Download task when app is inactive

 https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background

For the background download, there is beginBackgroundTaskWithExpirationHandler: that is specifically designed to do that. When you use it, you will get few more minutes to execute whatever you need (after that limit, your application will get terminated no matter what).

You can write following methods:

func beginBackgroundTask() -> UIBackgroundTaskIdentifier {
    return UIApplication.sharedApplication().beginBackgroundTaskWithExpirationHandler({})
}

func endBackgroundTask(taskID: UIBackgroundTaskIdentifier) {
    UIApplication.sharedApplication().endBackgroundTask(taskID)
}

When you want to use it, you just simple begin / end the task when starting / finishing the download call:

// Start task
let task = self.beginBackgroundTask()

// Do whatever you need
self.someBackgroundTask()

// End task
self.endBackgroundTask(task)

Hope it helps!