Make tear-offs cacheable for RenderSemanticsGestureHandler (#11556)

* Make tear-offs cachable for RenderSemanticsGestureHandler

Fixes https://github.com/flutter/flutter/issues/11552

* comment fix

* refactor

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

@@ -530,7 +530,7 @@ class RawGestureDetectorState extends State<RawGestureDetector> {
       final RenderSemanticsGestureHandler semanticsGestureHandler = context.findRenderObject();
       context.visitChildElements((Element element) {
         final _GestureSemantics widget = element.widget;
-        widget._updateHandlers(semanticsGestureHandler, _recognizers);
+        widget._updateHandlers(semanticsGestureHandler);
       });
     }
   }
@@ -600,45 +600,8 @@ class RawGestureDetectorState extends State<RawGestureDetector> {
     return widget.child == null ? HitTestBehavior.translucent : HitTestBehavior.deferToChild;
   }
 
-  @override
-  Widget build(BuildContext context) {
-    Widget result = new Listener(
-      onPointerDown: _handlePointerDown,
-      behavior: widget.behavior ?? _defaultBehavior,
-      child: widget.child
-    );
-    if (!widget.excludeFromSemantics)
-      result = new _GestureSemantics(owner: this, child: result);
-    return result;
-  }
-
-  @override
-  void debugFillProperties(DiagnosticPropertiesBuilder description) {
-    super.debugFillProperties(description);
-    if (_recognizers == null) {
-      description.add(new DiagnosticsNode.message('DISPOSED'));
-    } else {
-      final List<String> gestures = _recognizers.values.map<String>((GestureRecognizer recognizer) => recognizer.debugDescription).toList();
-      if (gestures.isEmpty)
-        gestures.add('<none>');
-      description.add(new IterableProperty<String>('gestures', gestures));
-      description.add(new IterableProperty<GestureRecognizer>('recognizers', _recognizers.values, hidden: true));
-    }
-    description.add(new EnumProperty<HitTestBehavior>('behavior', widget.behavior, defaultValue: null));
-  }
-}
-
-class _GestureSemantics extends SingleChildRenderObjectWidget {
-  const _GestureSemantics({
-    Key key,
-    Widget child,
-    this.owner
-  }) : super(key: key, child: child);
-
-  final RawGestureDetectorState owner;
-
-  void _handleTap() {
-    final TapGestureRecognizer recognizer = owner._recognizers[TapGestureRecognizer];
+  void _handleSemanticsTap() {
+    final TapGestureRecognizer recognizer = _recognizers[TapGestureRecognizer];
     assert(recognizer != null);
     if (recognizer.onTapDown != null)
       recognizer.onTapDown(new TapDownDetails());
@@ -648,16 +611,16 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
       recognizer.onTap();
   }
 
-  void _handleLongPress() {
-    final LongPressGestureRecognizer recognizer = owner._recognizers[LongPressGestureRecognizer];
+  void _handleSemanticsLongPress() {
+    final LongPressGestureRecognizer recognizer = _recognizers[LongPressGestureRecognizer];
     assert(recognizer != null);
     if (recognizer.onLongPress != null)
       recognizer.onLongPress();
   }
 
-  void _handleHorizontalDragUpdate(DragUpdateDetails updateDetails) {
+  void _handleSemanticsHorizontalDragUpdate(DragUpdateDetails updateDetails) {
     {
-      final HorizontalDragGestureRecognizer recognizer = owner._recognizers[HorizontalDragGestureRecognizer];
+      final HorizontalDragGestureRecognizer recognizer = _recognizers[HorizontalDragGestureRecognizer];
       if (recognizer != null) {
         if (recognizer.onDown != null)
           recognizer.onDown(new DragDownDetails());
@@ -671,7 +634,7 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
       }
     }
     {
-      final PanGestureRecognizer recognizer = owner._recognizers[PanGestureRecognizer];
+      final PanGestureRecognizer recognizer = _recognizers[PanGestureRecognizer];
       if (recognizer != null) {
         if (recognizer.onDown != null)
           recognizer.onDown(new DragDownDetails());
@@ -686,9 +649,9 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
     }
   }
 
-  void _handleVerticalDragUpdate(DragUpdateDetails updateDetails) {
+  void _handleSemanticsVerticalDragUpdate(DragUpdateDetails updateDetails) {
     {
-      final VerticalDragGestureRecognizer recognizer = owner._recognizers[VerticalDragGestureRecognizer];
+      final VerticalDragGestureRecognizer recognizer = _recognizers[VerticalDragGestureRecognizer];
       if (recognizer != null) {
         if (recognizer.onDown != null)
           recognizer.onDown(new DragDownDetails());
@@ -702,7 +665,7 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
       }
     }
     {
-      final PanGestureRecognizer recognizer = owner._recognizers[PanGestureRecognizer];
+      final PanGestureRecognizer recognizer = _recognizers[PanGestureRecognizer];
       if (recognizer != null) {
         if (recognizer.onDown != null)
           recognizer.onDown(new DragDownDetails());
@@ -718,20 +681,58 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
   }
 
