Core Data, iVars and Categories

Question

One of my core data subclasses has an NSSet of items. It is often (but not always, so no NSOrderedSet) useful to instead retrieve an ordered NSArray, so I added orderedItems to the class, which sorts them.

I then ran into performance issues so decided to try caching the orderedItems. My plan is to use an iVar, _cachedOrderedItems in the class, which I will return if it is not null.

The snag comes with my use of categories. I read some good advice about putting all of my custom code in a category so that I can re-generate the core data class if necessary and not lose all my customizations. One of those customizations is the orderedItems method.

It seems I can not declare an iVar in the category itself. And if I try to put it in the core data class instead, I can not access it in the category.

Do I need to move my custom code back into the core data class? Or am I missing something?

I have also heard about Mogenerator, and would consider learning to use this if it would help.

And maybe I should be using a subclass and not a category for my custom code? — Ben Packard, Oct 21 '12 at 16:41

score 1 · Answer 1 · answered Oct 21 '12 at 16:41

1

You can use associative references to add ivars to a class any time you can't modify the original class, including in categories. For a detailed example, see Faking instance variables in Objective-C categories with Associative References.

answered Oct 21 '12 at 16:41

jatoben

3,079
15
12

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

You own the class, so you can use a class continuation (discussed here) instead of a category. This allows you to add instance variables.

edited May 23 '17 at 11:56

Community

1
1

answered Oct 21 '12 at 17:28

jrturton

118,105
32
252
268

score 0 · Answer 3 · answered Oct 21 '12 at 20:54

You should definitely use mogenerator. See for example http://importantshock.wordpress.com/2006/12/19/mogenerator-or-how-i-nearly-abandoned-core-data/.

You can make an Aggregate target in XCode, add a Run Script with the following:

mogenerator -m path/to/your/datamodel.xcdatamodeld/version.xcdatamodel --template-var arc=true -M /CoreData/Generated -H /CoreData

For every NSManagedObject you get a class and a subclass. When updating your datamodel, run the script again and the base class will be updated, preserving all the changes you made to the managed object subclass. Remove --template-var arc=true for none arc.

Core Data, iVars and Categories

3 Answers3