }
 
   @override
   Iterable<SemanticsNode> compile({ _SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics }) sync* {
     assert(!_debugCompiled);
     assert(() { _debugCompiled = true; return true; });
-    SemanticsNode node = owner._semantics;
+    SemanticsNode node = renderObjectOwner._semantics;
     assert(node != null);
     if (geometry != null) {
       geometry.applyAncestorChain(_ancestorChain);
-      geometry.updateSemanticsNode(rendering: owner, semantics: node, parentSemantics: parentSemantics);
+      geometry.updateSemanticsNode(rendering: renderObjectOwner, semantics: node, parentSemantics: parentSemantics);
     } else {
       assert(_ancestorChain.length == 1);
     }
@@ -578,10 +578,10 @@ class _CleanSemanticsFragment extends _SemanticsFragment {
 
 abstract class _InterestingSemanticsFragment extends _SemanticsFragment {
   _InterestingSemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<SemanticAnnotator> annotators,
     Iterable<_SemanticsFragment> children
-  }) : super(owner: owner, annotators: annotators, children: children);
+  }) : super(renderObjectOwner: renderObjectOwner, annotators: annotators, children: children);
 
   bool get haveConcreteNode => true;
 
@@ -593,7 +593,7 @@ abstract class _InterestingSemanticsFragment extends _SemanticsFragment {
     for (SemanticAnnotator annotator in _annotators)
       annotator(node);
     for (_SemanticsFragment child in _children) {
-      assert(child._ancestorChain.last == owner);
+      assert(child._ancestorChain.last == renderObjectOwner);
       node.addChildren(child.compile(
         geometry: createSemanticsGeometryForChild(geometry),
         currentSemantics: _children.length > 1 ? null : node,
@@ -612,10 +612,10 @@ abstract class _InterestingSemanticsFragment extends _SemanticsFragment {
 
 class _RootSemanticsFragment extends _InterestingSemanticsFragment {
   _RootSemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<SemanticAnnotator> annotators,
     Iterable<_SemanticsFragment> children
-  }) : super(owner: owner, annotators: annotators, children: children);
+  }) : super(renderObjectOwner: renderObjectOwner, annotators: annotators, children: children);
 
   @override
   SemanticsNode establishSemanticsNode(_SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics) {
@@ -623,14 +623,14 @@ class _RootSemanticsFragment extends _InterestingSemanticsFragment {
     assert(geometry == null);
     assert(currentSemantics == null);
     assert(parentSemantics == null);
-    owner._semantics ??= new SemanticsNode.root(
-      handler: owner is SemanticActionHandler ? owner as dynamic : null,
-      owner: owner.owner
+    renderObjectOwner._semantics ??= new SemanticsNode.root(
+      handler: renderObjectOwner is SemanticActionHandler ? renderObjectOwner as dynamic : null,
+      owner: renderObjectOwner.owner.semanticsOwner
     );
-    SemanticsNode node = owner._semantics;
+    SemanticsNode node = renderObjectOwner._semantics;
     assert(MatrixUtils.matrixEquals(node.transform, new Matrix4.identity()));
     assert(!node.wasAffectedByClip);
-    node.rect = owner.semanticBounds;
+    node.rect = renderObjectOwner.semanticBounds;
     return node;
   }
 
@@ -642,20 +642,20 @@ class _RootSemanticsFragment extends _InterestingSemanticsFragment {
 
 class _ConcreteSemanticsFragment extends _InterestingSemanticsFragment {
   _ConcreteSemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<SemanticAnnotator> annotators,
     Iterable<_SemanticsFragment> children
-  }) : super(owner: owner, annotators: annotators, children: children);
+  }) : super(renderObjectOwner: renderObjectOwner, annotators: annotators, children: children);
 