   @override
-  RenderSemanticsGestureHandler createRenderObject(BuildContext context) {
-    final RenderSemanticsGestureHandler result = new RenderSemanticsGestureHandler();
-    updateRenderObject(context, result);
+  Widget build(BuildContext context) {
+    Widget result = new Listener(
+      onPointerDown: _handlePointerDown,
+      behavior: widget.behavior ?? _defaultBehavior,
+      child: widget.child
+    );
+    if (!widget.excludeFromSemantics)
+      result = new _GestureSemantics(owner: this, child: result);
     return result;
   }
 
-  void _updateHandlers(RenderSemanticsGestureHandler renderObject, Map<Type, GestureRecognizer> recognizers) {
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder description) {
+    super.debugFillProperties(description);
+    if (_recognizers == null) {
+      description.add(new DiagnosticsNode.message('DISPOSED'));
+    } else {
+      final List<String> gestures = _recognizers.values.map<String>((GestureRecognizer recognizer) => recognizer.debugDescription).toList();
+      if (gestures.isEmpty)
+        gestures.add('<none>');
+      description.add(new IterableProperty<String>('gestures', gestures));
+      description.add(new IterableProperty<GestureRecognizer>('recognizers', _recognizers.values, hidden: true));
+    }
+    description.add(new EnumProperty<HitTestBehavior>('behavior', widget.behavior, defaultValue: null));
+  }
+}
+
+class _GestureSemantics extends SingleChildRenderObjectWidget {
+  const _GestureSemantics({
+    Key key,
+    Widget child,
+    this.owner
+  }) : super(key: key, child: child);
+
+  final RawGestureDetectorState owner;
+
+  @override
+  RenderSemanticsGestureHandler createRenderObject(BuildContext context) {
+    final RenderSemanticsGestureHandler renderObject = new RenderSemanticsGestureHandler();
+    _updateHandlers(renderObject);
+    return renderObject;
+  }
+
+  void _updateHandlers(RenderSemanticsGestureHandler renderObject) {
+    final Map<Type, GestureRecognizer> recognizers = owner._recognizers;
     renderObject
-      ..onTap = recognizers.containsKey(TapGestureRecognizer) ? _handleTap : null
-      ..onLongPress = recognizers.containsKey(LongPressGestureRecognizer) ? _handleLongPress : null
+      ..onTap = recognizers.containsKey(TapGestureRecognizer) ? owner._handleSemanticsTap : null
+      ..onLongPress = recognizers.containsKey(LongPressGestureRecognizer) ? owner._handleSemanticsLongPress : null
       ..onHorizontalDragUpdate = recognizers.containsKey(HorizontalDragGestureRecognizer) ||
-          recognizers.containsKey(PanGestureRecognizer) ? _handleHorizontalDragUpdate : null
+          recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsHorizontalDragUpdate : null
       ..onVerticalDragUpdate = recognizers.containsKey(VerticalDragGestureRecognizer) ||
-          recognizers.containsKey(PanGestureRecognizer) ? _handleVerticalDragUpdate : null;
+          recognizers.containsKey(PanGestureRecognizer) ? owner._handleSemanticsVerticalDragUpdate : null;
   }
 
   void _updateSemanticsActions(RenderSemanticsGestureHandler renderObject, Set<SemanticsAction> actions) {
@@ -740,6 +741,6 @@ class _GestureSemantics extends SingleChildRenderObjectWidget {
 
   @override
   void updateRenderObject(BuildContext context, RenderSemanticsGestureHandler renderObject) {
-    _updateHandlers(renderObject, owner._recognizers);
+    _updateHandlers(renderObject);
   }
 }

packages/flutter/test/widgets/gesture_detector_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/gestures.dart';
 
 void main() {
@@ -216,4 +217,35 @@ void main() {
     await tester.tapAt(const Offset(10.0, 10.0));
     expect(didTap, isFalse);
   });
+
+  testWidgets('cache unchanged callbacks', (WidgetTester tester) async {
+    final GestureTapCallback inputCallback = () {};
+
+    await tester.pumpWidget(
+        new Center(
+            child: new GestureDetector(
+              onTap: inputCallback,
+              child: new Container(),
+            )
+        )
+    );
+
+    final RenderSemanticsGestureHandler renderObj1 = tester.renderObject(find.byType(GestureDetector));
+    final GestureTapCallback actualCallback1 = renderObj1.onTap;
+
+    await tester.pumpWidget(
+        new Center(
+            child: new GestureDetector(
+              onTap: inputCallback,
+              child: new Container(),
+            )
+        )
+    );
+
+    final RenderSemanticsGestureHandler renderObj2 = tester.renderObject(find.byType(GestureDetector));
+    final GestureTapCallback actualCallback2 = renderObj2.onTap;
+
+    expect(renderObj1, same(renderObj2));
+    expect(actualCallback1, same(actualCallback2)); // Should be cached.
+  });
 }