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.

Solution 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.

Solution 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);
  

Solution 3

To handle errors in an async and await function, use try-catch:

Run the following example to see how to handle an error from an asynchronous function.

Future<void> printOrderMessage() async {
  try {
    var order = await fetchUserOrder();
    print('Awaiting user order...');
    print(order);
  } catch (err) {
    print('Caught error: $err');
  }
}

Future<String> fetchUserOrder() {
  // Imagine that this function is more complex.
  var str = Future.delayed(
      Duration(seconds: 4),
      () => throw 'Cannot locate user order');
  return str;
}

Future<void> main() async {
  await printOrderMessage();
}

Within an async function, you can write try-catch clauses the same way you would in synchronous code.

Solution 4

I was trying to find this answer when got to this page, hope it helps: https://stackoverflow.com/a/57736915/12647239

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

View more solutions

Comments

[CopsOnRoad]

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.

[Günter Zöchbauer]

Thanks, should have been async