How to throw an exception from a server API code in Dart? - dart

Question

I'm developing a client-server application in Dart and have been following the tutorial. My server code is roughly based on it.
In my server API code, when something goes wrong, I want to throw an exception, for example:
void checkEverything() {
if(somethingWrong)
throw new RpcError(400, "Something Wrong", "Something went wrong!");
}
#ApiMethod(path: 'myservice/{arg}')
Future<String> myservice(String arg) async {
checkEverything();
// ...
return myServiceResponse;
}
and that exception should be processed in the main server, e.g.
// ...
var apiResponse;
try {
var apiRequest = new HttpApiRequest.fromHttpRequest(request);
apiResponse = await _apiServer.handleHttpApiRequest(apiRequest);
} catch (error, stack) {
var exception = error is Error ? new Exception(error.toString()) : error;
if((error is RpcError && error.statusCode==400) {
// My code for creating the HTTP response
apiResponse = new HttpApiResponse.error(
HttpStatus.BAD_REQUEST, "Something went wrong", exception, stack);
}
else {
// standard error processing from the Dart tutorial
apiResponse = new HttpApiResponse.error(
HttpStatus.INTERNAL_SERVER_ERROR, exception.toString(),
exception, stack);
}
}
(snippet, see the tutorial for the complete code sans my error handling).
However, my exception never reaches the above catch clause. Instead, it seems to get caught in _apiServer.handleHttpApiRequest(apiRequest);, which, in turns, throws INTERNAL_SERVER_ERROR (500):
[WARNING] rpc: Method myservice returned null instead of valid return value
[WARNING] rpc:
Response
Status Code: 500
Headers:
access-control-allow-credentials: true
access-control-allow-origin: *
cache-control: no-cache, no-store, must-revalidate
content-type: application/json; charset=utf-8
expires: 0
pragma: no-cache
Exception:
RPC Error with status: 500 and message: Method with non-void return type returned 'null'
Unhandled exception:
RPC Error with status: 400 and message: Something went wrong!
#0 MyApi.myservice (package:mypackage/server/myapi.dart:204:24)
[...]
This is not very specific for the client. I'd like to communicate that an error has happened, not to return a good-looking response. So what is the proper way of handling server-side exceptions in Dart and passing that information to the client?

Answer 1

OK, I think I solved the problem. The throw clause apparently has to be in the API method itself, and not in a subordinate method. I.e.:
#ApiMethod(path: 'myservice/{arg}')
Future<String> myservice(String arg) async {
if(somethingWrong)
throw new RpcError(400, "Something Wrong", "Something went wrong!");
// ...
return myServiceResponse;
}
and not:
void checkEverything() {
if(somethingWrong)
throw new RpcError(400, "Something Wrong", "Something went wrong!");
}
#ApiMethod(path: 'myservice/{arg}')
Future<String> myservice(String arg) async {
checkEverything();
// ...
return myServiceResponse;
}