NSURLSession authentication and downloading file in background - ios

Question

In iPad app, I need to connect to a server and download files that's using a self-signed SSL certificate with NSURLSession object in background mode:
static NSString *const URL = #"https://...";
- (void)testBackgroundDownloadTask {
if (self.downloadTask) {
return self;
}
self.session = [self backgroundSession];
NSURL *downloadURL = [NSURL URLWithString:URL];
NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
self.downloadTask = [self.session downloadTaskWithRequest:request];
[self.downloadTask resume];
}
- (NSURLSession *)backgroundSession{
static NSURLSession *sess = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:URL];
// Default configuration: working perfectly
//NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
configuration.sessionSendsLaunchEvents = YES;
//configuration.TLSMinimumSupportedProtocol = kSSLProtocolAll;
configuration.networkServiceType = NSURLNetworkServiceTypeBackground;
sess = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
});
return sess;
}
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
NSURLProtectionSpace *protectionSpace = challenge.protectionSpace;
NSString *authMethod = protectionSpace.authenticationMethod;
if ([authMethod isEqualToString:NSURLAuthenticationMethodClientCertificate]) {
// obtain challenge, it working with NSURLSession in default session config
[self.challengeHandler handleChallenge:challenge onCompletion:^(id obj) {
NSURLCredential *c = (NSURLCredential *)obj;
if (c) {
[challenge.sender useCredential:c forAuthenticationChallenge:challenge];
completionHandler(NSURLSessionAuthChallengeUseCredential, c);
}
else {
[challenge.sender cancelAuthenticationChallenge:challenge];
}
}];
}
else if ([authMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
completionHandler(NSURLSessionAuthChallengeUseCredential, [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust]);
}
}
If I use defaultSessionConfiguration, my app after call didReceiveChallenge method perform successfully download file (call NSURLSesionDownloadDelegate methods
– URLSession:downloadTask:didResumeAtOffset:expectedTotalBytes: 
– URLSession:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite: 
– URLSession:downloadTask:didFinishDownloadingToURL: 
But I use backgroundSessionConfiguration, after call didReceiveChallenge other delegate’s methods not called (file not downloading and didCompleteWithError not called)
Any ideas on how to resolve this problem?

Answer 1

Are you testing this in the background?
On your NSURLSessionDownloadDelegate, you should see what you get when you implement
- (void)URLSession:(NSURLSession *)session
task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
NSLog(#"%s %# %#", __PRETTY_FUNCTION__, task.response, error);
}
If any of that stuff in –URLSession:didReceiveChallenge:completionHandler: is async, then you need to surround it with a UIApplication/UIBackgroundTaskIdentifier {begin,end}BackgroundTask..., or iOS will kill your background process when the stack pops.