   @override
   SemanticsNode establishSemanticsNode(_SemanticsGeometry geometry, SemanticsNode currentSemantics, SemanticsNode parentSemantics) {
-    owner._semantics ??= new SemanticsNode(
-      handler: owner is SemanticActionHandler ? owner as dynamic : null
+    renderObjectOwner._semantics ??= new SemanticsNode(
+      handler: renderObjectOwner is SemanticActionHandler ? renderObjectOwner as dynamic : null
     );
-    SemanticsNode node = owner._semantics;
+    SemanticsNode node = renderObjectOwner._semantics;
     if (geometry != null) {
       geometry.applyAncestorChain(_ancestorChain);
-      geometry.updateSemanticsNode(rendering: owner, semantics: node, parentSemantics: parentSemantics);
+      geometry.updateSemanticsNode(rendering: renderObjectOwner, semantics: node, parentSemantics: parentSemantics);
     } else {
       assert(_ancestorChain.length == 1);
     }
@@ -670,10 +670,10 @@ class _ConcreteSemanticsFragment extends _InterestingSemanticsFragment {
 
 class _ImplicitSemanticsFragment extends _InterestingSemanticsFragment {
   _ImplicitSemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<SemanticAnnotator> annotators,
     Iterable<_SemanticsFragment> children
-  }) : super(owner: owner, annotators: annotators, children: children);
+  }) : super(renderObjectOwner: renderObjectOwner, annotators: annotators, children: children);
 
   @override
   bool get haveConcreteNode => _haveConcreteNode;
@@ -685,18 +685,18 @@ class _ImplicitSemanticsFragment extends _InterestingSemanticsFragment {
     assert(_haveConcreteNode == null);
     _haveConcreteNode = currentSemantics == null && _annotators.isNotEmpty;
     if (haveConcreteNode) {
-      owner._semantics ??= new SemanticsNode(
-        handler: owner is SemanticActionHandler ? owner as dynamic : null
+      renderObjectOwner._semantics ??= new SemanticsNode(
+        handler: renderObjectOwner is SemanticActionHandler ? renderObjectOwner as dynamic : null
       );
-      node = owner._semantics;
+      node = renderObjectOwner._semantics;
     } else {
-      owner._semantics = null;
+      renderObjectOwner._semantics = null;
       node = currentSemantics;
     }
     if (geometry != null) {
       geometry.applyAncestorChain(_ancestorChain);
       if (haveConcreteNode)
-        geometry.updateSemanticsNode(rendering: owner, semantics: node, parentSemantics: parentSemantics);
+        geometry.updateSemanticsNode(rendering: renderObjectOwner, semantics: node, parentSemantics: parentSemantics);
     } else {
       assert(_ancestorChain.length == 1);
     }
@@ -713,9 +713,9 @@ class _ImplicitSemanticsFragment extends _InterestingSemanticsFragment {
 
 class _ForkingSemanticsFragment extends _SemanticsFragment {
   _ForkingSemanticsFragment({
-    RenderObject owner,
+    RenderObject renderObjectOwner,
     Iterable<_SemanticsFragment> children
-  }) : super(owner: owner, children: children) {
+  }) : super(renderObjectOwner: renderObjectOwner, children: children) {
     assert(children != null);
     assert(children.length > 1);
   }
@@ -727,7 +727,7 @@ class _ForkingSemanticsFragment extends _SemanticsFragment {
     assert(geometry != null);
     geometry.applyAncestorChain(_ancestorChain);
     for (_SemanticsFragment child in _children) {
-      assert(child._ancestorChain.last == owner);
+      assert(child._ancestorChain.last == renderObjectOwner);
       yield* child.compile(
         geometry: new _SemanticsGeometry.copy(geometry),
         currentSemantics: null,
@@ -880,7 +880,8 @@ class PipelineOwner {
     }
   }
 
-  bool _semanticsEnabled = false;
+  SemanticsOwner get semanticsOwner => _semanticsOwner;
+  SemanticsOwner _semanticsOwner;
   bool _debugDoingSemantics = false;
   List<RenderObject> _nodesNeedingSemantics = <RenderObject>[];
 
@@ -892,8 +893,10 @@ class PipelineOwner {
   ///
   /// See [RendererBinding] for an example of how this function is used.
   void flushSemantics() {
+    if (_semanticsOwner == null)
+      return;
     Timeline.startSync('Semantics');
-    assert(_semanticsEnabled);
+    assert(_semanticsOwner != null);
     assert(() { _debugDoingSemantics = true; return true; });
     try {
       _nodesNeedingSemantics.sort((RenderObject a, RenderObject b) => a.depth - b.depth);
@@ -906,6 +909,7 @@ class PipelineOwner {
       assert(() { _debugDoingSemantics = false; return true; });
       Timeline.finishSync();
     }
+    _semanticsOwner.sendSemanticsTree();
   }
 
