---

This section details on the original issue you should resolve

<issue_title>Add error handler</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
At the moment, one of the challenges is that callers of the generated Retrofit.dart code have to know about things like a DioError. I feel like this is an implementation detail of the networking layer, and it would be nice to be able to (within the Retrofit file) catch and provide common error handling and/or possible modification to any errors.

Describe the solution you'd like
I was thinking that we could add another parameter to the RestApi annotation which is a function reference. Something like:

@RestApi(onError: handleError)
abstract class MyAPI {
  @POST('/login')
  Future<User> login({String userid, String password});

  Exception handleError(Exception e) {
    // this is where I would do any common error handling
    return MyNewException();
  }
}

And then, in the generated code, if the onError parameter has been set, then it would call this function before rethrowing.

Note that I think that the error handling should also encompass both the network call and also any calls that convert to/from JSON.

Describe alternatives you've considered
I did think about adding the following function in the generated class:

class _MyAPI implements MyAPI {
  ...

  Exception handleError(Exception e) {
    // standard error handling just returns the error untouched
    return e;
  }
}

And then the app's class could override handleError(). The same changes would be required in the generated code, however, I feel like that isn't super discoverable for users of the Retrofit library.

At the moment, the only "workaround" I can find is that every caller has to apply their own common error handling. That behaviour can be extracted into a common function, but I do believe that the domain-specific business logic should not know about things like DioError.

Additional context
None</issue_description>

Comments on the Issue (you are @copilot in this section)

@trevorwang > I also find myself wrapping every rest client method call into something like this: > > `_restClient.apiMethodCall(...).catchError((ex) => errorHandler(ex));` > > That is a lot of boiler plate code and the same `errorHandler` is shared between multiple unrelated services. > > Currently it is not possible to replace this with a Dio interceptor, because some exceptions, such as data conversions or `SocketException` are not being routed there. > > Also the proposed `@ErrorHandler()` methods in the rest client is not ideal, because those methods may have to use framework specific code, which shouldn't be in the rest client (eg. Bloc or other state UI or navigation) and also this won't allow to share error handlers. > > It would be neat if you could just specify an optional `onError` parameter in the rest client constructor declaration with the same signature as used by `Future.catchError()`: > > `factory RestClient(Dio dio, {String baseUrl, Function onError}) = _RestClient;` > > and then have `retrofit_generator` would generate and wire in the above `.catchError(onError)` in the rest client implementation.

It sounds like more ideal and makes a minimum change. @ekuleshov @TekExplorer </comment_new>
<comment_new>@trevorwang
I have the same opinion with @ekuleshov . It seems that It doesn't provide more benefits than a normal error handle from you proposal. @TekExplorer </comment_new>