[framework] allow other RenderObjects to behave like repaint boundaries (#101952)

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

@@ -843,7 +843,14 @@ class RenderOpacity extends RenderProxyBox {
        super(child);
 
   @override
-  bool get alwaysNeedsCompositing => child != null && (_alpha > 0);
+  bool get isRepaintBoundary => child != null && (_alpha > 0);
+
+  @override
+  OffsetLayer updateCompositedLayer({required covariant OpacityLayer? oldLayer}) {
+    final OpacityLayer updatedLayer = oldLayer ?? OpacityLayer();
+    updatedLayer.alpha = _alpha;
+    return updatedLayer;
+  }
 
   int _alpha;
 
@@ -864,13 +871,13 @@ class RenderOpacity extends RenderProxyBox {
     assert(value >= 0.0 && value <= 1.0);
     if (_opacity == value)
       return;
-    final bool didNeedCompositing = alwaysNeedsCompositing;
+    final bool wasRepaintBoundary = isRepaintBoundary;
     final bool wasVisible = _alpha != 0;
     _opacity = value;
     _alpha = ui.Color.getAlphaFromOpacity(_opacity);
-    if (didNeedCompositing != alwaysNeedsCompositing)
+    if (wasRepaintBoundary != isRepaintBoundary)
       markNeedsCompositingBitsUpdate();
-    markNeedsPaint();
+    markNeedsCompositedLayerUpdate();
     if (wasVisible != (_alpha != 0) && !alwaysIncludeSemantics)
       markNeedsSemanticsUpdate();
   }
@@ -891,19 +898,10 @@ class RenderOpacity extends RenderProxyBox {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (child != null) {
     if (_alpha == 0) {
-        // No need to keep the layer. We'll create a new one if necessary.
-        layer = null;
       return;
     }
-      assert(needsCompositing);
-      layer = context.pushOpacity(offset, _alpha, super.paint, oldLayer: layer as OpacityLayer?);
-      assert(() {
-        layer!.debugCreator = debugCreator;
-        return true;
-      }());
-    }
+    super.paint(context, offset);
   }
 
   @override
@@ -929,8 +927,15 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
   int? _alpha;
 
   @override
-  bool get alwaysNeedsCompositing => child != null && _currentlyNeedsCompositing!;
-  bool? _currentlyNeedsCompositing;
+  bool get isRepaintBoundary => child != null && _currentlyIsRepaintBoundary!;
+  bool? _currentlyIsRepaintBoundary;
+
+  @override
+  OffsetLayer updateCompositedLayer({required covariant OpacityLayer? oldLayer}) {
+    final OpacityLayer updatedLayer = oldLayer ?? OpacityLayer();
+    updatedLayer.alpha = _alpha;
+    return updatedLayer;
+  }
 
   /// The animation that drives this render object's opacity.
   ///
@@ -990,11 +995,11 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
     final int? oldAlpha = _alpha;
     _alpha = ui.Color.getAlphaFromOpacity(opacity.value);
     if (oldAlpha != _alpha) {
-      final bool? didNeedCompositing = _currentlyNeedsCompositing;
-      _currentlyNeedsCompositing = _alpha! > 0;
-      if (child != null && didNeedCompositing != _currentlyNeedsCompositing)
+      final bool? wasRepaintBoundary = _currentlyIsRepaintBoundary;
+      _currentlyIsRepaintBoundary = _alpha! > 0;
+      if (child != null && wasRepaintBoundary != _currentlyIsRepaintBoundary)
         markNeedsCompositingBitsUpdate();
-      markNeedsPaint();
+      markNeedsCompositedLayerUpdate();
       if (oldAlpha == 0 || _alpha == 0)
         markNeedsSemanticsUpdate();
     }
@@ -1002,19 +1007,10 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (child != null) {
     if (_alpha == 0) {
-        // No need to keep the layer. We'll create a new one if necessary.
-        layer = null;
       return;
     }
-      assert(needsCompositing);
-      layer = context.pushOpacity(offset, _alpha!, super.paint, oldLayer: layer as OpacityLayer?);
-      assert(() {
-        layer!.debugCreator = debugCreator;
-        return true;
-      }());
-    }
+    super.paint(context, offset);
   }
 
   @override

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

@@ -545,13 +545,11 @@ class _HeroFlight {
           bottom: offsets.bottom,
           left: offsets.left,
           child: IgnorePointer(
-            child: RepaintBoundary(
             child: FadeTransition(
               opacity: _heroOpacity,
               child: child,
             ),
           ),
-          ),
         );
       },
     );

packages/flutter/test/rendering/debug_test.dart

@@ -231,7 +231,8 @@ void main() {
       rootLayer,
       const Rect.fromLTWH(0, 0, 500, 500),
     );
-    root.paint(context, const Offset(40, 40));
+    context.paintChild(root, const Offset(40, 40));
+
     final OpacityLayer opacityLayer = rootLayer.firstChild! as OpacityLayer;
     expect(opacityLayer.offset, const Offset(40, 40));
     debugDisableOpacityLayers = false;

