# 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`