Showing the file download progress with NSURLSessionDataTask

You can also use NSURLSessionDownloadTask like following. Call startDownload methode.In .h file use this

- (void)startDownload
{
    NSString *s;
    s = @"http://www.nasa.gov/sites/default/files/styles/1600x1200_autoletterbox/public/pia17474_1.jpg?itok=4fyEwd02";
    NSURLSessionDownloadTask *task = [self.session downloadTaskWithURL:[NSURL URLWithString:s]];
    [task resume];
}

- (NSURLSession *) configureSession {
    NSURLSessionConfiguration *config =
    [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.neuburg.matt.ch37backgroundDownload"];
    config.allowsCellularAccess = NO;
    // ... could set config.discretionary here ...
    NSURLSession *session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    return session;
}

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    CGFloat prog = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"downloaded %d%%", (int)(100.0*prog));

}

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes {
    // unused in this example
}

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSData *d = [NSData dataWithContentsOfURL:location];
    UIImage *im = [UIImage imageWithData:d];
    dispatch_async(dispatch_get_main_queue(), ^{
        self.image = im;

    });
}

-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    NSLog(@"completed; error: %@", error);
}

Since iOS 11.0 and macOS 10.13, URLSessionTask (former NSURLSessionTask) adopted the ProgressReporting protocol. That means you can use the progress property to track the progress of a session task.

Expecting you already know how to use KVO observers, you can do something like:

task = session.downloadTask(with: url)
task.resume()
task.progress.addObserver(self, forKeyPath: "fractionCompleted", options: .new, context: &self.progressKVOContext)

and observe the value with:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if context == &self.progressKVOContext, let keyPath = keyPath {
        switch keyPath {
        case "fractionCompleted":
            guard let progress = object as? Progress else {
                return
            }
            DispatchQueue.main.async { [weak self] in
                self?.onDownloadProgress?(progress.fractionCompleted)
            }
        case "isCancelled":
            cancel()
        default:
            break
        }
    }
    else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    }
}

iOS 13 update

OperationQueue has now a progress property.

For example, the UIProgressView can observe that property and update the progress value automatically using the observedProgress. Full example in https://nshipster.com/ios-13/#track-the-progress-of-enqueued-operations.

You need to implement following delegates:

<NSURLSessionDataDelegate, NSURLSessionDelegate, NSURLSessionTaskDelegate>

Also need to create two properties:

@property (nonatomic, retain) NSMutableData *dataToDownload;
@property (nonatomic) float downloadSize;

- (void)viewDidLoad {
    [super viewDidLoad];

    NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];

    NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];

    NSURL *url = [NSURL URLWithString: @"your url"];
    NSURLSessionDataTask *dataTask = [defaultSession dataTaskWithURL: url];

    [dataTask resume];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    completionHandler(NSURLSessionResponseAllow);

    progressBar.progress=0.0f;
    _downloadSize=[response expectedContentLength];
    _dataToDownload=[[NSMutableData alloc]init];
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    [_dataToDownload appendData:data];
    progressBar.progress=[ _dataToDownload length ]/_downloadSize;
}