如果正在iOS中使用NSURLSession，app会在下载完成时自动重启。AppDelegate中的方法application:handleEventsForBackgroundURLSession:completionHandler:负责创建适当地session，保存completionHandler(这是该方法的传递的，需要我们去手动保存它)，并在session的代理方法URLSessionDidFinishEventsForBackgroundURLSession:去调用保存的completionHandler。下面的代码展示了一个创建和启动一个下载任务：

/// Objective-C
NSURL *url = [NSURL URLWithString:@"https://www.example.com/"];
NSURLSessionDownloadTask *backgroundDownloadTask = [backgroundSession downloadTaskWithURL:url];
[backgroundDownloadTask resume];

/// Swift
if let url = URL(string: "https://www.example.com/") {
    let backgroundDownloadTask = backgroundSession.downloadTask(with: url)
    backgroundDownloadTask.resume()
}

下面是iOS中后台下载的session代理方法：

/// Objective-C
- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    AppDelegate *appDelegate = (AppDelegate *)[[[UIApplication sharedApplication] delegate];
    if (appDelegate.backgroundSessionCompletionHandler) {
        CompletionHandler completionHandler = appDelegate.backgroundSessionCompletionHandler;
        appDelegate.backgroundSessionCompletionHandler = nil;
        completionHandler();
    }

    NSLog(@"All tasks are finished");
}

/// Swift
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
    guard let appDelegate = UIApplication.sharedApplication.delegate as? AppDelegate else {
        return
    }

    if let completionHandler = appDelegate.backgroundSessionCompletionHandler {
        appDelegate.backgroundSessionCompletionHandler = nil
        completionHandler()
    }
}

下面是AppDelegate中的代理方法：

/// Objective-C
@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@property (copy) CompletionHandler backgroundSessionCompletionHandler;

@end

@implementation AppDelegate

- (void)application:(UIApplication *)application
handleEventsForBackgroundURLSession:(NSString *)identifier
  completionHandler:(void (^)())completionHandler
{
    self.backgroundSessionCompletionHandler = completionHandler;
}

@end

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    var backgroundSessionCompletionHandler: CompletionHandler?
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        backgroundSessionCompletionHandler = completionHandler
    }
}