Core Data Multithreading / Concurrency with Core Data

Basic rules are:

1. Use one NSPersistentStoreCoordinator per program. You don't need them per thread.
2. Create one NSManagedObjectContext per thread.
3. Never pass an NSManagedObject on a thread to the other thread.
4. Instead, get the object IDs via -objectID and pass it to the other thread.

More rules:

1. Make sure you save the object into the store before getting the object ID. Until saved, they're temporary, and you can't access them from another thread.
2. And beware of the merge policies if you make changes to the managed objects from more than one thread.
3. NSManagedObjectContext's -mergeChangesFromContextDidSaveNotification: is helpful.

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