   /// Cause the entire subtree rooted at the given [RenderObject] to
@@ -1858,8 +1862,8 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     assert(!owner._debugDoingSemantics);
     assert(_semantics == null);
     assert(_needsSemanticsUpdate);
-    assert(owner._semanticsEnabled == false);
-    owner._semanticsEnabled = true;
+    assert(owner._semanticsOwner == null);
+    owner._semanticsOwner = new SemanticsOwner();
     owner._nodesNeedingSemantics.add(this);
     owner.requestVisualUpdate();
   }
@@ -1917,7 +1921,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// tree will be out of date.
   void markNeedsSemanticsUpdate({ bool onlyChanges: false, bool noGeometry: false }) {
     assert(!attached || !owner._debugDoingSemantics);
-    if ((attached && !owner._semanticsEnabled) || (_needsSemanticsUpdate && onlyChanges && (_needsSemanticsGeometryUpdate || noGeometry)))
+    if ((attached && owner._semanticsOwner == null) || (_needsSemanticsUpdate && onlyChanges && (_needsSemanticsGeometryUpdate || noGeometry)))
       return;
     if (!noGeometry && (_semantics == null || (_semantics.hasChildren && _semantics.wasAffectedByClip))) {
       // Since the geometry might have changed, we need to make sure to reapply any clips.
@@ -1981,7 +1985,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     // early-exit if we're not dirty and have our own semantics
     if (!_needsSemanticsUpdate && hasSemantics) {
       assert(_semantics != null);
-      return new _CleanSemanticsFragment(owner: this);
+      return new _CleanSemanticsFragment(renderObjectOwner: this);
     }
     List<_SemanticsFragment> children;
     visitChildrenForSemantics((RenderObject child) {
@@ -2004,16 +2008,16 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     _needsSemanticsGeometryUpdate = false;
     Iterable<SemanticAnnotator> annotators = getSemanticAnnotators();
     if (parent is! RenderObject)
-      return new _RootSemanticsFragment(owner: this, annotators: annotators, children: children);
+      return new _RootSemanticsFragment(renderObjectOwner: this, annotators: annotators, children: children);
     if (hasSemantics)
-      return new _ConcreteSemanticsFragment(owner: this, annotators: annotators, children: children);
+      return new _ConcreteSemanticsFragment(renderObjectOwner: this, annotators: annotators, children: children);
     if (annotators.isNotEmpty)
-      return new _ImplicitSemanticsFragment(owner: this, annotators: annotators, children: children);
+      return new _ImplicitSemanticsFragment(renderObjectOwner: this, annotators: annotators, children: children);
     _semantics = null;
     if (children == null)
       return null;
     if (children.length > 1)
-      return new _ForkingSemanticsFragment(owner: this, children: children);
+      return new _ForkingSemanticsFragment(renderObjectOwner: this, children: children);
     assert(children.length == 1);
     return children.single;
   }

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

@@ -78,7 +78,7 @@ class SemanticsNode extends AbstractNode {
   /// The root node is assigned an identifier of zero.
   SemanticsNode.root({
     SemanticActionHandler handler,
-    Object owner
+    SemanticsOwner owner
   }) : _id = 0,
        _actionHandler = handler {
     attach(owner);
@@ -131,21 +131,34 @@ class SemanticsNode extends AbstractNode {
 
   final Set<SemanticAction> _actions = new Set<SemanticAction>();
 
+  /// Adds the given action to the set of semantic actions.
+  ///
+  /// If the user chooses to perform an action,
+  /// [SemanticActionHandler.performAction] will be called with the chosen
+  /// action.
   void addAction(SemanticAction action) {
     if (_actions.add(action))
       _markDirty();
   }
 
+  /// Adds the [SemanticAction.scrollLeft] and [SemanticAction.scrollRight] actions.
   void addHorizontalScrollingActions() {
     addAction(SemanticAction.scrollLeft);
     addAction(SemanticAction.scrollRight);
   }
 
+  /// Adds the [SemanticAction.scrollUp] and [SemanticAction.scrollDown] actions.
   void addVerticalScrollingActions() {
     addAction(SemanticAction.scrollUp);
     addAction(SemanticAction.scrollDown);
   }
 
