Core Data

Build and maintain data persistence using Core Data for apps that have not adopted SwiftData. Covers stack setup, concurrency, batch operations, NSFetchedResultsController, persistent history tracking, staged migration, and testing.

Contents

• Stack Setup & Concurrency
• NSFetchedResultsController
• Batch Operations
• Persistent History Tracking
• Staged Migration & Coexistence
• Testing
• Common Mistakes
• Review Checklist
• References

Stack Setup & Concurrency

Core Data contexts are bound to queues: viewContext runs on the main queue, while background tasks use newBackgroundContext().

• Context Isolation: Always wrap access in context.perform(_:) or context.performAndWait(_:).
• Thread Crossing: Never pass NSManagedObject instances between contexts or threads; pass NSManagedObjectID and re-fetch via existingObject(with:).
• Automatic Merging: Set automaticallyMergesChangesFromParent = true and configure mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy on viewContext.

import CoreData

func updateTrip(id: NSManagedObjectID, name: String) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        guard let trip = try context.existingObject(with: id) as? CDTrip else { return }
        trip.name = name
        if context.hasChanges { try context.save() }
    }
}

NSFetchedResultsController

Drives UI collections from an NSFetchRequest with automatic change tracking:

• Always supply at least one sort descriptor in the fetch request.
• Use diffable snapshots (controller(_:didChangeContentWith:)) on iOS 13+ to animate UI changes.
• Delete the cache with deleteCache(withName:) before changing predicate or sort descriptors, or pass cacheName: nil.

Batch Operations

NSBatchInsertRequest, NSBatchUpdateRequest, and NSBatchDeleteRequest execute directly at the SQLite level, bypassing managed object contexts.

• Set resultType = .objectIDs / .resultTypeObjectIDs.
• Merge results manually into live contexts using NSManagedObjectContext.mergeChanges(fromRemoteContextSave:into:).
• Note that batch deletes bypass Core Data relationship delete rules (e.g. Deny).

Persistent History Tracking

Track store modifications across app extensions, widgets, and processes:

• Enable NSPersistentHistoryTrackingKey: true in store options.
• Query changes using NSPersistentHistoryChangeRequest and merge them into local contexts.
• Purge consumed transactions periodically with history truncation requests to prevent database bloat.
• Read references/persistent-history.md for full transaction tracking and purge loops.

Staged Migration & Coexistence

• Staged Migration (iOS 17+): Use NSStagedMigrationManager with lightweight and custom stages (NSCustomMigrationStage) for deterministic upgrades. Read references/staged-migration.md.
• SwiftData Boundary: When sharing a database file with SwiftData, point both to the same store URL, preserve model schemas and attribute names, and map renames with @Attribute(originalName:). Route pure SwiftData work to the swiftdata skill.

Testing

Use NSInMemoryStoreType for deterministic unit testing. Share a single compiled NSManagedObjectModel instance across tests to prevent duplicate entity runtime errors.

Common Mistakes

• Passing NSManagedObject across threads: Causes concurrency crashes. Always pass NSManagedObjectID and refetch with existingObject(with:).
• Missing mergeChanges after batch requests: Batch operations bypass memory contexts. Always merge returned object IDs.
• Calling save() without hasChanges: Avoid redundant I/O; guard saves with if context.hasChanges.
• Omitting merge policy on viewContext: Missing mergePolicy causes conflict save crashes. Set NSMergeByPropertyObjectTrumpMergePolicy.
• Marking NSManagedObject as Sendable: Managed objects are queue-bound. Do not mark @unchecked Sendable.

Review Checklist

• NSPersistentContainer created once and shared across the app
• Context access strictly guarded by perform(_:) or performAndWait(_:)
• No NSManagedObject instances cross thread or context boundaries
• viewContext.automaticallyMergesChangesFromParent is enabled
• mergePolicy configured on contexts to handle conflicts cleanly
• Batch operation results merged into active contexts via mergeChanges
• NSFetchedResultsController requests include explicit sort descriptors
• In-memory test stores reuse a shared NSManagedObjectModel

References

• Implementation patterns (Stack, FRC, Batch, Testing): references/core-data-patterns.md
• Cross-process tracking: references/persistent-history.md
• Staged migration: references/staged-migration.md
• Core Data
• NSPersistentContainer
• NSFetchedResultsController
• NSStagedMigrationManager