My existing strategy is to use the completion callback of one download to begin the next, continuing until the list is empty.
This approach is not without its pitfalls; see NSURLSession’s Resume Rate Limiter for details.
What I need is something that acts as if it were a resumeAfter: or suspendUntil: method on NSURLSessionTask.
To be clear, iOS has no general-purpose ‘resume me at this time’ mechanism.
However, I know that there's a very limited amount of time allowed before the app must return from the callback or it will be terminated. (10 seconds?)
Something like that. However, you can use a UIApplication background task to keep your app running after it’s returned from that callback. That’ll give you up to 30 seconds of execution time, which gets you close to the lower bound of your requirement.
Beyond that, I think we’re in enhancement request territory. For example, it would be nice if you could:
specify a target start time for a task in an NSURLSession background session
dependencies between such tasks (don’t start this until you’ve completed this)
priorities for tasks (do all the members of this group before starting on members of this group)
and so on.
ps If you do file any enhancements requests like this, please post your bug number, just for the record.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
I understand that the download strategy has problems but, since the files being downloaded are individually useful, batching the list would be a poor UX.
"To be clear, iOS has no general-purpose ‘resume me at this time’ mechanism."
Oh, we've noticed!
(It sometimes feels as if providing any feature that does not involve a human tapping on glass is a high-risk venture.)
Thank you very much (as usual!) for your clarification of the situation and suggestions.
I'll think about the enhancement request; it does seem that some queue-like control over the session might be generally useful.