+  /// Adds the [SemanticAction.increase] and [SemanticAction.decrease] actions.
+  void addAdjustmentActions() {
+    addAction(SemanticAction.increase);
+    addAction(SemanticAction.decrease);
+  }
+
   bool _hasAction(SemanticAction action) {
     return _actionHandler != null && _actions.contains(action);
   }
@@ -285,6 +298,9 @@ class SemanticsNode extends AbstractNode {
       _markDirty();
   }
 
+  @override
+  SemanticsOwner get owner => super.owner;
+
   @override
   SemanticsNode get parent => super.parent;
 
@@ -309,15 +325,16 @@ class SemanticsNode extends AbstractNode {
     return true;
   }
 
-  static Map<int, SemanticsNode> _nodes = <int, SemanticsNode>{};
-  static Set<SemanticsNode> _detachedNodes = new Set<SemanticsNode>();
-
   @override
-  void attach(Object owner) {
+  void attach(SemanticsOwner owner) {
     super.attach(owner);
-    assert(!_nodes.containsKey(_id));
-    _nodes[_id] = this;
-    _detachedNodes.remove(this);
+    assert(!owner._nodes.containsKey(_id));
+    owner._nodes[_id] = this;
+    owner._detachedNodes.remove(this);
+    if (_dirty) {
+      _dirty = false;
+      _markDirty();
+    }
     if (parent != null)
       _inheritedMergeAllDescendantsIntoThisNode = parent._shouldMergeAllDescendantsIntoThisNode;
     if (_children != null) {
@@ -328,26 +345,27 @@ class SemanticsNode extends AbstractNode {
 
   @override
   void detach() {
+    assert(owner._nodes.containsKey(_id));
+    assert(!owner._detachedNodes.contains(this));
+    owner._nodes.remove(_id);
+    owner._detachedNodes.add(this);
     super.detach();
-    assert(_nodes.containsKey(_id));
-    assert(!_detachedNodes.contains(this));
-    _nodes.remove(_id);
-    _detachedNodes.add(this);
     if (_children != null) {
       for (SemanticsNode child in _children)
         child.detach();
     }
   }
 
-  static List<SemanticsNode> _dirtyNodes = <SemanticsNode>[];
   bool _dirty = false;
   void _markDirty() {
     if (_dirty)
       return;
     _dirty = true;
-    assert(!_dirtyNodes.contains(this));
-    assert(!_detachedNodes.contains(this));
-    _dirtyNodes.add(this);
+    if (attached) {
+      assert(!owner._dirtyNodes.contains(this));
+      assert(!owner._detachedNodes.contains(this));
+      owner._dirtyNodes.add(this);
+    }
   }
 
   mojom.SemanticsNode _serialize() {
@@ -397,35 +415,71 @@ class SemanticsNode extends AbstractNode {
     return result;
   }
 
-  static List<mojom.SemanticsListener> _listeners;
+  @override
+  String toString() {
+    StringBuffer buffer = new StringBuffer();
+    buffer.write('$runtimeType($_id');
+    if (_dirty)
+      buffer.write(" (${ owner != null && owner._dirtyNodes.contains(this) ? 'dirty' : 'STALE' })");
+    if (_shouldMergeAllDescendantsIntoThisNode)
+      buffer.write(' (leaf merge)');
+    buffer.write('; $rect');
+    if (wasAffectedByClip)
+      buffer.write(' (clipped)');
+    for (SemanticAction action in _actions) {
+      buffer.write('; $action');
+    }
+    if (hasCheckedState) {
+      if (isChecked)
+        buffer.write('; checked');
+      else
+        buffer.write('; unchecked');
+    }
+    if (label.isNotEmpty)
+      buffer.write('; "$label"');
+    buffer.write(')');
+    return buffer.toString();
+  }
+
+  /// Returns a string representation of this node and its descendants.
+  String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {
+    String result = '$prefixLineOne$this\n';
+    if (_children != null && _children.isNotEmpty) {
+      for (int index = 0; index < _children.length - 1; index += 1) {
+        SemanticsNode child = _children[index];
+        result += '${child.toStringDeep("$prefixOtherLines \u251C", "$prefixOtherLines \u2502")}';
+      }
+      result += '${_children.last.toStringDeep("$prefixOtherLines \u2514", "$prefixOtherLines  ")}';
+    }
+    return result;
+  }
+}
+
+class SemanticsOwner {
+  final List<SemanticsNode> _dirtyNodes = <SemanticsNode>[];
+  final Map<int, SemanticsNode> _nodes = <int, SemanticsNode>{};
+  final Set<SemanticsNode> _detachedNodes = new Set<SemanticsNode>();
+
+  List<mojom.SemanticsListener> _listeners;
 
