asynchronous

- (void)doThisTask {
// For this snippet we are downloading the same in two tasks.
NSString *imageUrl = @"http://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png";
// You always start by creating an NSURLConfiguration.
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
// This creates a session using the current class as a delegate.
NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                      delegate:self
                                                 delegateQueue:nil];
//Tasks are always created by sessions. This one is created with the block-based method. Remember you could still use the NSURLSessionDownloadDelegate to track download progress.
NSURLSessionDownloadTask *getImageTask = [session downloadTaskWithURL:[NSURL URLWithString:imageUrl]
                                                    completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
                                                        // Here you use the location variable provided in the completion handler to get a pointer to the image
                                                        UIImage *downloadedImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:location]];
                                                        // Finally you could, for example, update UIImageView’s image to show the new file.
                                                        dispatch_async(dispatch_get_main_queue(), ^{_imageWithBlock.image = downloadedImage;});
                                                    }];

// You always have to start up the task!
[getImageTask resume];
}