flutter_flutter/packages/flutter/test/widgets/scrollable_animations_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/scheduler.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('Does not animate if already at target position', (WidgetTester tester) async {
    final ScrollController controller = ScrollController();
    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: ListView(
          controller: controller,
          children: List<Widget>.generate(80, (int i) => Text('$i', textDirection: TextDirection.ltr)),
        ),
      ),
    );

    expectNoAnimation();
    final double currentPosition = controller.position.pixels;
    controller.position.animateTo(currentPosition, duration: const Duration(seconds: 10), curve: Curves.linear);

    expectNoAnimation();
    expect(controller.position.pixels, currentPosition);
  });

  testWidgets('Does not animate if already at target position within tolerance', (WidgetTester tester) async {
    final ScrollController controller = ScrollController();
    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: ListView(
          controller: controller,
          children: List<Widget>.generate(80, (int i) => Text('$i', textDirection: TextDirection.ltr)),
        ),
      ),
    );

    expectNoAnimation();

    final double halfTolerance = controller.position.physics.tolerance.distance / 2;
    expect(halfTolerance, isNonZero);
    final double targetPosition = controller.position.pixels + halfTolerance;
    controller.position.animateTo(targetPosition, duration: const Duration(seconds: 10), curve: Curves.linear);

    expectNoAnimation();
    expect(controller.position.pixels, targetPosition);
  });

  testWidgets('Animates if going to a position outside of tolerance', (WidgetTester tester) async {
    final ScrollController controller = ScrollController();
    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: ListView(
          controller: controller,
          children: List<Widget>.generate(80, (int i) => Text('$i', textDirection: TextDirection.ltr)),
        ),
      ),
    );

    expectNoAnimation();

    final double doubleTolerance = controller.position.physics.tolerance.distance * 2;
    expect(doubleTolerance, isNonZero);
    final double targetPosition = controller.position.pixels + doubleTolerance;
    controller.position.animateTo(targetPosition, duration: const Duration(seconds: 10), curve: Curves.linear);

    expect(SchedulerBinding.instance.transientCallbackCount, equals(1), reason: 'Expected an animation.');
  });
}

void expectNoAnimation() {
  expect(SchedulerBinding.instance.transientCallbackCount, equals(0), reason: 'Expected no animation.');
}