   /// Whether there are currently any consumers of semantic data.
   ///
   /// If there are no consumers of semantic data, there is no need to compile
   /// semantic data into a [SemanticsNode] tree.
-  static bool get hasListeners => _listeners != null && _listeners.length > 0;
-
-  /// Called when the first consumer of semantic data arrives.
-  ///
-  /// Typically set by [RendererBinding].
-  static VoidCallback onSemanticsEnabled;
+  bool get hasListeners => _listeners != null && _listeners.length > 0;
 
   /// 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.
-  static void addListener(mojom.SemanticsListener listener) {
-    if (!hasListeners) {
-      assert(onSemanticsEnabled != null); // initialise the binding _before_ adding listeners
-      onSemanticsEnabled();
-    }
+  void addListener(mojom.SemanticsListener listener) {
     _listeners ??= <mojom.SemanticsListener>[];
     _listeners.add(listener);
   }
 
   /// Uploads the semantics tree to the listeners registered with [addListener].
-  static void sendSemanticsTree() {
+  void sendSemanticsTree() {
     assert(hasListeners);
     for (SemanticsNode oldNode in _detachedNodes) {
       // The other side will have forgotten this node if we even send
@@ -491,7 +545,7 @@ class SemanticsNode extends AbstractNode {
     _dirtyNodes.clear();
   }
 
-  static SemanticActionHandler _getSemanticActionHandlerForId(int id, { @required SemanticAction action }) {
+  SemanticActionHandler _getSemanticActionHandlerForId(int id, { @required SemanticAction action }) {
     assert(action != null);
     SemanticsNode result = _nodes[id];
     if (result != null && result._shouldMergeAllDescendantsIntoThisNode && !result._hasAction(action)) {
@@ -508,59 +562,29 @@ class SemanticsNode extends AbstractNode {
     return result._actionHandler;
   }
 
-  @override
-  String toString() {
-    StringBuffer buffer = new StringBuffer();
-    buffer.write('$runtimeType($_id');
-    if (_dirty)
-      buffer.write(" (${ _dirtyNodes.contains(this) ? 'dirty' : 'STALE' })");
-    if (_shouldMergeAllDescendantsIntoThisNode)
-      buffer.write(' (leaf merge)');
-    buffer.write('; $rect');
-    if (wasAffectedByClip)
-      buffer.write(' (clipped)');
-    for (SemanticAction action in _actions) {
-      buffer.write('; $action');
-    }
-    if (hasCheckedState) {
-      if (isChecked)
-        buffer.write('; checked');
-      else
-        buffer.write('; unchecked');
-    }
-    if (label.isNotEmpty)
-      buffer.write('; "$label"');
-    buffer.write(')');
-    return buffer.toString();
-  }
-
-  /// Returns a string representation of this node and its descendants.
-  String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {
-    String result = '$prefixLineOne$this\n';
-    if (_children != null && _children.isNotEmpty) {
-      for (int index = 0; index < _children.length - 1; index += 1) {
-        SemanticsNode child = _children[index];
-        result += '${child.toStringDeep("$prefixOtherLines \u251C", "$prefixOtherLines \u2502")}';
-      }
-      result += '${_children.last.toStringDeep("$prefixOtherLines \u2514", "$prefixOtherLines  ")}';
-    }
-    return result;
+  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>.
-    SemanticsNode.addListener(listener);
+    semanticsOwner.addListener(listener);
   }
 
   @override
   void performAction(int id, mojom.SemanticAction encodedAction) {
-    SemanticAction action = SemanticAction.values[encodedAction.mojoEnumValue];
-    SemanticActionHandler node = SemanticsNode._getSemanticActionHandlerForId(id, action: action);
-    node?.performAction(action);
+    semanticsOwner.performAction(id, SemanticAction.values[encodedAction.mojoEnumValue]);
   }
 }

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

@@ -9,6 +9,7 @@ import 'package:flutter/rendering.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
 import 'basic.dart';
+import 'binding.dart';
 import 'framework.dart';
 import 'gesture_detector.dart';
 