packages/flutter/test/rendering/proxy_box_test.dart

@@ -561,6 +561,178 @@ void main() {
     // The follower is still hit testable because there is a leader layer.
     expect(follower.hitTest(hitTestResult, position: Offset.zero), isTrue);
   });
+
+  test('RenderObject can become a repaint boundary', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary();
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+
+    renderBox.isRepaintBoundary = true;
+    renderBox.markNeedsCompositingBitsUpdate();
+    renderBox.markNeedsCompositedLayerUpdate();
+
+    pumpFrame(phase: EnginePhase.composite);
+
+    // The first time the render object becomes a repaint boundary
+    // we must repaint from the parent to allow the layer to be
+    // created.
+    expect(childBox.paintCount, 2);
+    expect(renderBox.paintCount, 2);
+    expect(renderBox.debugLayer, isA<OffsetLayer>());
+
+    renderBox.markNeedsCompositedLayerUpdate();
+    expect(renderBox.debugNeedsPaint, false);
+    expect(renderBox.debugNeedsCompositedLayerUpdate, true);
+
+    pumpFrame(phase: EnginePhase.composite);
+
+    // The second time the layer exists and we can skip paint.
+    expect(childBox.paintCount, 2);
+    expect(renderBox.paintCount, 2);
+    expect(renderBox.debugLayer, isA<OffsetLayer>());
+
+    renderBox.isRepaintBoundary = false;
+    renderBox.markNeedsCompositingBitsUpdate();
+
+    pumpFrame(phase: EnginePhase.composite);
+
+    // Once it stops being a repaint boundary we must repaint to
+    // remove the layer. its required that the render object
+    // perform this action in paint.
+    expect(childBox.paintCount, 3);
+    expect(renderBox.paintCount, 3);
+    expect(renderBox.debugLayer, null);
+
+    // When the render object is not a repaint boundary, calling
+    // markNeedsLayerPropertyUpdate is the same as calling
+    // markNeedsPaint.
+
+    renderBox.markNeedsCompositedLayerUpdate();
+    expect(renderBox.debugNeedsPaint, true);
+    expect(renderBox.debugNeedsCompositedLayerUpdate, true);
+  });
+
+  test('RenderObject with repaint boundary asserts when a composited layer is replaced during layer property update', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return TestOffsetLayerA();
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+
+    renderBox.markNeedsCompositedLayerUpdate();
+
+    pumpFrame(phase: EnginePhase.composite, onErrors: expectAssertionError);
+  }, skip: kIsWeb); // https://github.com/flutter/flutter/issues/102086
+
+  test('RenderObject with repaint boundary asserts when a composited layer is replaced during painting', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return TestOffsetLayerA();
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+    renderBox.markNeedsPaint();
+
+    pumpFrame(phase: EnginePhase.composite, onErrors: expectAssertionError);
+  }, skip: kIsWeb); // https://github.com/flutter/flutter/issues/102086
+
+  test('RenderObject with repaint boundary asserts when a composited layer tries to update its own offset', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return (oldLayer ?? TestOffsetLayerA())..offset = const Offset(2133, 4422);
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+    renderBox.markNeedsPaint();
+
+    pumpFrame(phase: EnginePhase.composite, onErrors: expectAssertionError);
+  }, skip: kIsWeb); // https://github.com/flutter/flutter/issues/102086
+
+  test('RenderObject markNeedsPaint while repaint boundary, and then updated to no longer be a repaint boundary with '
+    'calling markNeedsCompositingBitsUpdate 1', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return oldLayer ?? TestOffsetLayerA();
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+
+    childBox.markNeedsPaint();
+    childBox.isRepaintBoundary = false;
+    childBox.markNeedsCompositingBitsUpdate();
+
+    expect(() => pumpFrame(phase: EnginePhase.composite), returnsNormally);
+  });
+
+  test('RenderObject markNeedsPaint while repaint boundary, and then updated to no longer be a repaint boundary with '
+    'calling markNeedsCompositingBitsUpdate 2', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return oldLayer ?? TestOffsetLayerA();
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+
+    childBox.isRepaintBoundary = false;
+    childBox.markNeedsCompositingBitsUpdate();
+    childBox.markNeedsPaint();
+
+    expect(() => pumpFrame(phase: EnginePhase.composite), returnsNormally);
+  });
+
+  test('RenderObject markNeedsPaint while repaint boundary, and then updated to no longer be a repaint boundary with '
+    'calling markNeedsCompositingBitsUpdate 3', () {
+    final ConditionalRepaintBoundary childBox = ConditionalRepaintBoundary(isRepaintBoundary: true);
+    final ConditionalRepaintBoundary renderBox = ConditionalRepaintBoundary(child: childBox);
+    // Ignore old layer.
+    childBox.offsetLayerFactory = (OffsetLayer? oldLayer) {
+      return oldLayer ?? TestOffsetLayerA();
+    };
+
+    layout(renderBox, phase: EnginePhase.composite);
+
+    expect(childBox.paintCount, 1);
+    expect(renderBox.paintCount, 1);
+
+    childBox.isRepaintBoundary = false;
+    childBox.markNeedsCompositedLayerUpdate();
+    childBox.markNeedsCompositingBitsUpdate();
+
+    expect(() => pumpFrame(phase: EnginePhase.composite), returnsNormally);
+  });
 }
 
