Ideal time to invalidate cached data in an iOS app

Question

So I'm working on an iOS app which uses Core Data as a local, offline store for data stored on a remote server. Think like the way Mail.app keeps your most recent n messages. Now right now the app quite naively stores all of this data without deleting any of the old data.

My question is this: what's the best time in an iOS app's lifecycle to take care of tasks like removing cached data? I already know how I'm going to delete this old data, but doing so is an expensive operation. So what I want to know is when the best time to perform this sort of expensive operation is.

Answer 1

If it is not too much trouble, then doing so when the application goes into the background would be a nice time to do it. If it takes around 10 seconds or more, though, be sure to set up a background task to allow you to run for a bit more time.

Answer 2

You can run this operation in background with GCD or NSOperationQueue. I would do it after I get new data from server, then delete old cache, and build new one. If you move expensive operation to background (threads, block, NSOperation, or what ever you prefer) then it better to use child NSMagaedObjects for synchronization.