iOS Timed Background Processing

How long does Apple permit a background task to run?

Correct me if I am wrong, but I have stumbled upon a perfect article from Xamarin that discusses iOS backgrounding feature.

I will simply break down to two parts, ios pre 7 and ios 7+:

iOS version pre 7

The answer is simply 600 seconds (10 minutes), reason is provided by
the article above.

iOS version 7+

The answer is that the time system allocates you is opportunistic. You
will have to use @Gary Riches's suggestion

NSLog(@"Time Remaining: %f", [[UIApplication sharedApplication] backgroundTimeRemaining]);

to find out. The reason for it being opportunistic is the way iOS 7+
handles background tasks is completely different, certainly optimised. To
be exact, It has an intermittent behaviour, and therefore, if you need
background tasks such as downloading a big chuck of data, it will be
much more effective if you use `NSURLSession` instead.

However, in my special case, I am uploading one single object that contains one file to be exact. I do not have to consider NSURLSession for uploading a small amount of data. And besides, it's uploading task, it can take as much time as it wants. :-)

For these TL;DR visitors, the answer above should be sufficient. For more details, please refer to the article above.

How do you extend an iOS app's background execution time when continuing an upload operation?

You have stumbled across a less-than-stellar code sample in Apple’s documentation.

• First, if you perform a synchronous network request, you definitely should dispatch it to a background queue. If you don't, you risk having the watchdog process kill your app. But you shouldn’t dispatch the network request synchronously to the global queue, but rather asynchronously, or else you just end up with the same problem, namely blocking the main thread.

• That having been said, one really should never perform network requests synchronously. You should perform them asynchronously and end the background task in the completion handler.

• In that example, they use NSData, which we don’t use anymore.

• Also UIBackgroundTaskInvalid doesn’t exist anymore. It is now UIBackgroundTaskIdentifier.invalid.

• In Apple’s defense, the point of this code sample is the background task, not the network code. They really didn’t want to get into the weeds of the implementation of this network code and were trying to keep it simple. That having been said, it really is a horrible and outdated code example.

See this answer for a better example of how one might use background tasks. Also, if the upload might take more than 30 seconds, we wouldn’t use the background task at all, but a proper (but more complicated) background URLSession. (See Downloading files in the background. Upload tasks follow the same basic pattern outlined there, though make sure to upload from a file, not a Data.)

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