Flutter API异步错误导致getUsers接口失败测试未通过

我正在用测试驱动开发（TDD）实现AuthenticationRemoteDataSource，这是和API交互的上层实现，包含createUser和getUsers两个方法。createUser的成功、失败测试都通过了，getUsers的成功测试也没问题，但失败状态的测试始终失败，找不到问题所在。

实现代码 #

AuthenticationRemoteDataSourceImplementation中的getUsers方法：

Future<List<UserModel>> getUsers() async {
    try {
      final response =
          await _client.get(Uri.parse('$kBaseUrl$kGetUsersEndpoint'));

      if (response.statusCode != 200) {
        throw (
          APIException(
            message: response.body,
            statusCode: response.statusCode,
          ),
        );
      }

      return List<DataMap>.from(jsonDecode(response.body) as List)
          .map((userdata) => UserModel.fromMap(userdata))
          .toList();
    } on APIException {
      rethrow;
    } catch (e) {
      throw APIException(message: e.toString(), statusCode: 505);
    }
  }

注：DataMap即Map<String, dynamic>，_client是http.Client实例，APIException是包含消息和状态码的自定义异常。

测试代码 #

getUsers的测试用例：

group('getUsers', () {
    const testList = [UserModel.empty()];
    test('Should return a [List<UserModel>] when the status code is 200',
        () async {
      // stubbing
      when(
        () => client.get(
          any(),
        ),
      ).thenAnswer((_) async =>
          http.Response(jsonEncode([testList.first.toMap()]), 200));

      // act
      final result = await remoteDataSource.getUsers();

      // assert
      expect(result, equals(testList));

      verify(() => client.get(Uri.parse('$kBaseUrl$kGetUsersEndpoint')))
          .called(1);
      verifyNoMoreInteractions(client);
    });

    // 失败的测试用例
    test('Should throw an [APIException] when the status code is not 200',
        () async {
      // stubbing
      when(() => client.get(any())).thenAnswer(
        (_) async => http.Response(
          'Server is down',
          505,
        ),
      );

      // act
      final methodCall = remoteDataSource.getUsers;

      // assert
      expect(
        () => methodCall(),
        throwsA(
          const APIException(
            message: 'Server is down',
            statusCode: 505,
          ),
        ),
      );

      verify(
        () => client.get(
          Uri.parse('$kBaseUrl$kGetUsersEndpoint'),
        ),
      ).called(1);

      verifyNoMoreInteractions(client);
    });
  });

报错信息 #

Expected: throws APIException:<APIException(Server is down, 505)>
  Actual: <Closure: () => Future<List<UserModel>>>
   Which: threw APIException:<APIException((APIException(Server is down, 505)), 505)>
          stack package:cleanarchtdd/src/authentication/data/datasources/authentication_remote_data_source.dart 78:7  AuthenticationRemoteDataSourceImplementation.getUsers
                ===== asynchronous gap ===========================
                package:matcher                                                                                       expect
                package:flutter_test/src/widget_tester.dart 454:18                                                    expect
                test\src\authentication\data\datasources\authentication_remote_data_source_test.dart 147:7            main.<fn>.<fn>

package:matcher                                                                             expect
package:flutter_test/src/widget_tester.dart 454:18                                          expect
test\src\authentication\data\datasources\authentication_remote_data_source_test.dart 147:7  main.<fn>.<fn>

我试过在when和verify里加async关键字，但引发了其他错误，而且这个测试的写法和createUser的失败测试完全一致。

问题原因及修复方案 #

1. 核心问题：throw语句的多余括号
   从报错信息里的APIException((APIException(Server is down, 505)), 505)能看出，实际抛出的异常是把原本的APIException作为了新异常的message。原因是getUsers里的throw语句多了一对括号：

   // 错误写法：抛出的是一个包含APIException的元组
   throw (
     APIException(
       message: response.body,
       statusCode: response.statusCode,
     ),
   );
   

   这会导致抛出的不是APIException实例，而是一个元组，这个元组会被后续的catch (e)捕获，然后被包装成一个新的APIException，message就是元组的字符串形式。

   修复成：

   // 正确写法：直接抛出APIException实例
   throw APIException(
     message: response.body,
     statusCode: response.statusCode,
   );
   

2. 异步测试的写法优化
   测试中的异步异常断言，建议用expectLater来确保异步操作完成后再断言，避免时序问题：

   // 替换原来的expect代码
   await expectLater(
     methodCall(),
     throwsA(const APIException(message: 'Server is down', statusCode: 505)),
   );
   

完成这两处修改后，失败的测试应该就能通过了。

内容的提问来源于stack exchange，提问作者byraychen