Use a specific instance of a class inside an isolate

Question

I am using an isolate through the compute() method to fetch, parse and sort datas from an API (around 10k entries).

My method getAllCards() is defined inside a class YgoProRepositoryImpl which has an instance of my remote datasource class YgoProRemoteDataSource it is in this class that the method to call my API is defined (it is a simple GET request).

Code Sample

ygopro_repository_impl.dart

class YgoProRepositoryImpl implements YgoProRepository {
  final YgoProRemoteDataSource remoteDataSource;

  // ...

  YgoProRepositoryImpl({
    required this.remoteDataSource,
    // ...
  });

  // ...

  static Future> _fetchCards(_) async {
    // As I'm inside an isolate I need to re-setup my locator
    setupLocator();
    final cards = await sl()
        .getCardInfo(GetCardInfoRequest(misc: true));
    cards.sort((a, b) => a.name.compareTo(b.name));
    return cards;
  }

  @override
  Future> getAllCards() async {
    final cards = await compute(_fetchCards, null);
    return cards;
  }

  // ...
}

service_locator.dart

import 'package:get_it/get_it.dart';

import 'data/api/api.dart';
import 'data/datasources/remote/ygopro_remote_data_source.dart';
import 'data/repository/ygopro_repository_impl.dart';
import 'domain/repository/ygopro_repository.dart';

final sl = GetIt.instance;

void setupLocator() {
  // ...

  _configDomain();
  _configData();

  // ...

  _configExternal();
}

void _configDomain() {
  //! Domain
  
  // ...

  // Repository
  sl.registerLazySingleton(
    () => YgoProRepositoryImpl(
      remoteDataSource: sl(),
      // ...
    ),
  );
}

void _configData() {
  //! Data
  // Data sources
  sl.registerLazySingleton(
    () => YgoProRemoteDataSourceImpl(sl()),
  );

  // ...
}

void _configExternal() {
  //! External
  sl.registerLazySingleton(() => DioClient());
  
  // ...
}

The code is working properly but getAllCards() is not testable as I cannot inject a mocked class of YgoProRemoteDataSource inside my isolate because it will always get a reference from my service locator.

How can I do to not rely on my service locator to inject YgoProRemoteDataSource inside my isolate and make getAllCards() testable ?

user_2553215 · Answer

Do you really need to test the getCards() function? What are you really testing there? That compute works, sure hope the Dart SDK team has a test for this.

That leaves _fetchCards(), and setupLocator() doesn't need to be tested either, it is precondition for your test-logic. You want to change the setup for the test anyways.

So what you actually want to test is the fetching & sorting. Restructure this into a testable static function and setup your locator beforehand. Put a @visibleForTesting annotation on it.

And on a side-note, depending on how much you bind in your service locator, this could be huge overhead for just using the one repository afterwards.

Example:

  static Future> _fetchCards(_) async {
    // As I'm inside an isolate I need to re-setup my locator
    setupLocator();
    return reallyFetchCards();
  }

  @visibleForTesting
  static Future> reallyFetchCards() async {
    final cards = await sl()
        .getCardInfo(GetCardInfoRequest(misc: true));
    cards.sort((a, b) => a.name.compareTo(b.name));
    return cards;
  }

  @override
  Future> getAllCards() async {
    final cards = await compute(_fetchCards, null);
    return cards;
  }

Test:


// Setup SL and datasource
...

final cards = await YgoProRepositoryImpl.reallyFetchCrads();

// Expect stuff

user_440696 · Answer

Did a more serious attempt, please see the repo: https://github.com/maxim-saplin/compute_sl_test_sample

Essentially with the current state of affairs with Flutter/Dart you can't pass neither closures nor classes containing closures across isolates boundaries (yet that might change when newer features in Dart land Flutter https://github.com/dart-lang/sdk/issues/46623#issuecomment-916161528). That means there's no way you can pass service locator (which contains closures) or trick the isolate to instantiate a test version of locator via closure IF you don't want any test code to be part of the release build. Yet you can easily pass data source instance to isolate to be used at its entry point as a param.

Beside, I don't think asking isolate to rebuild the entire service locator makes sense. The whole idea behind compute() is to create a short leaving isolate, run the computation, return the result and terminate the isolate. Initialising the locator is an overhead which is better to be avoided. Besides it seems the whole concept of compute() is being as isolated from the rest of the app as possible.

