gistfile1.m

// Objective-C:
user.userId = @"unknown";

if ([object isKinfOfClass:[NSDictionary class]]) {

  if ([[((NSDictionary *)object) objectForKey:@"user_id"] isKindOfClass:[NSString class]]) {
  
    user.userId = (NSString *)[((NSDictionary *)object) objectForKey:@"user_id"];
  }
}


// Swift:
user.userId = (object as? NSDictionary)?["user_id"] as? String ?? "unknown"
// that's all :)