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I was sure that I've posted here the solution, but better later than never... Since it's a pretty long code, I've made a gist for it (hopefully it doesn't missing any custom helper): https://gist.github.com/Idomo/ece6683348372b54e470fd630b1fb76e Beside that, to recreate the saved unfinished sessions, so it'll fire urlSession(_:task:didCompleteWithError:) and re-upload, you should call this function in application(_:, didFinishLaunchingWithOptions:): func recreateFilesUploadTasks() { if let filesUploadTasks = UserDefaults.standard.stringArray(forKey: UserDefaultsKeys.filesUploadTasks) { let filesManager = FilesManager() for sessionId in filesUploadTasks { filesManager.recreateSession(id: sessionId) } } }

Got it, but I'm missing the answer for "Where should I recreate it and how?"Also, I'm not sure that I understood this:It’s common for folks to make the mistake of creating their URLSession lazily, either deliberately (as an optimisation) or accidentally (because they’ve tied the session to a view controller, which is a really bad idea).What do you mean by "creating URLSession lazily"?How can I create it in another way? I know that I need to create it only after some action the user performed, so it will always be "lazily" (or I didn't got it correctly).I'm creating it and attaching the delegate to a class that I've created (class FilesManager: NSObject, URLSessionDataDelegate, URLSessionDelegate{}).If not using `self` for the delegate, what else can I use?How can I pass delegate as some "`static class`" so it will always be avaliable to fire delegate's methods?Sorry for asking so much, but I couldn't found answers for this thing anywhere and the docs are missing most of the things you've mentioned since your first comment.

I saw your post and also tried to use `os_log` after one of your comments, but it didn't worked for me from some reason so I switched to `NSLog`.What do you mean by "Are you user you re-created your background session?" where should I recreate it and how? as much as I understood it should call the delegate automatically, 'cause it is a background process that can't "die" (even after force quit).

OK, thats a good direction.Can I retry to execute the `uploadTask` from the delegate?Will calling `resume()` on `task: URLSessionTask` work or after the task has been canceled it useless?Edit:It doesn't seems to fire `urlSession(session:task:didCompleteWithError:)` after opening the app after force quitting it (I'm able to see `NSLog` output from other methods, but not from the delegate),I can't even log the error to see if it contains the keys you've mentioned.You may see my logs:16:47:19.249330 +0300 My app Debug: viewDidLoad() 16:51:57.128801 +0300 My app Debug: uploadImage(_:to:completionBlock:), Upload task resumed 16:52:07.594877 +0300 My app Debug: urlSession(_:task:didCompleteWithError:) 16:52:40.115190 +0300 My app Debug: uploadImage(_:to:completionBlock:), Upload task resumed ### Force Quit here, not getting to `urlSession(_:task:didCompleteWithError:)` ### Open the app again: 16:52:49.592161 +0300 My app Debug: viewDidLoad()

After some searching I found that background uploads works only when uploading from file, so I saved the `URLRequest.httpBody` as a file and uploaded it:static func urlSessionConfiguration(for id: String) -> URLSessionConfiguration { let configuration = URLSessionConfiguration.background(withIdentifier: FilesManager.urlSessionIdentifierPrefix + id) configuration.sharedContainerIdentifier = FilesManager.urlSessionIdentifierPrefix return configuration } let session = URLSession(configuration: FilesManager.urlSessionConfiguration(for: fileName), delegate: self, delegateQueue: nil) let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0] let filePath = "\(documentsPath)/\(fileName)" DispatchQueue.main.async { if let data = urlRequest.httpBody, (data as NSData).write(toFile: filePath, atomically: true) { urlRequest.httpBody = nil // Use only file's content let fileUrl = NSURL(fileURLWithPath: filePath) as URL session.uploadTask(with: urlRequest, fromFile: fileUrl).resume() } else { session.uploadTask(withStreamedRequest: urlRequest).resume() } }Now I'm facing an issue that if the user is manually quit the app (from the app switcher), the background uploads are cancled (that I've found in some article: "if you manually force quit the app then iOS will take this as definite confirmation that you are finished with the app and so cancel any scheduled background-transfers.").Well, as one of many users how quiting apps like this when we were done with (or at least we're think so, 'cause we're not aware to background tasks), I can tell that it is a really big issue.The article I've mentioned does suggest a way to "overcome" this issue, but it only for testing, I can't really detect when the app has been quit from the app switcher (as much as I know).So my question is:Can I prevent from background tasks to be cancelled when the app is quit from the app switcher (or at least to detect that from `AppDelegate` and run `exit()` from code to overcome this)?

It really depends on the resolution of the image, it can be between 0.1Mb to 1MB (maybe a bit more).Anyway, I got some suggestion that based on what you've said (that only GET requests are retrying to be sent 'till success), and tried to send some `ping` with GET (tried also with HEAD) request so only when `urlSession(_:task:didCompleteWithError:)` will beign called, I'll send the POST request (and know that there is an internet connection), but from some reason I'm getting timeout also for the GET request (that uses the same background configuration).Please tell me, am I doing something wrong?This is my code:In class:static var requests = [URLRequest]() static let urlSessionConfiguration: URLSessionConfiguration = { let configuration = URLSessionConfiguration.background(withIdentifier: FilesManager.urlSessionIdentifier) configuration.sessionSendsLaunchEvents = false configuration.sharedContainerIdentifier = "myApp" return configuration }() In upload func:let url = URL(string: "https://url.com/path")! var urlRequest = URLRequest(url: url) urlRequest.httpMethod = "POST" urlRequest.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type") urlRequest.httpBody = body FilesManager.requests.append(urlRequest) URLSession(configuration: FilesManager.urlSessionConfiguration, delegate: self, delegateQueue: nil).dataTask(with: URL(string: "https://url.com/ping.txt")!).resume()Delegate:extension FilesManager: URLSessionDataDelegate, URLSessionDelegate { func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { print("\(#funcion) \(error), \(task.originalRequest?.httpMethod)") guard FilesManager.requests.count > 0 else { return } let urlRequest = FilesManager.requests.removeFirst() FilesManager.urlSession.dataTask(with: urlRequest).resume() } }Console:urlSession(_:task:didCompleteWithError:) Optional(Error Domain=NSURLErrorDomain Code=-1001 "The request timed out." UserInfo={_kCFStreamErrorCodeKey=-2104, _NSURLErrorFailingURLSessionTaskErrorKey=BackgroundDataTask .<1>, _NSURLErrorRelatedURLSessionTaskErrorKey=( "BackgroundDataTask .<1>", "LocalDataTask .<1>" ), NSLocalizedDescription=The request timed out., _kCFStreamErrorDomainKey=4, NSErrorFailingURLStringKey=https://url.com/ping.txt, NSErrorFailingURLKey=https://url.com/ping.txt}), Optional("GET") urlSession(_:task:didCompleteWithError:) Optional(Error Domain=NSURLErrorDomain Code=-1001 "The request timed out." UserInfo={_kCFStreamErrorCodeKey=-2104, _NSURLErrorFailingURLSessionTaskErrorKey=BackgroundDataTask .<2>, _NSURLErrorRelatedURLSessionTaskErrorKey=( "BackgroundDataTask .<2>", "LocalDataTask .<2>" ), NSLocalizedDescription=The request timed out., _kCFStreamErrorDomainKey=4, NSErrorFailingURLStringKey=https://url.com/path, NSErrorFailingURLKey=https://url.com/path}), Optional("POST")

Oh, so do you have another way to achieve this goal? (I have to use POST 'cause I'm using `multipart/form-data`)