下面的代码确实调用了一个网络服务，但我不完全理解它的结构。

NSError *error;
NSData *postData = [NSJSONSerialization dataWithJSONObject: requestData options:0 error:&error];
NSString *postLength = [NSString stringWithFormat:@"%lu", (unsigned long)[postData length]];
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
[request setURL:url];
[request setHTTPMethod:@"POST"];
[request setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
[request setValue:@"private" forHTTPHeaderField:@"Cache-Control"];
[request setValue:@"Microsoft-IIS/7.5" forHTTPHeaderField:@"Server"];
[request setValue:@"Basic realm='TimeBoxWebAPI'" forHTTPHeaderField:@"WWW-Authenticate"];

[request setValue:postLength forHTTPHeaderField:@"Content-Length"];
[request setValue:@"application/json" forHTTPHeaderField:@"Accept"];
[request setHTTPBody:postData];

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[NSURLConnection sendAsynchronousRequest:request 
                                   queue:queue         
                       completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) 
{
    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
    if ([data length] >0 && error == nil){
        dispatch_queue_t myQueue = dispatch_queue_create("My Queue", DISPATCH_QUEUE_SERIAL);

            dispatch_async(myQueue, ^{

                [self parseGetSchedulingData:data];
                dispatch_async(dispatch_get_main_queue(), ^{
                     [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
                         [tableHolidays reloadData];
                         [refreshControl endRefreshing];

                     if(activityIndicator.isAnimating)
                     {
                         [activityIndicator stopAnimating];
                         [activityIndicator removeFromSuperview];
                     }
                 });
             });
         }
         else if ([data length] == 0 && error == nil){
             NSLog(@"Empty Response, not sure why?");

             [[[UIAlertView alloc] initWithTitle: nil
                                         message:@"Error while trying to log in. Please try later"
                                        delegate:nil
                               cancelButtonTitle:@"OK"
                               otherButtonTitles:nil] show];

             dispatch_async(dispatch_get_main_queue(), ^{
                 [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
                 if(activityIndicator.isAnimating)
                 {
                     [activityIndicator stopAnimating];
                     [activityIndicator removeFromSuperview];
                 }
             });

         }


         else if (error != nil){
             NSLog(@"%@", error.description);
             dispatch_async(dispatch_get_main_queue(), ^{
                 [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
                 if(activityIndicator.isAnimating)
                 {
                     [activityIndicator stopAnimating];
                     [activityIndicator removeFromSuperview];
                 }
             });
         }
     }];
}

特别是，我不确定对 setValue:forHTTPHeaderField: 和 sendAsynchronousRequest:queue:completion: 的调用到底做了什么。