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.

Stack Setup
Concurrency and Threading
NSFetchedResultsController
Batch Operations
Persistent History Tracking
Staged Migration
Composite Attributes
Testing
Common Mistakes
Review Checklist
References

Stack Setup

NSPersistentContainer

encapsulates the Core Data stack.

Docs: NSPersistentContainer

swift

import CoreData

final class CoreDataStack: @unchecked Sendable {
    static let shared = CoreDataStack()

    let container: NSPersistentContainer

    private init() {
        container = NSPersistentContainer(name: "MyAppModel")
        container.loadPersistentStores { _, error in
            if let error { fatalError("Core Data store failed: \(error)") }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
    }

    var viewContext: NSManagedObjectContext { container.viewContext }

    func newBackgroundContext() -> NSManagedObjectContext {
        container.newBackgroundContext()
    }
}

For CloudKit sync, use

NSPersistentCloudKitContainer

instead.

Concurrency and Threading

Core Data contexts are bound to queues. The

viewContext

is on the main queue; background contexts operate on private queues.

Docs: NSManagedObjectContext

Rules:

Always use
```
perform(_:)
```
or
```
performAndWait(_:)
```
when accessing a context off its own queue.
Never pass
```
NSManagedObject
```
instances across context or thread boundaries. Pass
```
NSManagedObjectID
```
instead and re-fetch.

Set

automaticallyMergesChangesFromParent = true

on the

viewContext

swift

// Writing on a background context
func updateTrip(id: NSManagedObjectID, newName: String) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        guard let trip = try context.existingObject(with: id) as? CDTrip else {
            throw PersistenceError.notFound
        }
        trip.name = newName
        try context.save()
    }
}

Swift Concurrency Integration

NSManagedObjectContext.perform(_:)

has an

async throws

overload (iOS 15+). Avoid marking

NSManagedObject

subclasses as

Sendable

swift

func importItems(_ records: [ItemRecord]) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        for record in records {
            let item = CDItem(context: context)
            item.id = record.id
            item.title = record.title
        }
        try context.save()
    }
    // After save completes, viewContext auto-merges if configured
}

Do not use
@unchecked Sendable
on managed objects. If you need cross-boundary communication, pass the

objectID

(which is

Sendable

) and re-fetch:

swift

let objectID = trip.objectID  // Sendable
Task.detached {
    let bgContext = CoreDataStack.shared.newBackgroundContext()
    try await bgContext.perform {
        let trip = try bgContext.existingObject(with: objectID) as! CDTrip
        trip.isFavorite = true
        try bgContext.save()
    }
}

NSFetchedResultsController

Efficiently drives

UITableView

UICollectionView

from a Core Data fetch request, with built-in change tracking and optional caching.

Docs: NSFetchedResultsController

swift

import CoreData
import UIKit

class TripsViewController: UITableViewController, NSFetchedResultsControllerDelegate {

    private lazy var fetchedResultsController: NSFetchedResultsController<CDTrip> = {
        let request: NSFetchRequest<CDTrip> = CDTrip.fetchRequest()
        request.sortDescriptors = [
            NSSortDescriptor(keyPath: \CDTrip.startDate, ascending: false)
        ]
        request.fetchBatchSize = 20

        let controller = NSFetchedResultsController(
            fetchRequest: request,
            managedObjectContext: CoreDataStack.shared.viewContext,
            sectionNameKeyPath: nil,
            cacheName: "TripsCache"
        )
        controller.delegate = self
        return controller
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        try? fetchedResultsController.performFetch()
    }

    // MARK: - UITableViewDataSource

    override func numberOfSections(in tableView: UITableView) -> Int {
        fetchedResultsController.sections?.count ?? 0
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        fetchedResultsController.sections?[section].numberOfObjects ?? 0
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "TripCell", for: indexPath)
        let trip = fetchedResultsController.object(at: indexPath)
        cell.textLabel?.text = trip.name
        return cell
    }

    // MARK: - NSFetchedResultsControllerDelegate (diffable)

    func controller(
        _ controller: NSFetchedResultsController<any NSFetchRequestResult>,
        didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference
    ) {
        let snapshot = snapshot as NSDiffableDataSourceSnapshot<String, NSManagedObjectID>
        dataSource.apply(snapshot, animatingDifferences: true)
    }
}

Key points:

The fetch request must have at least one sort descriptor.
Call
```
deleteCache(withName:)
```
before changing the fetch request predicate or sort descriptors, or set
```
cacheName
```
to
```
nil
```
.
The diffable snapshot delegate method (
```
didChangeContentWith:
```
) is available iOS 13+ and is preferred over the older per-change callbacks.
After a context
```
reset()
```
, call
```
performFetch()
```
again.

Batch Operations

Batch operations execute at the SQL level, bypassing the managed object context. They are fast but don't trigger context notifications automatically.

