How to catch exception in flutter?

Question

This is my exception class. Exception class has been implemented by the abstract exception class of flutter. Am I missing something?

class FetchDataException implements Exception {
 final _message;
 FetchDataException([this._message]);

String toString() {
if (_message == null) return "Exception";
  return "Exception: $_message";
 }
}


void loginUser(String email, String password) {
  _data
    .userLogin(email, password)
    .then((user) => _view.onLoginComplete(user))
    .catchError((onError) => {
       print('error caught');
       _view.onLoginError();
    });
}

Future < User > userLogin(email, password) async {
  Map body = {
    'username': email,
    'password': password
  };
  http.Response response = await http.post(apiUrl, body: body);
  final responseBody = json.decode(response.body);
  final statusCode = response.statusCode;
  if (statusCode != HTTP_200_OK || responseBody == null) {
    throw new FetchDataException(
      "An error occured : [Status Code : $statusCode]");
   }
  return new User.fromMap(responseBody);
}

CatchError doesn't catch the error when the status is not 200. In short error caught is not printed.

Watthour

Your question How to catch exception in flutter? is general, but your code is personal. Can someone explain how to catch and print errors in Flutter??? – Isolate

Answer 1

Try

void loginUser(String email, String password) async {
  try {
    var user = await _data
      .userLogin(email, password);
    _view.onLoginComplete(user);
      });
  } on FetchDataException catch(e) {
    print('error caught: $e');
    _view.onLoginError();
  }
}

catchError is sometimes a bit tricky to get right. With async/await you can use try/catch like with sync code and it is usually much easier to get right.

Plug

Günter, I think we can't use await after method parameters ends. You should use async there. I am not having IDE right now, so can't check if await is valid there. – Prettify

Thanks, should have been async – Undis

Answer 2

Let's say this is your function which throws an exception:

Future<void> foo() async {
  throw Exception('FooException');
}

You can either use try-catch block or catchError on the Future since both do the same thing.

• Using try-catch

  try {
    await foo();
  } on Exception catch (e) {
    print(e); // Only catches an exception of type `Exception`.
  } catch (e) {
    print(e); // Catches all types of `Exception` and `Error`.
  }
  

• Use catchError

  await foo().catchError(print);
  

Prettify

how to catch a specific exception with the catchError? – Dropforge

Answer 3

I was trying to find this answer when got to this page, hope it helps: https://mcmap.net/q/274624/-how-do-i-return-error-from-a-future-in-dart

Basicly i was just trying to catch an error message from a method, but i was calling

throw Exception("message")

And in "catchError" i was getting "Exception: message" instead of "message".

catchError(
  (error) => print(error)
);

fixed with the return in the above reference

Degraw

Answer 4

Future < User > userLogin(email, password) async { try {
  Map body = {
    'username': email,
    'password': password
  };
  http.Response response = await http.post(apiUrl, body: body);
  final responseBody = json.decode(response.body);
  final statusCode = response.statusCode;
  if (statusCode != HTTP_200_OK || responseBody == null) {
    throw new FetchDataException(
      "An error occured : [Status Code : $statusCode]");
   }
  return new User.fromMap(responseBody); }
   catch (e){
    print(e.toString());
}

Yeisk

Answer 5

You can throw exception as below as well.

 Future.error('Location permissions are denied');

More important, Sometimes you may face to a situation where you want to take action on certain exceptions.

Example:

  _getCurrentLocation() async {
    try {
      LocationPermission permission = await Geolocator.checkPermission();
      if (permission == LocationPermission.denied) {
        permission = await Geolocator.requestPermission();
        if (permission == LocationPermission.denied) {
          throw Exception("Location permissions are denied");
          //Future.error('Location permissions are denied');
        }
      }

      if (permission == LocationPermission.deniedForever) {
        throw Exception("Location permissions permanently denied.");
        //Future.error('Location permissions permanently denied.');
      }

      Position pos = await Geolocator.getCurrentPosition();
      _currentPosition = LatLng(pos.latitude, pos.longitude);
    } on Exception catch (e) {
      String message = e.toString();
      if (message == "Location permissions are denied") {
        print("Action on denied location");
        return;
      }
      if (message == "Location permissions permanently denied.") {
        print("Action on permanently denied location");
        return;
      }

    }

Middleoftheroad