How to handle exeptions in flutter - flutter

Question

How to handle exeptions in flutter?
Why i cant have response.data?
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Future addFine(String apiToken, String uin, int type) async {
try {
Response response = await _dio.post(apiEndpoint + "fines",
options: Options(headers: {"Authorization": apiToken}),
data: {"uin": uin, "type": type});
print(response.data);
print(response.statusMessage);
print(response.statusCode);
} catch (error, stacktrace) {
print(error);
print(stacktrace);
return null;
}
}
my stack trace
I/flutter ( 5580): DioError [DioErrorType.RESPONSE]: Http status error [422]
I/flutter ( 5580): #0 DioMixin._dispatchRequest (package:dio/src/dio.dart:966:7)
I/flutter ( 5580): <asynchronous suspension>
I/flutter ( 5580): #1 DioMixin._request._interceptorWrapper.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:dio/src/dio.dart:849:37)
I/flutter ( 5580): #2 DioMixin.checkIfNeedEnqueue (package:dio/src/dio.dart:1121:22)
I/flutter ( 5580): #3 DioMixin._request._interceptorWrapper.<anonymous closure>.<anonymous closure> (package:dio/src/dio.dart:846:22)
I/flutter ( 5580): #4 new Future.<anonymous closure> (dart:async/future.dart:175:37)
I/flutter ( 5580): #5 _rootRun (dart:async/zone.dart:1182:47)
I/flutter ( 5580): #6 _CustomZone.run (dart:async/zone.dart:1093:19)
I/flutter ( 5580): #7 _CustomZone.runGuarded (dart:async/zone.dart:997:7)
I/flutter ( 5580): #8 _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
I/flutter ( 5580): #9 _rootRun (dart:async/zone.dart:1190:13)
I/flutter ( 5580): #10 _CustomZone.run (dart:async/zone.dart:1093:19)
I/flutter ( 5580): #11 _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:1021:23)
I/flutter ( 5580): #12
Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18:15

Answer 1

If you consider codes that indicate an error valid information and want to access the information anyway, you can decide yourself, what you consider an error:
Response response = await _dio.post(apiEndpoint + "fines",
options: Options(
headers: {"Authorization": apiToken},
validateStatus: (status) => true),
data: {"uin": uin, "type": type});
In the options, you can pass a validator that decides what status codes are valid. In this case I just made it use any code. any code at all. You may want to restrict that further to your usecase, maybe even per call. For example, you could restrict it to "OK" and "Unprocessable Entity", where for example a 401 would still throw an exception:
validateStatus: (status) => { return status == 200 || status == 422; }