경량 마이그레이션과 CloudKit 동기화

// Without migration, app crashes on launch:
// "The model used to open the store is incompatible with the one used to create the store"

let desc = NSPersistentStoreDescription(url: storeURL)
desc.shouldMigrateStoreAutomatically = true
desc.shouldInferMappingModelAutomatically = true
container.persistentStoreDescriptions = [desc]

// In MyModel.xcdatamodeld:
// ├── MyModel.xcdatamodel  (v1 - old)
// └── MyModel 2.xcdatamodel (v2 - current)
// Set current version to v2 in File Inspector

enum AppSchemaV1: VersionedSchema {
  static var versionIdentifier = Schema.Version(1,0,0)
  static var models: [any PersistentModel.Type] { [ItemV1.self] }
  @Model final class ItemV1 { var name: String; init(name: String) { self.name = name } }
}

enum AppMigrationPlan: SchemaMigrationPlan {
  static var schemas: [any VersionedSchema.Type] { [AppSchemaV1.self, AppSchemaV2.self] }
  static var stages: [MigrationStage] { [migrateV1toV2] }
  static let migrateV1toV2 = MigrationStage.lightweight(fromVersion: AppSchemaV1.self, toVersion: AppSchemaV2.self)
}

static let migrateV1toV2 = MigrationStage.custom(
  fromVersion: AppSchemaV1.self,
  toVersion: AppSchemaV2.self,
  willMigrate: { context in
    // transform old data
  },
  didMigrate: nil
)

let container = NSPersistentCloudKitContainer(name: "MyModel")
container.loadPersistentStores { _, error in
  if let error { fatalError("\(error)") }
}
container.viewContext.automaticallyMergesChangesFromParent = true

// Xcode: Signing & Capabilities → + → iCloud → CloudKit
// Enable "Use CloudKit" in Core Data model inspector

let config = ModelConfiguration(
  schema: schema,
  cloudKitDatabase: .automatic
)
let container = try ModelContainer(for: schema, configurations: config)

// Best practice: keep models granular
// One attribute per write concern
// Avoid large blobs that frequently change

// Copy production store to a temp location
// Load it with the new container and verify objects are intact

NotificationCenter.default.addObserver(
  forName: .NSPersistentStoreCoordinatorStoresWillChange,
  object: container.persistentStoreCoordinator,
  queue: .main
) { _ in showMigrationProgress() }