在iCloud中使用CoreData时，iCloud是否会同步所有CoreData数据

Question

I have an application that I want to add iCloud support to. This app loads data from a server, and the data is stored in CoreData so that NSFetchedResultsController s can manage the UITableView s.

Almost all the data stored is deleted when the app is terminated, because the data is rather time sensitive and it does not make sense to store it. There are however a few items in CoreData that are saved, such as the order of certain items, and user selected favorites.

Would it make sense to use Core Data in iCloud to sync these few saved items across devices? My concern is that once all the table views are populated with the temporary data, then the device will use bandwidth syncing this information when that is unwanted.

Answer 1

Yes, it syncs all of the data in the store. But there's more to the story than that.

You tell Core Data to use iCloud when you add the persistent store to the persistent store coordinator-- by using the right option values when calling addPersistentStoreWithType:configuration:URL:options:error: . If you tell Core Data to use iCloud here, all of the data in that persistent store will be synced.

However, you can call this method more than once, using different persistent store files. It's called a persistent store coordinator because it can coordinate among more than one persistent store. Most apps use only one, but you can use as many as you need.

There are a couple of different ways to deal with this, depending on how your app works.

• If you use the same Core Data entity types in both persistent stores (ie synced and non-synced data all uses the same entities), you'll need to tell your managed object context which store it should use for each new object. You do this by calling assignObject:toPersistentStore: on the context when creating the object.
• If you use different entities for different stores (ie synced data uses one subset of your data model, non-synced data uses a different subset), you can use different configurations in the data model, and the store will be chosen automatically. Each configuration effectively defines a named subset of the model that contains only some of the entities. Use the right configuration name when adding the persistent store, and new objects will go to the appropriate store.

If the data that shouldn't sync is really transient, consider using NSInMemoryStoreType when setting up the non-syncing persistent store. That uses more memory, but it also saves you from needing to delete the store contents. When the app exists, an in-memory store just disappears. If you can spare the memory, this can be a lot more convenient to use. They work pretty much the same (you can still use NSFetchedResultsController , for example) but they never get saved to a file.