Question

So I’m building a document-based application in Cocoa and trying to understand NSManagedObjectContext. I’ve read Apple’s “Core Data Basics” but can’t wrap my head around it. In my application I created a custom CoreDataUtility class so that all of my classes can get the context via the following lines of code:

MyCoreDataUtility *coreData = [MyCoreDataUtility sharedCoreDataUtility];
NSManagedObjectContext *context = [coreData context];

I got this idea from the site “Cocoa is my girlfriend”. Where I get confused is when I run my application and I create a new window (my file owner class is a subclass of NSPersistentDocument) if I make changes to the context on the new window, the view reflects the changes on the old window and as well as the new one. Based on this I’m assuming that both windows are referring to the same context or at least the same data in some way.

So my questions are:

1. I have the assumption that each time I create a new window a new
   context is created, is this correct?
2. If a new context is created and they are different (I actually
   printed out the memory address of the two contexts and they
   addresses are different) does this have something to do with the
   persistent store coordinator or persistent object store (both of
   these concepts are also nebulous in my mind)?
3. If all documents refer to the same context is it my responsibility to
   create a new context every time a document is created?

Thanks all!

Answer 1

I have the assumption that each time I create a new window a new
context is created, is this correct?

No and yes. Creating a new window does not create a new context, but I suppose that what you see as “a new window” is actually your action of “creating a new document”. In a document based app with CoreDate you are likely to have a persistent store which creates a new context for every document by itself.

That is also the answer to your second question. The store coordinator is just the central point that manages all documents of your application. It takes care of some menu related actions, such as opening, saving and all side effects.

If all documents refer to the same context is it my responsibility to
create a new context every time a document is created?

That is not possible. One context can be related with only one file (or “store”), hence every document must have “at its minimum” one context. If you are using the template for a CoreData and Multi-document based app, you do not have to worry about creating a new context. As I said before, the store coordinator will take care of that. However as you learn more about CoreData, and specially if you do multithreading, you will have to create more than one context because because a context cannot cross threads.

Could you provide the link from Cocoa is my girlfriend where you took that code? It seems you want a singleton to store your context and usually that is a bad idea in a document based app because it is extremely easy to mess up things giving one function the context of a different document. Your application should have a good flow where the context is passed. However I always use a singleton if my application is not document based.