totalBytesExpectedToWrite is -1 in NSURLSessionDownloadTask - ios

Question

I faced with a strange problem. I load file from the Internet using NSURLSession and NSURLSessionDownloadTask. Here is the code
NSURLSessionConfiguration *sessionConfiguration =
[NSURLSessionConfiguration backgroundSessionConfiguration:kSessionId];
self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
delegate:self
delegateQueue:[NSOperationQueue new]];
NSURL *url = [NSURL URLWithString:urlString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionDownloadTask *downloadTask = [self.session downloadTaskWithRequest:request];
[downloadTask resume];
My class is declared as NSURLSessionDownloadDelegate and I get callbacks well. But when the system calls the delegate method
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
NSLog(#"totalBytesExpectedToWrite: %lld", totalBytesExpectedToWrite);
NSLog(#"%lld", totalBytesWritten);
}
totalBytesExpectedToWrite always equal -1 and I have no ability to show a progress to user because I don't know the downloading file's size.
Could you prompt me where I made a mistake?

Answer 1

-1 is NSURLSessionTransferSizeUnknown, which means that the http server did not provide
a "Content-Length" header (and the data is sent using "Transfer-Encoding: chunked").
There is probably not much that you can do. You could try if the workaround from https://stackoverflow.com/a/12599242/1187415 works in your case as well:
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:anURL];
[request addValue:#"" forHTTPHeaderField:#"Accept-Encoding"];

Answer 2

The web service may not be providing the total size in the header field Content-Length.
If the total size is not provided there is no way for your app to know the length and this provide a progress bar.
Check what is coming from the web server with a analyzer such as Charles Proxy.

Answer 3

The Content-Length can be non 0 and totalBytesExpectedToWrite:-1
//TRACK PROGRESS - MOVED DOWN as also used in BACKGROUND REFRESH > DOWNLOAD FILE > CALL DELEGATE
-(void)URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
//to see response header
NSLog(#"downloadTask.response:%#\n", downloadTask.response);
// { status code: 200, headers {
// "Cache-Control" = "no-cache";
// "Content-Disposition" = "attachment; filename=Directory.zip";
// "Content-Encoding" = gzip;
// "Content-Length" = 33666264;
// "Content-Type" = "application/octet-stream";
// Date = "Tue, 27 Oct 2015 15:50:01 GMT";
// Expires = "-1";
// Pragma = "no-cache";
// Server = "Microsoft-IIS/8.5";
// "X-AspNet-Version" = "4.0.30319";
// "X-Powered-By" = "ASP.NET";
// } }
NSDictionary *responseHeaders = ((NSHTTPURLResponse *)downloadTask.response).allHeaderFields;
NSString * contentLengthString = responseHeaders[#"Content-Length"];
double contentLengthDouble = 0.0f;
if (contentLengthString) {
NSNumberFormatter *f = [[NSNumberFormatter alloc] init];
NSNumber *contentLengthNumber = [f numberFromString:contentLengthString];
contentLengthDouble = [contentLengthNumber doubleValue];
}else{
}
NSLog(#"contentLengthString:[%#]", contentLengthString);
//You can get progress her
NSLog(#"bytesWritten:%lld", bytesWritten);
NSLog(#"totalBytesWritten:%lld", totalBytesWritten);
//DONT USE CAN BE ALWAYS -1 for Gzip
NSLog(#"totalBytesExpectedToWrite:%lld", totalBytesExpectedToWrite);
//avoid DIV by 0
if (contentLengthDouble > 0.0) {
double percentage1 = (totalBytesWritten / contentLengthDouble);
double percentage = percentage1 * 100.0;
NSLog(#"PERCENTAGE DOWNLOADED:[%f%%]", percentage);
}else{
NSLog(#"PERCENTAGE DOWNLOADED:[contentLengthDouble is 0]");
}
NSLog(#"=========");
}
The following is Output over and over as zip is downloaded.
but totalBytesExpectedToWrite:-1
So you need to check Content-Length in downloadTask.response
2015-10-27 16:04:18.580 ClarksonsDirectory[89873:15495901] downloadTask.response:<NSHTTPURLResponse: 0x7f9eabaae750> { URL: http://asset10232:50/api/1/dataexport/ios/?lastUpdatedDate=01012014000000 } { status code: 200, headers {
"Cache-Control" = "no-cache";
"Content-Disposition" = "attachment; filename=Directory.zip";
"Content-Encoding" = gzip;
"Content-Length" = 33666264;
"Content-Type" = "application/octet-stream";
Date = "Tue, 27 Oct 2015 16:03:55 GMT";
Expires = "-1";
Pragma = "no-cache";
Server = "Microsoft-IIS/8.5";
"X-AspNet-Version" = "4.0.30319";
"X-Powered-By" = "ASP.NET";
} }
contentLengthString:[33666264]
bytesWritten:47278
totalBytesWritten:33606690
totalBytesExpectedToWrite:-1
PERCENTAGE DOWNLOADED:[99.823045%]