What’s new in Core Data

Composite attributes

• New type of attribute that encapsulates complex and custom data types
  • You can nest them and now create them in the Core Data model editor
  • There is a demo of how to adopt these new Composite Attributes
  • Watching the demo I can only think, how will SwiftData handle this, and if it does already, how much easier will it be

Stage your migrations

• It is always preferable to do Lightweight migrations – it is built in and will happen automatically.  Check out last year’s Evolve your app’s Schema – I captured my thoughts on that session here in the section on Core Data.
• When it is to complex, you can do a staged migration (this is new),
  • For migration of non-conforming lightweight changes
  • Simplify your app
  • Provides some opportunities for your app to gain execution control during the migration to perform specific tasks
    • You need to identify when Changes don’t conform with light weight migration
      • You can manually review the chains
      • You can try to open the store with the lightweight option using NSMigratePersistentStoresAutomaticallyOption and NSInferMappingModelAutomaticallyOption set to true.
      • You’ll receive an hash error if they are not compatible
      • Or you can use NSMappingModel.inferredMappingModel(forSourceModel:destinationModel:) which will either return the inferred model or nil if it cannot 
    • Decompose into a series of comforting migrations
    • Describe the sequence of ordering using the new APIs
    • Have Core Data execute an event loop to do the migration
  • The complex model change that is described may work for my migration, I will have to see if I can recreate this with multiple models
• Adding Staged migration allows you to mix and match (in sequence) your model migration from both lightweight and custom migration stages.
  • You create a .willMigrateHandler to do custom work during migration, for each fetch entity you will copy in the data and relate them.

Defer your migrations

• If your lightweight migrations take too much time, which can frustrate your users.
  • You can now do deferred migrations at a later date, for example dropping indices or deleting a column – you can defer this clean up by setting
  • NSPersistentStoreDeferredLightweightMigrationOptionKey to True – whit is only available for Sqlight, but is available back to iOS 14
  • You can check the store’s metadata if that value is set and you can then process them by running NSPersistentStoreCoordinator.finishDeferredLightweightMigration()
  • Consider using Background Task API to schedule this work
• You can combine deferred and staged migrations