networkSignal.m

RACSignal *networkSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        NetworkOperation *operation = [NetworkOperation getJSONOperationForURL:@"http://someurl"];
        [operation setCompletionBlockWithSuccess:^(NetworkOperation *theOperation, id *result) {
            [subscriber sendNext:result];
            [subscriber sendCompleted];
        } failure:^(NetworkOperation *theOperation, NSError *error) {
            [subscriber sendError:error];
        }];