ArrangeBuilder with widget tests

example.dart

/// Please first check out how ArrangeBuilder should be used in a simpler example:
/// https://gist.github.com/tomaszpolanski/abd4725d4e07ba637d5540b5fe10aa1e
abstract class ExampleUsecase {
  Future<String> fetchMessage();
}

example_usecase_mixin.dart

/// Please first check out how ArrangeBuilder should be used in a simpler example:
/// https://gist.github.com/tomaszpolanski/abd4725d4e07ba637d5540b5fe10aa1e
import 'package:flutter/cupertino.dart';

import 'example.dart';
import 'package:mocktail/mocktail.dart';

/// An example of a mixing that needs to be implemented once and can be used
/// in multiple tests without doing `when(() => ...)` in every test file
mixin ExampleUsecaseMixin {
  ExampleUsecase exampleUsecase = _MockExampleUsecase();

  /// Function that prepares the mixin to be in a usable state.
  /// This would be mixin's constructor if mixins would support constructor
  void arrangeExampleUsecase() {
    withExampleUsecaseFetchMessage();
  }

  void withExampleUsecaseFetchMessage([ValueGetter<Future<String>>? message]) {
    when(() => exampleUsecase.fetchMessage()).thenAnswer(
        (_) => message != null ? message.call() : Future.value('empty'));
  }
}

class _MockExampleUsecase extends Mock implements ExampleUsecase {}

example_widget.dart

/// Please first check out how ArrangeBuilder should be used in a simpler example:
/// https://gist.github.com/tomaszpolanski/abd4725d4e07ba637d5540b5fe10aa1e
import 'package:flutter/widgets.dart';

import 'example.dart';

class Example extends StatefulWidget {
  const Example({
    super.key,
    required this.usecase,
  });

  /// This usually would come as injection of some sort, not a parameter for a widget
  final ExampleUsecase usecase;

  @override
  State<Example> createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  late final _fetch = widget.usecase.fetchMessage();

  @override
  Widget build(BuildContext context) => FutureBuilder(
        future: _fetch,
        builder: (_, snapshot) {
          if (snapshot.hasError) {
            return const Text('Something went wrong');
          } else if (snapshot.hasData) {
            return const Text('some message');
          } else {
            return const Text('Loading');
          }
        },
      );
}

example_widget_test.dart

/// Please first check out how ArrangeBuilder should be used in a simpler example:
/// https://gist.github.com/tomaszpolanski/abd4725d4e07ba637d5540b5fe10aa1e
import 'dart:async';

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

import 'example_usecase_mixin.dart';
import 'example_widget.dart';

late _ArrangeBuilder _builder;

void main() {
  setUp(() {
    _builder = _ArrangeBuilder();
  });

  testWidgets('when loaded then show the message', (tester) async {
    const expected = 'some message';
    _builder.withExampleUsecaseFetchMessage(() => Future.value(expected));

    await tester.pumpTested();
    await tester.pumpAndSettle();

    expect(find.text(expected), findsOneWidget);
  });

  testWidgets('when failed then show error message', (tester) async {
    final message = () => Future<String>.error(Exception('error'));
    _builder.withExampleUsecaseFetchMessage(message);

    await tester.pumpTested();
    await tester.pumpAndSettle();

    expect(find.text('Something went wrong'), findsOneWidget);
  });

  testWidgets('when not completed then show loading message', (tester) async {
    final message = () => Completer<String>().future;
    _builder.withExampleUsecaseFetchMessage(message);

    await tester.pumpTested();
    await tester.pumpAndSettle();

    expect(find.text('Loading'), findsOneWidget);
  });
}

extension on WidgetTester {
  Future<void> pumpTested() => pumpWidget(
        Directionality(
          textDirection: TextDirection.ltr,
          child: Example(usecase: _builder.exampleUsecase),
        ),
      );
}

/// ArrangeBuilder can use multiple mixins
class _ArrangeBuilder with ExampleUsecaseMixin {
  _ArrangeBuilder() {
    arrangeExampleUsecase();
  }
}