What are causes of freezing in an iOS app after returning from background?

This is very difficult issue, since it doesn't occur every time.

When an app is just launched, returning from background, it is fine without any freezing. After some time being in background, when I re-activate it, the whole UI is frozen for while, even the activity indicator is not animating. As time goes on, this freezing gets longer and finally causes the app to be terminated by iOS for not resuming in time.

I would like to learn what are the generic causes of an app to be unresponsive, especially if the app is supporting background running feature and re-activating after a period of being in background.

It looks like I have missed some fundamental techniques that should be respected to avoid freezing prob开发者_运维技巧lems.

UPDATE: I am suspicious if saving UIImage into Core Data object can be the main reason. At launch, it's not visible, however, as time passes, numerous saveManagedObjectContext are performed while UIImage is still referenced. Maybe I should have listened to the instruction not to save any image in Core Data?

check for your crash log if you are getting this

Exception Codes: 0x8badf00d

then its most probably your application is taking much time either in launch or while moving in background or terminate, actually app take much time in launch/shut iOS suspend it and our app freeze, there is no such time define in documents but as i experienced it should not be more then 4-5 seconds. check your app delegate applicationDidFinishLaunch or multitasking delegates there is some code which is taking time in completion. For more info lokk at these

http://developer.apple.com/library/ios/#technotes/tn2151/_index.html

http://en.wikipedia.org/wiki/Hexspeak