Question

I have some NSManagedObject subclass in my app, and I’m trying to understand when and how to save changes.
I will try to explain myself, for example class A is NSManagedObject subclass.
During app life cycle I do:

App launched
...
Create an instance of class A
...
Change some properties of A instance
...
App go to background
...
App becomes active again
...
Change some more properties of A instance
...
App terminates

When do I need to call [context save:] ??
Do I call it after every change in A instance? Or maybe I call it only when app go to background?
Should I call it after creation or deletion of any A instance?

Answer 1

A nice approach is place UIManagedDocument in your AppDelegate. Then you can call [context save] whenever some change occurs in the app (like a crash). The order I like to follow is something like:

1. Create UIManagedDocument object (in application did load or wherever)
   and assign it to a property
2. Setup the document (check whether it exist on disk or is already open, etc.. and respond accordingly)
3. Pass the UIManagedObjectContext to the initial UIViewController in your app (from there you can pass the context to other view controllers)

UIManaged document will save the context for you.

Take a look at the UIManagedDocument documentation to configure persistent store options (you send an NSDictionary of options to your UIManagedDocument instance, see the first example through the link below).

UIManagedDocument documentation:
http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIManagedDocument_Class/Reference/Reference.html

Also see the CoreData lecture and demo (lectures 13 and 14) of the iPhone and iPad application development course with Paul Hegarty available free on iTunesU (Fall 2011).