@@ -30,16 +31,23 @@ class SemanticsDebugger extends StatefulWidget {
 }
 
 class _SemanticsDebuggerState extends State<SemanticsDebugger> {
+  _SemanticsClient _client;
+
   @override
   void initState() {
     super.initState();
-    _SemanticsDebuggerListener.ensureInstantiated();
-    _SemanticsDebuggerListener.instance.addListener(_update);
+    // TODO(abarth): We shouldn't reach out to the WidgetsBinding.instance
+    // 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)
+      ..addListener(_update);
   }
 
   @override
   void dispose() {
-    _SemanticsDebuggerListener.instance.removeListener(_update);
+    _client.removeListener(_update);
     super.dispose();
   }
 
@@ -58,21 +66,21 @@ class _SemanticsDebuggerState extends State<SemanticsDebugger> {
 
   void _handleTap() {
     assert(_lastPointerDownLocation != null);
-    _SemanticsDebuggerListener.instance._performAction(_lastPointerDownLocation, SemanticAction.tap);
+    _client._performAction(_lastPointerDownLocation, SemanticAction.tap);
     setState(() {
       _lastPointerDownLocation = null;
     });
   }
   void _handleLongPress() {
     assert(_lastPointerDownLocation != null);
-    _SemanticsDebuggerListener.instance._performAction(_lastPointerDownLocation, SemanticAction.longPress);
+    _client._performAction(_lastPointerDownLocation, SemanticAction.longPress);
     setState(() {
       _lastPointerDownLocation = null;
     });
   }
   void _handlePanEnd(DragEndDetails details) {
     assert(_lastPointerDownLocation != null);
-    _SemanticsDebuggerListener.instance.handlePanEnd(_lastPointerDownLocation, details.velocity);
+    _client.handlePanEnd(_lastPointerDownLocation, details.velocity);
     setState(() {
       _lastPointerDownLocation = null;
     });
@@ -81,7 +89,7 @@ class _SemanticsDebuggerState extends State<SemanticsDebugger> {
   @override
   Widget build(BuildContext context) {
     return new CustomPaint(
-      foregroundPainter: new _SemanticsDebuggerPainter(_SemanticsDebuggerListener.instance.generation, _lastPointerDownLocation),
+      foregroundPainter: new _SemanticsDebuggerPainter(_client.generation, _client, _lastPointerDownLocation),
       child: new GestureDetector(
         behavior: HitTestBehavior.opaque,
         onTap: _handleTap,
@@ -196,6 +204,11 @@ class _SemanticsDebuggerEntry {
         || actions.contains(SemanticAction.scrollDown);
   }
 
+  bool get _isAdjustable {
+    return actions.contains(SemanticAction.increase)
+        || actions.contains(SemanticAction.decrease);
+  }
+
   TextPainter textPainter;
   void _updateMessage() {
     List<String> annotations = <String>[];
@@ -215,6 +228,8 @@ class _SemanticsDebuggerEntry {
       annotations.add('long-pressable');
     if (_isScrollable)
       annotations.add('scrollable');
+    if (_isAdjustable)
+      annotations.add('adjustable');
     String message;
     if (annotations.isEmpty) {
       assert(label != null);
@@ -295,16 +310,12 @@ class _SemanticsDebuggerEntry {
   }
 }
 
-class _SemanticsDebuggerListener extends ChangeNotifier implements mojom.SemanticsListener {
-  _SemanticsDebuggerListener._() {
-    SemanticsNode.addListener(this);
+class _SemanticsClient extends ChangeNotifier implements mojom.SemanticsListener {
+  _SemanticsClient(this.semanticsOwner) {
+    semanticsOwner.addListener(this);
   }
 
-  static _SemanticsDebuggerListener instance;
-  static final SemanticsServer _server = new SemanticsServer();
-  static void ensureInstantiated() {
-    instance ??= new _SemanticsDebuggerListener._();
-  }
+  final SemanticsOwner semanticsOwner;
 
