为什么我的后台任务不会马上退出？

Question

当我运行程序时，无论其中的代码是否在一秒钟内完成，后台任务似乎都会运行20秒左右。这是我的viewdidload：

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.yesButton.layer.cornerRadius = 20;
    self.yesButton.layer.borderColor=[UIColor blackColor].CGColor;
    self.yesButton.layer.borderWidth= 2.0f;

    self.noButton.layer.cornerRadius = 20;
    self.noButton.layer.borderColor=[UIColor blackColor].CGColor;
    self.noButton.layer.borderWidth= 2.0f;

    self.invalidISBNImage.hidden = true;
    self.noInternetImage.hidden = true;
    [self.yesButton setEnabled:NO];
    [self.noButton setEnabled:NO];


    if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
    {
        if([[UIDevice currentDevice] isMultitaskingSupported])
        {
            UIApplication *application = [UIApplication sharedApplication];

            __block UIBackgroundTaskIdentifier background_task;

            background_task = [application beginBackgroundTaskWithExpirationHandler: ^ {
                [application endBackgroundTask: background_task];
                background_task = UIBackgroundTaskInvalid;
            }];

            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

                if([self canReachInternet])
                {
                    NSString *siteUrl = [NSString stringWithFormat:@"http://www.isbnsearch.org/isbn/%@", isbnSelling];
                    NSURL *url = [NSURL URLWithString:siteUrl];
                    NSString *webDataString = [NSString stringWithContentsOfURL:url];

                    if(webDataString == nil || [isbnSelling isEqualToString:@""])
                    {
                        NSLog(@"ISBN not recognized.");
                        invalidISBN = true;
                        self.invalidISBNImage.hidden = false;
                    }

                    else
                    {
                        // This is where I parse some html
                    }
                }

                else
                {
                    NSLog(@"Failed to establish a connection to the internet.");
                    noInternetConnection = true;
                    self.noInternetImage.hidden = false;
                }

                [application endBackgroundTask: background_task];
                background_task = UIBackgroundTaskInvalid;


                self.titleLabel.text = title;
                self.authorLabel.text = author;
                self.editionLabel.text = edition;
                self.publisherLabel.text = publisher;
                self.datePublishedLabel.text = datePublished;
                self.bindingLabel.text = binding;

                self.loadingImage.hidden = true;

                if(!invalidISBN && !noInternetConnection)
                {
                    [self.yesButton setEnabled:YES];
                    [self.noButton setEnabled:YES];
                }
            });
        }
    }
}

例如，当满足以下条件时： if（webDataString == nil || [isbnSelling isEqualToString：@“”]）

此行不能立即生效： self.invalidISBNImage.hidden = false; 我想要的时候。它发生在20秒之后......我认为程序的流程几乎会立即结束后台任务，但即使它在一秒内到达这个部分，它似乎也会运行后台任务还有20秒。是否有一行代码可以完全退出后台任务？