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 does not drop layer when animating to 1', (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 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: 0.99); // Layer is dropped at 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: 0.99, 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);
  }
}