   _SemanticsDebuggerEntry get rootNode => _nodes[0];
   final Map<int, _SemanticsDebuggerEntry> _nodes = <int, _SemanticsDebuggerEntry>{};
@@ -357,7 +368,7 @@ class _SemanticsDebuggerListener extends ChangeNotifier implements mojom.Semanti
 
   void _performAction(Point position, SemanticAction action) {
     _SemanticsDebuggerEntry entry = _hitTest(position, (_SemanticsDebuggerEntry entry) => entry.actions.contains(action));
-    _server.performAction(entry?.id ?? 0, mojom.SemanticAction.values[action.index]);
+    semanticsOwner.performAction(entry?.id ?? 0, action);
   }
 
   void handlePanEnd(Point position, Velocity velocity) {
@@ -366,10 +377,13 @@ class _SemanticsDebuggerListener extends ChangeNotifier implements mojom.Semanti
     if (vx.abs() == vy.abs())
       return;
     if (vx.abs() > vy.abs()) {
-      if (vx.sign < 0)
+      if (vx.sign < 0) {
+        _performAction(position, SemanticAction.decrease);
         _performAction(position, SemanticAction.scrollLeft);
-      else
+      } else {
+        _performAction(position, SemanticAction.increase);
         _performAction(position, SemanticAction.scrollRight);
+      }
     } else {
       if (vy.sign < 0)
         _performAction(position, SemanticAction.scrollUp);
@@ -380,14 +394,15 @@ class _SemanticsDebuggerListener extends ChangeNotifier implements mojom.Semanti
 }
 
 class _SemanticsDebuggerPainter extends CustomPainter {
-  const _SemanticsDebuggerPainter(this.generation, this.pointerPosition);
+  const _SemanticsDebuggerPainter(this.generation, this.client, this.pointerPosition);
 
   final int generation;
+  final _SemanticsClient client;
   final Point pointerPosition;
 
   @override
   void paint(Canvas canvas, Size size) {
-    _SemanticsDebuggerEntry rootNode = _SemanticsDebuggerListener.instance.rootNode;
+    _SemanticsDebuggerEntry rootNode = client.rootNode;
     rootNode?.paint(canvas, rootNode.findDepth());
     if (pointerPosition != null) {
       Paint paint = new Paint();
@@ -399,6 +414,7 @@ class _SemanticsDebuggerPainter extends CustomPainter {
   @override
   bool shouldRepaint(_SemanticsDebuggerPainter oldDelegate) {
     return generation != oldDelegate.generation
+        || client != oldDelegate.client
         || pointerPosition != oldDelegate.pointerPosition;
   }
 }

packages/flutter/test/material/tooltip_test.dart

@@ -394,7 +394,7 @@ void main() {
   });
 
   testWidgets('Does tooltip contribute semantics', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
     GlobalKey key = new GlobalKey();
     await tester.pumpWidget(
       new Overlay(

packages/flutter/test/rendering/reattach_test.dart

@@ -136,9 +136,10 @@ void main() {
   test('objects can be detached and re-attached: semantics', () {
     TestTree testTree = new TestTree();
     TestSemanticsListener listener = new TestSemanticsListener();
-    SemanticsNode.addListener(listener);
+    renderer.ensureSemantics();
+    renderer.pipelineOwner.semanticsOwner.addListener(listener);
     // Lay out, composite, paint, and update semantics
-    layout(testTree.root, phase: EnginePhase.sendSemanticsTree);
+    layout(testTree.root, phase: EnginePhase.flushSemantics);
     expect(listener.updates.length, equals(1));
     // Remove testTree from the custom render view
     renderer.renderView.child = null;
@@ -148,7 +149,7 @@ void main() {
     testTree.child.markNeedsSemanticsUpdate();
     expect(listener.updates.length, equals(0));
     // Lay out, composite, paint, and update semantics again
-    layout(testTree.root, phase: EnginePhase.sendSemanticsTree);
+    layout(testTree.root, phase: EnginePhase.flushSemantics);
     expect(listener.updates.length, equals(1));
   });
 }

packages/flutter/test/rendering/rendering_tester.dart

@@ -12,8 +12,7 @@ enum EnginePhase {
   compositingBits,
   paint,
   composite,
-  flushSemantics,
-  sendSemanticsTree
+  flushSemantics
 }
 
 class TestRenderingFlutterBinding extends BindingBase with SchedulerBinding, ServicesBinding, RendererBinding, GestureBinding {
@@ -33,24 +32,21 @@ class TestRenderingFlutterBinding extends BindingBase with SchedulerBinding, Ser
     renderView.compositeFrame();
     if (phase == EnginePhase.composite)
       return;
-    if (SemanticsNode.hasListeners) {
-      pipelineOwner.flushSemantics();
-      if (phase == EnginePhase.flushSemantics)
-        return;
-      SemanticsNode.sendSemanticsTree();
-    }
+    pipelineOwner.flushSemantics();
+    assert(phase == EnginePhase.flushSemantics);
   }
 }
 
