Q:

Best Life Cycle Method to Place Main Thread-Blocking Operation in Objective-C

My app syncs with a server similar to the Apple Mail app. While the sync takes place on a background thread, because it is hitting Core Data rather hard, I've found it necessary to block interaction with user controls during the sync, lest some other operation hit core data and create problems.
I had been putting the sync in View Will Appear to keep the phone and server in constant sync.  However, with larger amounts of data, I'm noticing that the sync is unacceptably long...that is it ties up the thread for five or ten seconds. I tried placing it in viewdidload so it gets called less often but it is still annoying to wait when you have just opened app.
I've noticed that Apple does not sync mail immediately but waits a few seconds so as not to tie up the app at first.  This gives you the illusion, you don't have to wait (although in reality you usually do).
I'm wondering if there is a place in the life cycle that would be better for syncing such as viewdidappear and also whether there is a way to use a delay to launch the sync five or ten seconds after you are in the view controller when it's less conspicuous.
Thanks in advance for any suggestions.

A:

Firstly blocking the main thread isn't preferred under any circumstances  for asynchronous operations , as the user will think that the app is hanging and will quit it 
Secondly viewDidAppear is meant for updates when say it's in a vc that navigation returns to with back to refresh content or dismissing a model , other than those 2 things it will act like viewDidLoad with the overhead of delay
Finally if you need to sync the mails with server you have 2 options 

grab data every fixed time ( not recommeneded ) say with a timer
use silent push notification to notify the app with server new content and initiating the pull process upon receiving it

