I have implemented
If an iOS app is terminated by the system and relaunched, the app can use the same identifier to create a new configuration object and session and retrieve the status of transfers that were in progress at the time of termination. This behavior applies only for normal termination of the app by the system. If the user terminates the app from the multitasking screen, the system cancels all of the session’s background transfers. In addition, the system does not automatically relaunch apps that were force quit by the user. The user must explicitly relaunch the app before transfers can begin again.
After the app has been killed by the user, downloads do not continue in the background. On iOS 7 (and later) resume data is passed back.
After a force-quit the:
NSURLSessionTaskDelegate - (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
delegate method will be called when the app is restarted. If the download task can be resumed the error object will contain the resume data:
Using this data you can resume the download creating a
More info about that can be found in Life Cycle of a URL Session with Custom Delegates, step 13.