 TestRenderingFlutterBinding _renderer;
-TestRenderingFlutterBinding get renderer => _renderer;
+TestRenderingFlutterBinding get renderer {
+  _renderer ??= new TestRenderingFlutterBinding();
+  return _renderer;
+}
 
 void layout(RenderBox box, { BoxConstraints constraints, EnginePhase phase: EnginePhase.layout }) {
   assert(box != null); // If you want to just repump the last box, call pumpFrame().
   assert(box.parent == null); // We stick the box in another, so you can't reuse it easily, sorry.
 
-  _renderer ??= new TestRenderingFlutterBinding();
-
   renderer.renderView.child = null;
   if (constraints != null) {
     box = new RenderPositionedBox(

packages/flutter/test/widget/buttons_test.dart

@@ -11,7 +11,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Does FlatButton contribute semantics', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
     await tester.pumpWidget(
       new Material(
         child: new Center(

packages/flutter/test/widget/independent_widget_layout_test.dart

@@ -43,10 +43,7 @@ class OffscreenWidgetTree {
     pipelineOwner.flushCompositingBits();
     pipelineOwner.flushPaint();
     renderView.compositeFrame();
-    if (SemanticsNode.hasListeners) {
-      pipelineOwner.flushSemantics();
-      SemanticsNode.sendSemanticsTree();
-    }
+    pipelineOwner.flushSemantics();
     buildOwner.finalizeTree();
   }
 

packages/flutter/test/widget/semantics_1_test.dart

@@ -11,7 +11,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 1', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     // smoketest
     await tester.pumpWidget(

packages/flutter/test/widget/semantics_2_test.dart

@@ -11,7 +11,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 2', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     // this test is the same as the test in Semantics 1, but
     // starting with the second branch being ignored and then

packages/flutter/test/widget/semantics_3_test.dart

@@ -10,7 +10,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 3', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     // implicit annotators
     await tester.pumpWidget(

packages/flutter/test/widget/semantics_4_test.dart

@@ -10,7 +10,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 4', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     //    O
     //   / \       O=root

packages/flutter/test/widget/semantics_5_test.dart

@@ -10,7 +10,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 5', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     await tester.pumpWidget(
       new Stack(

packages/flutter/test/widget/semantics_7_test.dart

@@ -11,7 +11,7 @@ import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
 void main() {
   testWidgets('Semantics 7 - Merging', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     String label;
 

packages/flutter/test/widget/semantics_8_test.dart

@@ -10,7 +10,7 @@ import 'test_semantics.dart';
 
 void main() {
   testWidgets('Semantics 8 - Merging with reset', (WidgetTester tester) async {
-    TestSemanticsListener client = new TestSemanticsListener();
+    TestSemanticsListener client = new TestSemanticsListener(tester);
 
     await tester.pumpWidget(
       new MergeSemantics(

packages/flutter/test/widget/test_semantics.dart

@@ -2,12 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
 class TestSemanticsListener implements mojom.SemanticsListener {
-  TestSemanticsListener() {
-    SemanticsNode.addListener(this);
+  TestSemanticsListener(WidgetTester tester) {
+    tester.binding.ensureSemantics();
+    tester.binding.pipelineOwner.semanticsOwner.addListener(this);
   }
 
   final List<mojom.SemanticsNode> updates = <mojom.SemanticsNode>[];

packages/flutter_test/lib/src/binding.dart

@@ -463,12 +463,9 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     renderView.compositeFrame(); // this sends the bits to the GPU
     if (_phase == EnginePhase.composite)
       return;
-    if (SemanticsNode.hasListeners) {
-      pipelineOwner.flushSemantics();
-      if (_phase == EnginePhase.flushSemantics)
-        return;
-      SemanticsNode.sendSemanticsTree();
-    }
+    pipelineOwner.flushSemantics();
+    if (_phase == EnginePhase.flushSemantics)
+      return;
     buildOwner.finalizeTree();
   }