title: Change value of NSString inside block
body: <p>I have a problem with setting value to NSString inside block.</p>
  
  <pre><code>__block NSString *cityID = [[NSString alloc] init];
  AFJSONRequestOperation *operation = [AFJSONRequestOperation
                                       JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id json) {
                                           cityID = [NSString stringWithFormat:@"%@",[[json valueForKeyPath:@"location"] valueForKey:@"id"]];
                                           NSLog(@"city id for 621352674 = %@",cityID);
                                       } failure:nil]; 
  [operation start];
  NSLog(@"city id for 621352674 = %@",cityID);
  </code></pre>
  
  <p>First NSLog shows me right value of cityID. But second shows me nothing. How to ix this problem?</p>