How long does Apple permit a background task to run?

Question

I have to upload an array of image files to database, therefore, I stumbled upon Apple's background execution guide to make sure the app still uploads the data when user suspends or terminates my app.

But in the desciption, it says giving it a little extra time to finish its work if we call beginBackgroundTaskWithName:expirationHandler: or beginBackgroundTaskWithExpirationHandler: to start a background task.

How long is little extra time precisely?

Answer 1

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.