Question 76526 in Flutter Question posted in
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The return type 'Null' isn't a 'Future<Message>', as required by the closure's context – Flutter

i am using Mockitou package to Test my MessageBloc and the logic of my Message application in Flutter but i have common issue in all my tests mentioned in the title (The return type ‘Null’ isn’t a ‘Future<Message>’, as required by the closure’s context.) i Comment in the code where the error is

import 'package:chat/chat.dart';
import 'package:flutter_newapp/src/blocs/message/message_bloc.dart';
import 'package:flutter_newapp/src/blocs/message/message_event.dart';
import 'package:flutter_newapp/src/blocs/message/message_state.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';

class FakeMessageService extends Mock implements IMessageService {}

void main() {
  late MessageBloc sut;
  late IMessageService messageService;
  User user;

  setUp(() {
    messageService = FakeMessageService();
    user = User(
        username: 'test', photoUrl: '', active: true, lastSeen: DateTime.now());
    sut = MessageBloc(messageService);
  });

  tearDown(() => sut.close());

  test('it should emit initial state before Subscription',
      () => expect(sut.state, MessageInitial()));

  test('should emit message sent state when message is sent', () {
    final message = Message(
        from: '1234',
        to: '123',
        timestamp: DateTime.now(),
        contents: 'Hellllo');
    when(messageService.send([message])).thenAnswer((_) async => null); // Here is the Error
    sut.add(MessageEvent.onMessageSent([message]));
    expectLater(sut.stream, emits(MessageState.sent(message)));
  });
}

So if anyone have a Solution i will be So grateful

Do i have to Override the Methods of IMessageService or Something ? i think that this is the problem

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2

Answers


  1. 0

    I think messageService.send([message]) returns a type of Future<Message>, that’s why you’re getting this error.

    You can modify that method and set the type to Future<Message?> or provide a dummy Future<Message> value in the test itself.

    Because right now your method can’t return null and you are returning null from the test that’s why you’re facing the issue.

    Hope it helps.

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  2. 0

    This error is occurring because the closure you are passing to then fun is not returning the correct type. The closure is expected to return a Future, but it’s returning null instead.

    To resolve this error, you need to make sure that the closure you pass to then fun returns a Future that completes successfully. You can do this by returning a Future. value(null) instead of just null.

    when(messageService.send([message])).thenAnswer((_) async => Future.value(null));
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