Mockito + Dio + Flutter

かなり苦労しましたが、こんな感じで実装可能です

import 'package:http_mock_adapter/http_mock_adapter.dart';
import 'package:mockito/mockito.dart';
// その他mockとかimportしてるファイルありますが、割愛しました


void main() {
  WidgetsFlutterBinding.ensureInitialized();

  GroupRepository repository;
  MockHttpClient httpClient;
  FirebaseAuth firebase;
  DioAdapterMockito dioAdapterMockito;
  Dio dio;

  setUp(() async {
    dio = Dio();
    dioAdapterMockito = DioAdapterMockito();
    dio.httpClientAdapter = dioAdapterMockito;
    httpClient = MockHttpClient(dio: dio);
    firebase = MockFirebaseAuth();
    await firebase.signInWithEmailAndPassword(
      email: 'test@gmail.com',
      password: '11111111',
    );
    repository = GroupRepository(
      httpClient: httpClient,
      fb: firebase,
    );
  });

  test('findAll()', () async {
    const body = [
      {'id': 1, 'name': '青年会議所', 'description': 'test'},
      {'id': 2, 'name': 'Vertech', 'description': 'test'}
    ];

    final responsePayload = jsonEncode(body);
    final responseBody = ResponseBody.fromString(
      responsePayload,
      200,
      headers: {
        Headers.contentTypeHeader: [Headers.jsonContentType],
      },
    );

    when(dioAdapterMockito.fetch(any, any, any))
        .thenAnswer((_) async => responseBody);

    final groups = await repository.findAll();

    expect(groups.length, 2);
    expect(groups[0].name, '青年会議所');
    expect(groups[0].description, 'test');
    expect(groups[1].name, 'Vertech');
    expect(groups[1].description, 'test');
  });
}

参考記事

Simulating HTTP request/response workflow for effective testing in Dart/Flutter via http-mock-adapter