The task bypasses calls to delegate methods for response and data delivery, and instead provides any resulting NSData, URLResponse, and NSError objects inside the completion handler. Delegate methods for handling authentication challenges, however, are still called.

class func backgroundSessionConfigurationWithIdentifier(_ identifier: String) -> NSURLSessionConfiguration

Use this method to initialize a configuration object suitable for transferring data files while the app runs in the background. A session configured with this object hands control of the transfers over to the system, which handles the transfers in a separate process. In iOS, this configuration makes it possible for transfers to continue even when the app itself is suspended or terminated.

If an iOS app is terminated by the system and relaunched, the app can use the same identifier to create a new configuration object and session and retrieve the status of transfers that were in progress at the time of termination. This behavior applies only for normal termination of the app by the system. If the user terminates the app from the multitasking screen, the system cancels all of the session’s background transfers. In addition, the system does not automatically relaunch apps that were force quit by the user. The user must explicitly relaunch the app before transfers can begin again.

optional func URLSessionDidFinishEventsForBackgroundURLSession(_ session: NSURLSession)

In iOS, when a background transfer completes or requires credentials, if your app is no longer running, your app is automatically relaunched in the background, and the app’s UIApplicationDelegate is sent an application:handleEventsForBackgroundURLSession:completionHandler: message. This call contains the identifier of the session that caused your app to be launched. Your app should then store that completion handler before creating a background configuration object with the same identifier, and creating a session with that configuration. The newly created session is automatically reassociated with ongoing background activity.

When your app later receives a URLSessionDidFinishEventsForBackgroundURLSession: message, this indicates that all messages previously enqueued for this session have been delivered, and that it is now safe to invoke the previously stored completion handler or to begin any internal updates that may result in invoking the completion handler.

           optional func application(_ application: UIApplication,

handleEventsForBackgroundURLSession identifier: String,

                  completionHandler completionHandler: () -> Void)

completionHandler

The completion handler to call when you finish processing the events. Calling this completion handler lets the system know that your app’s user interface is updated and a new snapshot can be taken.

whether they finished successfully or resulted in an error. The app also calls this method if authentication is required for one or more transfers.

Use this method to reconnect any URL sessions and to update your app’s user interface. For example, you might use this method to update progress indicators or to incorporate new content into your views. After processing the events, execute the block in the completionHandler parameter so that the app can take a new snapshot of your user interface.

If a URL session finishes its work when your app is not running, the system launches your app in the background so that it can process the event. In that situation, use the provided identifier to create a new NSURLSessionConfiguration and NSURLSession object. You must configure the other options of your NSURLSessionConfiguration object in the same way that you did when you started the uploads or downloads. Upon creating and configuring the new NSURLSession object, that object calls the appropriate delegate methods to process the events.

If your app already has a session object with the specified identifier and is running or suspended, you do not need to create a new session object using this method. Suspended apps are moved into the background. As soon as the app is running again, the NSURLSession object with the identifier receives the events and processes them normally.

At launch time, the app does not call this method if there are uploads or downloads in progress but not yet finished. If you want to display the current progress of those transfers in your app’s user interface, you must recreate the session object yourself. In that situation, cache the identifier value persistently and use it to recreate your session object.
//

NSMutableArray *mutableOperations = [NSMutableArray array];

for (NSURL *fileURL in filesToUpload) {

    NSURLRequest *request = [[AFHTTPRequestSerializer serializer] multipartFormRequestWithMethod:@"POST" URLString:@"http://example.com/upload" parameters:nil constructingBodyWithBlock:^(id<AFMultipartFormData> formData) {

        [formData appendPartWithFileURL:fileURL name:@"images[]" error:nil];

    }];

    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];

    [mutableOperations addObject:operation];

}

NSArray *operations = [AFURLConnectionOperation batchOfRequestOperations:@[...] progressBlock:^(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations) {

    NSLog(@"%lu of %lu complete", numberOfFinishedOperations, totalNumberOfOperations);

} completionBlock:^(NSArray *operations) {

    NSLog(@"All operations in batch complete");

}];

[[NSOperationQueue mainQueue] addOperations:operations waitUntilFinished:NO];

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];

AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];

NSURL *URL = [NSURL URLWithString:@"http://example.com/upload"];

NSURLRequest *request = [NSURLRequest requestWithURL:URL];

NSURL *filePath = [NSURL fileURLWithPath:@"file://path/to/image.png"];

NSURLSessionUploadTask *uploadTask = [manager uploadTaskWithRequest:request fromFile:filePath progress:nil completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {

    if (error) {

        NSLog(@"Error: %@", error);

    } else {

        NSLog(@"Success: %@ %@", response, responseObject);

    }

}];

[uploadTask resume];