 class _TestRectClipper extends CustomClipper<Rect> {
@@ -631,3 +803,38 @@ class _TestSemanticsUpdateRenderFractionalTranslation extends RenderFractionalTr
     super.markNeedsSemanticsUpdate();
   }
 }
+
+class ConditionalRepaintBoundary extends RenderProxyBox {
+  ConditionalRepaintBoundary({this.isRepaintBoundary = false, RenderBox? child}) : super(child);
+
+  @override
+  bool isRepaintBoundary = false;
+
+  OffsetLayer Function(OffsetLayer?)? offsetLayerFactory;
+
+  int paintCount = 0;
+
+  @override
+  OffsetLayer updateCompositedLayer({required covariant OffsetLayer? oldLayer}) {
+    if (offsetLayerFactory != null) {
+      return offsetLayerFactory!.call(oldLayer);
+    }
+    return super.updateCompositedLayer(oldLayer: oldLayer);
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    paintCount += 1;
+    super.paint(context, offset);
+  }
+}
+
+class TestOffsetLayerA extends OffsetLayer {}
+
+void expectAssertionError() {
+  final FlutterErrorDetails errorDetails = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails()!;
+  final bool asserted = errorDetails.toString().contains('Failed assertion');
+  if (!asserted) {
+    FlutterError.reportError(errorDetails);
+  }
+}

packages/flutter/test/widgets/animated_opacity_repaint_test.dart

+// Copyright 2014 The Flutter 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/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('RenderAnimatedOpacityMixin avoids repainting child as it animates', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    final AnimationController controller = AnimationController(vsync: const TestVSync(), duration: const Duration(seconds: 1));
+    final Tween<double> opacityTween = Tween<double>(begin: 0, end: 1);
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: FadeTransition(
+          opacity: controller.drive(opacityTween),
+          child: const TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 0);
+    controller.forward();
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(RenderTestObject.paintCount, 1);
+
+    controller.stop();
+    await tester.pump();
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderAnimatedOpacityMixin allows opacity layer to be disposed when animating to 0 opacity', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    final AnimationController controller = AnimationController(vsync: const TestVSync(), duration: const Duration(seconds: 1));
+    final Tween<double> opacityTween = Tween<double>(begin: 1, end: 0);
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: FadeTransition(
+          opacity: controller.drive(opacityTween),
+          child: const TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+    expect(tester.layers, contains(isA<OpacityLayer>()));
+    controller.forward();
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 2));
+
+    expect(RenderTestObject.paintCount, 1);
+
+    controller.stop();
+    await tester.pump();
+
+    expect(tester.layers, isNot(contains(isA<OpacityLayer>())));
+  });
+}
+
+class TestWidget extends SingleChildRenderObjectWidget {
+  const TestWidget({super.key, super.child});
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return RenderTestObject();
+  }
+}
+
+class RenderTestObject extends RenderProxyBox {
+  static int paintCount = 0;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    paintCount += 1;
+    super.paint(context, offset);
+  }
+}

packages/flutter/test/widgets/opacity_repaint_test.dart

+// Copyright 2014 The Flutter 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/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('RenderOpacity acts as a repaint boundary for changes above the widget when partially opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity acts as a repaint boundary for changes above the widget when fully opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity can update its opacity without repainting its child - partially opaque to partially opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0.9,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity can update its opacity without repainting its child - partially opaque to fully opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity can update its opacity without repainting its child - fully opaque to partially opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity can update its opacity without repainting its child - fully opaque to fully transparent', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 1,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity must paint child - fully transparent to partially opaque', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 0);
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity allows child to update without updating parent', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      TestWidget(
+        child: Opacity(
+          opacity: 0.5,
+          child: Container(
+            color: Colors.red,
+          ),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+
+    await tester.pumpWidget(
+      TestWidget(
+        child: Opacity(
+          opacity: 0.5,
+          child: Container(
+            color: Colors.blue,
+          ),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+  });
+
+  testWidgets('RenderOpacity disposes of opacity layer when opacity is updated to 0', (WidgetTester tester) async {
+    RenderTestObject.paintCount = 0;
+    await tester.pumpWidget(
+      Container(
+        color: Colors.red,
+        child: const Opacity(
+          opacity: 0.5,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+    expect(tester.layers, contains(isA<OpacityLayer>()));
+
+    await tester.pumpWidget(
+      Container(
+        color: Colors.blue,
+        child: const Opacity(
+          opacity: 0,
+          child: TestWidget(),
+        ),
+      )
+    );
+
+    expect(RenderTestObject.paintCount, 1);
+    expect(tester.layers, isNot(contains(isA<OpacityLayer>())));
+  });
+}
+
+class TestWidget extends SingleChildRenderObjectWidget {
+  const TestWidget({super.key, super.child});
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return RenderTestObject();
+  }
+}
+
+class RenderTestObject extends RenderProxyBox {
+  static int paintCount = 0;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    paintCount += 1;
+    super.paint(context, offset);
+  }
+}