How to catch exception in flutter? - 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.

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.

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);

Answer 3

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

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());
}