Step 1 - ApiResponse and client contract

class ApiResponse<T> {
  final int status;
  final T? data;
  final String? error;

  ApiResponse({required this.status, this.data, this.error});

  bool get isSuccess => status >= 200 && status < 300;
}

abstract class ApiClient {
  Future<ApiResponse<Map<String, dynamic>>> post(
    String path, {
    Map<String, dynamic>? body,
  });
}

Step 2 - Fake HTTP client

class FakeApiClient implements ApiClient {
  ApiResponse<Map<String, dynamic>>? next;

  @override
  Future<ApiResponse<Map<String, dynamic>>> post(
    String path, {
    Map<String, dynamic>? body,
  }) async {
    return next ?? ApiResponse(status: 500, error: 'unset');
  }
}

Step 3 - Repository

abstract class AuthRepository {
  Future<ApiResponse<Map<String, dynamic>>> login(String email, String password);
}

class RemoteAuthRepository implements AuthRepository {
  final ApiClient _api;

  RemoteAuthRepository(this._api);

  @override
  Future<ApiResponse<Map<String, dynamic>>> login(String email, String password) {
    return _api.post(
      '/login',
      body: <String, dynamic>{'email': email, 'password': password},
    );
  }
}

Step 4 - Service

class AuthService {
  final AuthRepository _repo;

  AuthService(this._repo);

  Future<bool> signIn(String email, String password) async {
    final r = await _repo.login(email, password);
    return r.isSuccess;
  }
}

Step 5 - Composition root

Future<void> main() async {
  final api = FakeApiClient()
    ..next = ApiResponse(status: 200, data: {'token': 'abc'});
  final repo = RemoteAuthRepository(api);
  final auth = AuthService(repo);
  print(await auth.signIn('a@b.com', 'x'));
}

Practice tasks

• Add HttpApiClient implements ApiClient with dart:io HttpClient or package:http if you want real I/O.
• Register the same graph in a get_it setup function without changing AuthService’s constructor.
• Add Result<bool> from Level 6 instead of bare bool for signIn outcomes.