Response Parser makes it easier to parse data and error responses from the server.

Getting started

Do you want to write pretty functions like this...

Future<Either<ApiFailure, User>> fetchUser() async {
  return parseApiResponse(
    requestAction: () => dio.get('/user'),
    mapper: User.fromJson,
  );
}

Future<Either<ApiFailure, User>> fetchUser() async {
  final dio = Dio(BaseOptions(baseUrl: 'https://example.com'));
  try {
    final request = await dio.get('/user');
    final data = request.data?['data'];
    if (data == null) {
      final error = request.data?['error'];
      if (error != null) {
        return left(ApiFailure.serverFailure(error['message']));
      } else {
        return left(ApiFailure.unknown());
      }
    } else {
      return right(User.fromJson(data));
    }
  } catch (error, st) {
    ApiFailure? apiFailure;
    if (error is DioError) {
      final responseFailure = error.response?.data;
      if (responseFailure is Map<String, dynamic>) {
        apiFailure = ApiFailure.serverFailure(responseFailure['message']);
      } else {
        apiFailure = ApiFailure.httpError(error.response?.statusCode);
      }
    }
    return left(apiFailure ?? ApiFailure.unknown());
  }
}

Then continue reading!

Usage

To do so, you need to do a little preparation.
For example, let's assume your server returns such response:

{
  "data": {
    // Data you requested
  },
  "error": {
    // Server error which you should parse and show to user
    "message": "Something went wrong"
  }
}

And your error model looks this way:

class ApiFailure {
  factory ApiFailure.unknown() = _UnknownApiFailure;
  factory ApiFailure.serverFailure(String errorMessage) = _ServerFailure;
  factory ApiFailure.httpError(int? statusCode) = _HttpError;
}

Then you need to implement dataExtractor, failureParser and errorCatcher this way:

final _exampleResponseParser = ResponseParser<Response, ApiFailure>(
  dataExtractor: (response) => response.data['data']!,
  failureParser: (response) {
    final error = json['error'];
    if (error is Map<String, dynamic>) {
      return ApiFailure.serverFailure(error['message']);
    } else {
      return null;
    }
  },
  errorCatcher: (error, stackTrace) {
    ApiFailure? apiFailure;
    if (error is DioError) {
      apiFailure = ApiFailure.httpError(error.response?.statusCode);
    }
    return apiFailure ?? ApiFailure.unknown();
  },
);

And create top level parseApiResponse, parseListApiResponse and parseEmptyApiResponse functions.

final parseApiResponse = _exampleResponseParser.parseApiResponse;
final parseListApiResponse = _exampleResponseParser.parseListApiResponse;
final parseEmptyApiResponse = _exampleResponseParser.parseEmptyApiResponse;

How it works

This diagram shows how parseApiResponse works under the hood:

Actually, everything in the parseApiResponse method is wrapped with the try-catch block. So this method is safe and can't throw any exceptions.

Another way to use

Instead of creating top-level functions, you can create a class which extends ResponseParserBase and overrides it's methods:

class DefaultResponseParser extends ResponseParser<Response, ApiFailure>{
  Object extractData(Response response) => response.data['data']!;

  Failure? parseFailure(Response response) {
    final error = json['error'];
    if (error is Map<String, dynamic>) {
      return ApiFailure.serverFailure(error['message']);
    } else {
      return null;
    }
  };

  Failure catchError(Object error, StackTrace stackTrace) {
    ApiFailure? apiFailure;
    if (error is DioError) {
      apiFailure = ApiFailure.httpError(error.response?.statusCode);
    }

    return apiFailure ?? ApiFailure.unknown();
  };
}

That's all!
For more info, you can take a look at the example.