User Profile

To download files, we have two NSURLSession objects. One configured for foreground downloads and one for background downloads. Initially, we download user-requested files in the foreground using downloadTaskWithRequest, because foreground downloads are faster than background downloads. We then also start a background task for each download using beginBackgroundTaskWithName:expirationHandler:. This background task's expiration handler starts a background download if the download didn't complete in the foreground. It cancels the foreground download with resume data using cancelByProducingResumeData. A background download task is then started using downloadTaskWithResumeData. Now, testing has shown that background download tasks resume correctly (the urlSession(_:downloadTask:didResumeAtOffset:expectedTotalBytes:) callback is called by iOS) if the background task is started immediately. However, iOS can decide to start these background download tasks later. As a sidenote, the isDiscretionary property of our background download session is set to the default, which should be false. The issue we have is that background downloads that are resumed several minutes (6,5 minutes in the session I'm currently looking at) after their original foreground download was cancelled, are not resumed properly. They download the remaining part of the file, but the first part that was downloaded in the foreground is lost. What could have happened? Perhaps the temporary file has been deleted in the meantime? Is there some way to maintain this file or detect this case?