CODEWITHSUNDEEP
iosuiapplicationdelegatebackground-tasknsurlsessionuploadtaskurlsession
Guig
Guig

Reputation: 10197

using begin​Background​Task​With​Expiration​Handler​ for upload

From the doc it looks like uploading a file is a good use case for using begin​Background​Task​With​Expiration​Handler​. I've found that using

let uploadTask = session.uploadTask(with: request as URLRequest, fromFile: file)
uploadTask.resume()

will already run while the app is backgrounded (I'm getting upload progress pings for a while). Additionally I can set the URLSession to be backgrounded:

let config = URLSessionConfiguration.background(withIdentifier: "uploads")
session = URLSession(configuration: config, delegate: self, delegateQueue: nil)

So what is the advantage of using begin​Background​Task​With​Expiration​Handler ? Will it extend the time I've to finish my upload? If so can I know by how much (didn't see anything about this in the doc)? Or is it just that I'll get pinged before the app stops? Should I use it in combination with a background URLSession?

Additionally the docs says that the handler will be called shortly before the app’s remaining background time reaches 0 Does it mean that the app will be terminated after that? ie can I assume that the next call will be application:didFinishLaunchingWithOptions or can it be applicationDidBecomeActive ?

Upvotes: 3

Views: 1810

Answers (2)

Rob
Rob

Reputation: 437552

This background task will let your app continue to run in background after the user leaves your app for an extra 3 minutes or so (check background​Time​Remaining for actual value) to let your request finish. And, yes, near the end of that 3 minutes, the timeout handler will be called if you haven't yet ended the background task.

So, if you end the background task during the normal flow of your app, this timeout closure won't need to be called. This closure is solely for any quick, last minute cleanup, that you might need to do before your app stops running in the background because it timed out before you had a chance to indicate that the background task ended. It's not for starting anything new, but just any last second clean-up. And make sure to end the background task in this timeout handler ... if you don't end the background task, the OS will summarily kill your app rather than just suspending it. Often, the only thing you need to do in this timeout closure is end the background task, but if you need to do any other cleanup, this is where you can do it.

Needless to say, you must end your background task (either when the network request finishes, or in the timeout handler if your app didn't yet get a chance to end the background task in its normal flow). If you don't, your app won't just be suspended, but rather it will be killed.

Regarding making assumptions about what happens when your app is restarted by the user later, you can't make any assumes about which app delegate method will be called. Even if you gracefully ended the background task, you have no assurances that it won't get jettisoned for other reasons (e.g. memory pressure). So don't assume anything.

Upvotes: 3

matt
matt

Reputation: 535178

So what is the advantage of using begin​Background​Task​With​Expiration​Handler ?

If you are going to use URLSessionConfiguration.background, there is no such advantage and you should not use beginBackgroundTask(expirationHandler:) at all. Your entire premise (your very first sentence) was wrong. Uploading a file is not a good use case for beginBackgroundTask(expirationHandler:). It's a good use case for URLSessionConfiguration.background. The two things have nothing to do with each other.

Upvotes: 3

Related Questions