Do not do semantics for detached objects (#15320)

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

@@ -2222,8 +2222,10 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   /// any way to update the semantics tree.
   void markNeedsSemanticsUpdate() {
     assert(!attached || !owner._debugDoingSemantics);
-    if (attached && owner._semanticsOwner == null)
+    if (!attached || owner._semanticsOwner == null) {
+      _cachedSemanticsConfiguration = null;
       return;
+    }
 
     // Dirty the semantics tree starting at `this` until we have reached a
     // RenderObject that is a semantics boundary. All semantics past this

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

@@ -723,24 +723,42 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
 
   @override
   RenderSemanticsGestureHandler createRenderObject(BuildContext context) {
-    final RenderSemanticsGestureHandler renderObject = new RenderSemanticsGestureHandler();
-    _updateHandlers(renderObject);
-    return renderObject;
+    return new RenderSemanticsGestureHandler(
+      onTap: _onTapHandler,
+      onLongPress: _onLongPressHandler,
+      onHorizontalDragUpdate: _onHorizontalDragUpdateHandler,
+      onVerticalDragUpdate: _onVerticalDragUpdateHandler,
+    );
   }
 
   void _updateHandlers(RenderSemanticsGestureHandler renderObject) {
-    final Map<Type, GestureRecognizer> recognizers = owner._recognizers;
     renderObject
-      ..onTap = recognizers.containsKey(TapGestureRecognizer) ? owner._handleSemanticsTap : null
-      ..onLongPress = recognizers.containsKey(LongPressGestureRecognizer) ? owner._handleSemanticsLongPress : null
-      ..onHorizontalDragUpdate = recognizers.containsKey(HorizontalDragGestureRecognizer) ||
-          recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsHorizontalDragUpdate : null
-      ..onVerticalDragUpdate = recognizers.containsKey(VerticalDragGestureRecognizer) ||
-          recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsVerticalDragUpdate : null;
+      ..onTap = _onTapHandler
+      ..onLongPress = _onLongPressHandler
+      ..onHorizontalDragUpdate = _onHorizontalDragUpdateHandler
+      ..onVerticalDragUpdate = _onVerticalDragUpdateHandler;
   }
 
   @override
   void updateRenderObject(BuildContext context, RenderSemanticsGestureHandler renderObject) {
     _updateHandlers(renderObject);
   }
+
+  GestureTapCallback get _onTapHandler {
+    return owner._recognizers.containsKey(TapGestureRecognizer) ? owner._handleSemanticsTap : null;
+  }
+
+  GestureTapCallback get _onLongPressHandler {
+    return owner._recognizers.containsKey(LongPressGestureRecognizer) ? owner._handleSemanticsLongPress : null;
+  }
+
+  GestureDragUpdateCallback get _onHorizontalDragUpdateHandler {
+    return owner._recognizers.containsKey(HorizontalDragGestureRecognizer) ||
+        owner._recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsHorizontalDragUpdate : null;
+  }
+
+  GestureDragUpdateCallback get _onVerticalDragUpdateHandler {
+    return owner._recognizers.containsKey(VerticalDragGestureRecognizer) ||
+        owner._recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsVerticalDragUpdate : null;
+  }
 }

packages/flutter/test/rendering/object_test.dart

@@ -21,6 +21,15 @@ void main() {
     renderObject.markNeedsSemanticsUpdate();
     expect(onNeedVisualUpdateCallCount, 2);
   });
+
+  test('detached RenderObject does not do semantics', () {
+    final TestRenderObject renderObject = new TestRenderObject();
+    expect(renderObject.attached, isFalse);
+    expect(renderObject.describeSemanticsConfigurationCallCount, 0);
+
+    renderObject.markNeedsSemanticsUpdate();
+    expect(renderObject.describeSemanticsConfigurationCallCount, 0);
+  });
 }
 
 class TestRenderObject extends RenderObject {
@@ -39,10 +48,13 @@ class TestRenderObject extends RenderObject {
   @override
   Rect get semanticBounds => new Rect.fromLTWH(0.0, 0.0, 10.0, 20.0);
 
+  int describeSemanticsConfigurationCallCount = 0;
+
   @override
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);
     config.isSemanticBoundary = true;
+    describeSemanticsConfigurationCallCount++;
   }
 }
 

packages/flutter/test/widgets/scrollable_semantics_test.dart

@@ -12,6 +12,10 @@ import 'semantics_tester.dart';
 void main() {
   SemanticsTester semantics;
 
+  setUp(() {
+    debugResetSemanticsIdCounter();
+  });
+
   tearDown(() {
     semantics?.dispose();
     semantics = null;
@@ -327,13 +331,35 @@ void main() {
           children: new List<Widget>.generate(40, (int i) {
             return new Container(
               child: new Text('item $i'),
-              height: 40.0,
+              height: 400.0,
             );
           }),
         ),
       ),
     );
 
+    final TestSemantics expectedSemantics = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          children: <TestSemantics>[
+            new TestSemantics(
+              actions: <SemanticsAction>[SemanticsAction.scrollUp],
+              children: <TestSemantics>[
+                new TestSemantics(
+                  label: r'item 0',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  label: r'item 1',
+                  textDirection: TextDirection.ltr,
+                ),
+              ],
+            ),
+          ],
+        ),
+      ],
+    );
+
     // Start with semantics off.
     expect(tester.binding.pipelineOwner.semanticsOwner, isNull);
 
@@ -341,6 +367,7 @@ void main() {
     semantics = new SemanticsTester(tester);
     await tester.pumpAndSettle();
     expect(tester.binding.pipelineOwner.semanticsOwner, isNotNull);
+    expect(semantics, hasSemantics(expectedSemantics, ignoreId: true, ignoreRect: true, ignoreTransform: true));
 
     // Semantics off
     semantics.dispose();
@@ -351,6 +378,7 @@ void main() {
     semantics = new SemanticsTester(tester);
     await tester.pumpAndSettle();
     expect(tester.binding.pipelineOwner.semanticsOwner, isNotNull);
+    expect(semantics, hasSemantics(expectedSemantics, ignoreId: true, ignoreRect: true, ignoreTransform: true));
   });
 }
 

packages/flutter/test/widgets/semantics_tester.dart

@@ -265,9 +265,9 @@ class TestSemantics {
       return fail('expected node id $id to have hint "$hint" but found hint "${nodeData.hint}".');
     if (textDirection != null && textDirection != nodeData.textDirection)
       return fail('expected node id $id to have textDirection "$textDirection" but found "${nodeData.textDirection}".');
-    if (nextNodeId != null && nextNodeId != nodeData.nextNodeId)
+    if (!ignoreId && nextNodeId != null && nextNodeId != nodeData.nextNodeId)
       return fail('expected node id $id to have nextNodeId "$nextNodeId" but found "${nodeData.nextNodeId}".');
-    if (previousNodeId != null && previousNodeId != nodeData.previousNodeId)
+    if (!ignoreId && previousNodeId != null && previousNodeId != nodeData.previousNodeId)
       return fail('expected node id $id to have previousNodeId "$previousNodeId" but found "${nodeData.previousNodeId}".');
     if ((nodeData.label != '' || nodeData.value != '' || nodeData.hint != '' || node.increasedValue != '' || node.decreasedValue != '') && nodeData.textDirection == null)
       return fail('expected node id $id, which has a label, value, or hint, to have a textDirection, but it did not.');