import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

const _viewportWidth = 300.0;

class FooApp extends StatelessWidget {
  final ScrollController scrollController;
  final ValueNotifier<double> contentWidth;

  FooApp(this.scrollController, this.contentWidth);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Material(
        child: Wrap(
          children: [
            Container(
              height: 40,
              width: _viewportWidth,
              child: ListView(
                controller: scrollController,
                scrollDirection: Axis.horizontal,
                shrinkWrap: true,
                children: [
                  ValueListenableBuilder(
                    valueListenable: contentWidth,
                    builder: (context, contentWidth, _) =>
                        SizedBox(height: 40, width: contentWidth),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

void main() {
  group('Scroll issues', () {
    testWidgets(
        'No scroll notification when scroll offset changes due to content shrinking',
        (tester) async {
      final contentWidth = ValueNotifier(500.0);
      final scrollController = ScrollController();

      // Listen to [scrollController] update notifications.
      var scrollOffset = 0.0;
      scrollController
          .addListener(() => scrollOffset = scrollController.offset);

      await tester.pumpWidget(FooApp(scrollController, contentWidth));

      // When moving the scroll controller all the way to the end.
      scrollController.jumpTo(scrollController.position.maxScrollExtent);
      var expectedScrollPosition = contentWidth.value - _viewportWidth;
      await tester.pumpAndSettle();
      expect(scrollController.offset, expectedScrollPosition,
          reason: 'Expect scrollController to update to scroll to the end.');
      expect(scrollOffset, expectedScrollPosition,
          reason: 'Expect scrollController to have broadcast notifications.');

      // When shrinking the content so that the maxScrollExtent decreases.
      contentWidth.value -= 100;
      expectedScrollPosition = contentWidth.value - _viewportWidth;
      // Wait as long as needed for everything to settle.
      await tester.pumpAndSettle();
      await tester.pumpAndSettle();
      await tester.pumpAndSettle();
      expect(scrollController.offset, expectedScrollPosition,
          reason: 'Expect scrollController to update to the new end.');
      // >>>>>>>>>> FAILS HERE: <<<<<<<<<<
      expect(scrollOffset, expectedScrollPosition,
          reason: 'Expect scrollController to have broadcast a notification.');
    });
  });
}