iOS URLSession Initiate DownloadTask in Background - ios

Question

I am trying to build a sequential download manager where the user can initiate up to 1000 download tasks at once but only 2 will actually download and the rest will be put on hold, until one of the two finishes so that another one of the 998 can start.
Because there is no way to add a downloadTask then have that task wait for others to complete then automatically start, I have built a download queue myself and will only create new downloadTask when the old one completes.
I have read the documentations and am well aware of the mechanism of iOS's handling of background download event, and what methods I need to implement in order to handle them correctly. However, I am unable to find anything on whether it is safe and reliable to start a new downloadTask when the old one completes IN BACKGROUND.
The Main Question:
When iOS relaunches my app in background to inform me that my download tasks are finished, can I reliably create a new downloadTask and add it to the current session? If so, when that task finishes as well, will the system relaunch my app AGAIN to tell me it's finished? If so, then I can create an infinite loop of adding new tasks when the old backgroundTask finish.
Code Example
Would the following code reliable to get the entire download queue downloaded(say, 1000 items) if I download ONLY one item at a time and meanwhile the app stays entirely in background?
extension MyClass: URLSessionDelegate {
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
// This is called when background tasks are done
// Let's add a new background tasks WHILE app has just been
// relaunched IN BACKGROUND
if let nextURL = myURLQueue.removeFirst() {
session.downloadTask(with: URLRequest(url: nextURL))
}
DispatchQueue.main.async {
if let appDelegate = UIApplication.shared.delegate as? AppDelegate,
let completionHandler = appDelegate.backgroundSessionCompletionHandler {
appDelegate.backgroundSessionCompletionHandler = nil
completionHandler()
}
}
}
}

Answer 1

You are creating the URLSessionDownloadTask, but never starting it with resume().
FWIW, even if you fix the above, you should recognize that requests that are submitted while the app is running in background mode will always be submitted in discretionary mode. As the docs for isDiscretionary say:
For transfers started while your app is in the background, the system always starts transfers at its discretion—in other words, the system assumes this property is true and ignores any value you specified.
Also, recognize that having the system fire up your app every time a request is done in order to submit the next request is both inefficient (because your app will constantly fire up and go back into background) and slow (because concurrent requests reduce latency effects). It’s generally better to just submit all of the requests up front, while the app is active, and let the background session manage how many it will allow to run concurrently.