beginBackgroundTask expirationHandler never called - ios

Question

I'm trying to complete a task in the background using UIApplication.shared.beginBackgroundTask but for some reason the expirationHandler is never called. The task I'm trying to complete is a video export from photo library but sometimes the export cannot be completed in time while the user is using the app in the foreground.
This is the code I'm using :
func applicationDidEnterBackground(_ application: UIApplication) {
if backgroundTask == .invalid && UploadQueue.instance.hasMoreWork() {
backgroundTask = UIApplication.shared.beginBackgroundTask(withName: "ExportQueue") {
NSLog("DriveLog - System is requesting end. Still more work to do ...")
self.endBackgroundTask()
}
print("Invalid? \(backgroundTask == .invalid)")
NSLog("DriveLog - Starting background task: %i", backgroundTask.rawValue)
}
}
func endBackgroundTask() {
NSLog("DriveLog - End called")
UIApplication.shared.endBackgroundTask(backgroundTask)
backgroundTask = .invalid
}
I'm also calling :
(UIApplication.shared.delegate as! AppDelegate).endBackgroundTask()
during my task if I finish it earlier.
However I never see my expirationHandler being called in the log.
I have also tried putting beginBackgroundTask when starting the task in foreground but I get a warning message about task expiration while being in foreground.

Answer 1

You have not understood what the expiration handler is. It is called only if your time expires. Hence the name.
As soon as you call begin, start your task in the next line (not in the expiration handler). And when you are finished, call end.
You thus need to end the background task in two places: in the expiration handler, and outside it after actually performing your task.
It is very important to call end in both places, because if you fail to do so, the system will decide that you are a bad citizen and will never grant you any extra background time at all.
So, this is the diagram of the flow you need to construct:
Also note that this has nothing to do with UIBackgroundModes. That's a totally different mechanism.

Answer 2

matt's answer covers everything. I'm just going to try to give the same answer in different words because your edit suggests that matt's answer wasn't clear to you. (Read it again, though, it really does cover everything I'm going to say here, just in different words.)
You should not call beginBackgroundTask in applicationDidEnterBackground. You call it when you start whatever task you want time for. In your example that's going to be somewhere inside of UploadQueue. You don't call beginBackgroundTask when going into the background. You call it when you're starting a task that you would like to finish even if you go into the background.
Background tasks generally do not belong to the UIAppDelegate. They belong to the thing that creates the task (in your case: UploadQueue). You can create all the background tasks you want. They cost almost nothing. It's not just one "I want background" at the app level. Read matt's flow chart closely.
It's unclear from your question why you expect the expiration handler to be called. Do you expect your task to task to take so long that the OS forces you to stop it? That's what the expiration handler is for. If you've built your system correctly, it should rarely be called. Your task should end long before it's expired.
For full docs on how to do this, see Extending Your App's Background Execution Time. In particular note the caution:
Don’t wait until your app moves to the background to call the beginBackgroundTask(withName:expirationHandler:) method. Call the method before performing any long-running task.