CODEWITHSUNDEEP
iphoneobjective-cioscore-data
boo-urns
boo-urns

Reputation: 10376

How to write custom accessors for an NSManagedObject subclass?

I have a view called Cart. It displays a table of Items. These Items need to persist, so Item subclasses NSManagedObject; values like id, price, etc. are properties whose accessors are @dynamic so that they are automagically generated.

I have another view called Favorites. It displays a table of Items, but they don't need to persist. In fact, this view changes whenever the user logs in with different credentials.

The connection between the two views is that the user can add items to his cart from his favorites. A cart can store Items from different Favorites lists. Favorites lists don't change when the item is added to the cart.

Initially, I made the model for the Favorites view be an NSArray of NSDictionary objects. When the user adds the item to his cart, I create and save the item in Core Data from the NSDictionary key-value pairs. This approach doesn't look very clean or very DRY. Wouldn't it make more sense to make the Favorites view's model be an NSArray of Items?

So now my intent is to implement the Item class so that it will represent the Core Data model (NSManagedObject), but also work with the Favorites view. Being new to Objective-C and iOS development, I really don't know how this would work or look like. It seems that I would need to override the accessors that are magically created for me, but I can't call them at compile time using a super call... Can anyone give me a rough outline of when it would know to return the NSDictionary data or the Core Data data? In the case that it's Core Data data, how do I maintain the same level of efficiency that the magically-generated accessors would have?

Even better, is there a better implementation that is just as DRY or makes more sense? Or am I trying to combine too much functionality into one class? Is the NSArray of NSDictionary objects the best way to go in this case?

Upvotes: 2

Views: 291

Answers (1)

Jody Hagins
Jody Hagins

Reputation: 28409

You can specify a result type on your fetch request (object, objectID, count, dictionary).

Also, I would not use NSManagedObjects outside of a MOC. You should either have a separate object that you use for memory stuff, or you can use an in-memory persistent store for those objects... or, you can just create a separate MOC as a child to your main database MOC that you use for your in-memory objects.

The advantage of these approaches is that your code does not have to know whether they are backed to the disk or not.

As long as you do not save the MOC, the changes to those objects will never go to disk.

EDIT

NSFetchRequest *fetchRequest = // create the fetch request...
fetchRequest.resultType = NSDictionaryResultType;

Now, when you make the fetch, instead of getting back an array of NSManagedObject you will get back an array of NSDictionary.

Upvotes: 1

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