You can clone the repo and run the tests. Few words about the sample:

Based on Flutter counter starter app
lib/classes.dart recreates the code snippet you provided
test/widget_test.dart verifies that YgoProRepositoryImpl is working fine with isolate running fake version of data source
YgoProRemoteDataSourceImpl mimics real implementation and is located at classes.dart and YgoProRemoteDataSourceFake mimics test version
Running isolates under flutter_test requires wrapping test body in tester.runAsync() in order to have real time async execution (rather than fake async used by default by tests and relying on pumping to progress test time). Running tests in this mode can be slow (there's actual 0.5 second wait), structuring the tests in a way when compute() is not used or tested not in many tests is reasonable

classes.dart

import 'package:flutter/foundation.dart';
import 'package:get_it/get_it.dart';

final sl = GetIt.instance;

class YgoCard {
  YgoCard(this.name);

  final String name;
}

abstract class YgoProRemoteDataSource {
  Future> getCardInfo();
}

class YgoProRemoteDataSourceImpl extends YgoProRemoteDataSource {
  @override
  Future> getCardInfo() {
    return Future.delayed(Duration.zero,
        () => List.generate(5, (index) => YgoCard("Impl $index")));
  }
}

abstract class YgoProRepository {
  Future> getAllCards();
}

class YgoProRepositoryImpl implements YgoProRepository {
  final YgoProRemoteDataSource remoteDataSource;

  YgoProRepositoryImpl({
    required this.remoteDataSource,
  });

  static Future> _fetchCards(
      YgoProRemoteDataSource dataSource) async {
    final cards = await dataSource.getCardInfo();
    cards.sort((a, b) => a.name.compareTo(b.name));
    return cards;
  }

  @override
  Future> getAllCards() async {
    final cards = await compute(_fetchCards, remoteDataSource);
    return cards;
  }
}

void setupLocator() {
  sl.registerLazySingleton(
    () => YgoProRepositoryImpl(
      remoteDataSource: sl(),
    ),
  );

  sl.registerLazySingleton(
    () => YgoProRemoteDataSourceImpl(),
  );
}

widget_test.dart

import 'package:flutter_test/flutter_test.dart';
import 'package:test_sample/classes.dart';

import 'package:test_sample/main.dart';

void main() {
  setUpAll(() async {
    setupFakeLocator();
  });

  testWidgets('Test mocked data source', (WidgetTester tester) async {
    // Wrapping with runAync() is required to have real async in place
    await tester.runAsync(() async {
      await tester.pumpWidget(const MyApp());
      // Let the isolate spawned by compute() complete, Debug run might require longer wait
      await Future.delayed(const Duration(milliseconds: 500));
      await tester.pumpAndSettle();
      expect(find.text('Fake 9'), findsOneWidget);
    });
  });
}

class YgoProRemoteDataSourceFake extends YgoProRemoteDataSource {
  @override
  Future> getCardInfo() {
    return Future.delayed(Duration.zero,
        () => List.generate(10, (index) => YgoCard("Fake $index")));
  }
}

void setupFakeLocator() {
  sl.registerLazySingleton(
    () => YgoProRepositoryImpl(
      remoteDataSource: sl(),
    ),
  );

  sl.registerLazySingleton(
    () => YgoProRemoteDataSourceFake(),
  );
}

user_11086844 · Answer

As I understand you have two options, either inject the dependencies needed for static Future> _fetchCards(_) async via parameters, or mock the object in the locator itself. I would go for the fist option, and have something like :

  static Future> _fetchCards(_,YgoProRemoteDataSource remote) async {
    // No need to set up locator as you passed the needed dependencies
    // setupLocator();
    final cards = await remote
        .getCardInfo(GetCardInfoRequest(misc: true));
    cards.sort((a, b) => a.name.compareTo(b.name));
    return cards;
  }

  @override
  Future> getAllCards() async {
    final cards = await compute(_fetchCards, null);
    return cards;
  }

Edit

just updated the answer as its easier to edit this here than in the comments...

Hmm, the only workaround that I can think of is to pass the setupLocator() function as an argument to the class YgoProRepositoryImpl :

final Function setupLocator;
  YgoProRepositoryImpl({
    required this.remoteDataSource,
required this.setupLocator;
    // ...
  });

This way you could pass a mock that sets up your mock classes or the real setupLocator of your service_locator.dart. This might not be to elegant. But it should make it testable as now you can mock the setup and its not hardcoded in the function

Use a specific instance of a class inside an isolate

Code Sample

3 Answers

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