iOS，Core Data和iCloud-切换上下文

Question

I am creating an app where data needs to be displayed right away from the local datastore. I fire off a background thread when the app starts to determine if iCloud is available.

I looked everywhere and can't find the solution to this: when iCloud becomes available, can I change the "options" on the persistentStore to start using iCloud transactions?

I'm not sure what the proper approach is in this situation. Everything I try causes the application to crash.

Originally I had it so the iCloud checking wasn't in a background thread and the app worked fine, but occasionally timed out.

Answer 1

You have not to know when iCloud becomes available. You just work with data but you don't send them directly to iCloud. iOS does it instead you. So only it knows when and how it should send data.

Answer 2

No, you can't change the options on an NSPersistentStore object once it exists. You can only specify the options when adding the persistent store to the NSPersistentStoreCoordinator . The closest you could get to changing options would be to tear down the entire Core Data stack and start over with different options.

That wouldn't help, though, because:

• Even if you have detected that iCloud is available (I'm guessing using NSFileManager , either via its ubiquityIdentityToken or by calling URLForUbiquityContainerIdentifier: ), your call to addPersistentStoreWithType:configuration:URL:options:error: might still block for a while. If there's new data available in iCloud, it doesn't start downloading until you add the persistent store, and that method doesn't return until the download process is finished. And sometimes, iCloud just makes that method block for a while for no readily apparent reason.

• If you let the user make any changes to the data while using non-iCloud options, those changes will not get automatically sent to the cloud later. Core Data only sends changes to iCloud when the data changes while iCloud is active-- which makes it generate a transaction. You'd have to load and re-save any changes the user made, or those changes would never make it to the cloud.

You have, unfortunately, hit on one of the major stumbling points when using Core Data with iCloud. You can't make the full data store available until Core Data finishes communicating with iCloud-- because your call to add the persistent store doesn't return until then. And you can't do anything to speed up that process. This is just one of the headaches you'll run into if you continue trying to use iCloud with Core Data.

Depending on the nature of your data you might be able to use two data stores, one purely local and one synced via iCloud. You could make the purely local data store available while the iCloud one tries to get its act together well enough to be useful. If you stick with one data store though, you're stuck with the delay.