Return value for function inside a block

You can’t use the completion Block to create a return value for your method. The AFJSONRequestOperation does its work asynchronously. someFunction is going to return while the operation is still working. The success and failure Blocks are how you get resulting values where they need to go.

One option here is to pass in the caller as an argument to your wrapper method so that the completion Block can hand the array off.

- (void)goFetch:(id)caller
{
    AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request
    success: ^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {

        [caller takeThisArrayAndShoveIt:[JSON valueForKey:@"posts"]];
    }
    failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id JSON) {}
}

You could also make your caller create and pass a Block to be run on success. Then goFetch: no longer needs to know what properties exist on the caller.

- (void)goFetch:(void(^)(NSArray *))completion
{
    AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request
    success: ^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
        if( completion ) completion([JSON valueForKey:@"posts"]);
    }
    failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id JSON) {}
}