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benjaminbojko/iOS Background Download Gotchas.md

Last active August 10, 2019 00:38
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iOS Background Download Gotchas
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iOS Background Download Gotchas.md

iOS Background Download Gotchas

I've been doing some thorough investigations into setting up a solid background download process. Here are some of the gotchas I stumbled upon and wanted to capture:

Background Tasks

  • Only download and upload tasks are allowed to run in the background (no data tasks)
  • Once a download task completes, your app will have to move that task from its temporary location to a permament location (or process the data somehow); The temporary file will be deleted once the URLSession:downloadTask:didFinishDownloadingToURL: delegate method returns.

App Suspension

  • If your app is suspended, it will be re-launched whenever a background task completes
  • If your app is (re-) launched through a completed background task, you will have to create a NSURLSession instance with the same identifier as passed into application:handleEventsForBackgroundURLSession:completionHandler: and assign a delegate to receive individual download task delegate messages

Cancelled Tasks

  • If a user force-quits your app (double-tapping home and swiping up), your download tasks will be cancelled
  • didCompleteWithError: will be called if a task completes successfully or with an error
  • If a task is cancelled (manually or by force quitting) then task.error.code will have the value NSURLErrorCancelled, but task.state will be NSURLSessionTaskStateCompleted
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