NSBatchInsertRequest (iOS 13+)

Docs: NSBatchInsertRequest

swift

func batchImport(_ records: [[String: Any]]) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        let request = NSBatchInsertRequest(
            entity: CDTrip.entity(),
            objects: records
        )
        request.resultType = .objectIDs
        let result = try context.execute(request) as? NSBatchInsertResult
        if let ids = result?.result as? [NSManagedObjectID] {
            NSManagedObjectContext.mergeChanges(
                fromRemoteContextSave: [NSInsertedObjectsKey: ids],
                into: [CoreDataStack.shared.viewContext]
            )
        }
    }
}

NSBatchDeleteRequest (iOS 9+)

Docs: NSBatchDeleteRequest

swift

func deleteOldTrips(before cutoff: Date) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        let fetchRequest: NSFetchRequest<NSFetchRequestResult> = CDTrip.fetchRequest()
        fetchRequest.predicate = NSPredicate(format: "endDate < %@", cutoff as NSDate)
        let request = NSBatchDeleteRequest(fetchRequest: fetchRequest)
        request.resultType = .resultTypeObjectIDs
        let result = try context.execute(request) as? NSBatchDeleteResult
        if let ids = result?.result as? [NSManagedObjectID] {
            NSManagedObjectContext.mergeChanges(
                fromRemoteContextSave: [NSDeletedObjectsKey: ids],
                into: [CoreDataStack.shared.viewContext]
            )
        }
    }
}

NSBatchUpdateRequest (iOS 8+)

swift

func markAllTripsAsNotFavorite() async throws {
    let context = CoreDataStack.shared.newBackgroundContext()
    try await context.perform {
        let request = NSBatchUpdateRequest(entity: CDTrip.entity())
        request.propertiesToUpdate = ["isFavorite": false]
        request.resultType = .updatedObjectIDsResultType
        let result = try context.execute(request) as? NSBatchUpdateResult
        if let ids = result?.result as? [NSManagedObjectID] {
            NSManagedObjectContext.mergeChanges(
                fromRemoteContextSave: [NSUpdatedObjectsKey: ids],
                into: [CoreDataStack.shared.viewContext]
            )
        }
    }
}

Always merge changes back into relevant contexts after batch operations. Batch delete does not enforce the Deny delete rule.

Persistent History Tracking

Track store-level changes across targets (app, extensions, widgets) and processes.

Docs: NSPersistentHistoryChangeRequest

Enable History Tracking

swift

let description = NSPersistentStoreDescription()
description.setOption(true as NSNumber, forKey: NSPersistentHistoryTrackingKey)
description.setOption(true as NSNumber,
    forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
container.persistentStoreDescriptions = [description]

Observe, Fetch, Merge, and Purge

swift

// 1. Observe remote change notifications
NotificationCenter.default.addObserver(
    self, selector: #selector(storeRemoteChange(_:)),
    name: .NSPersistentStoreRemoteChange, object: container.persistentStoreCoordinator
)

// 2. Fetch history since last token
@objc func storeRemoteChange(_ notification: Notification) {
    let context = container.newBackgroundContext()
    context.perform {
        let request = NSPersistentHistoryChangeRequest.fetchHistory(after: self.lastToken)
        if let result = try? context.execute(request) as? NSPersistentHistoryResult,
           let transactions = result.result as? [NSPersistentHistoryTransaction] {
            // 3. Merge into viewContext
            for transaction in transactions {
                self.container.viewContext.mergeChanges(fromContextDidSave: transaction.objectIDNotification())
                self.lastToken = transaction.token
            }
        }
        // 4. Purge old history
        let purgeRequest = NSPersistentHistoryChangeRequest.deleteHistory(before: self.lastToken)
        try? context.execute(purgeRequest)
    }
}

Store

lastToken

UserDefaults

(per target) so history is processed correctly across launches.

Staged Migration

NSStagedMigrationManager

(iOS 17+) sequences schema migrations through ordered stages, each lightweight or custom.

Docs: NSStagedMigrationManager

swift

import CoreData

// Define migration stages
// Use version checksums from the compiled model versions, not model names.
let checksumV1 = "<ModelV1 version checksum>"
let checksumV2 = "<ModelV2 version checksum>"
let stage1to2 = NSLightweightMigrationStage([checksumV1, checksumV2])
stage1to2.label = "Add isFavorite property"

let stage2to3 = NSCustomMigrationStage(
    migratingFrom: NSManagedObjectModelReference(
        named: "ModelV2", in: Bundle.main),
    to: NSManagedObjectModelReference(
        named: "ModelV3", in: Bundle.main)
)
stage2to3.label = "Split name into firstName/lastName"
stage2to3.willMigrateHandler = { migrationManager, currentStage in
    guard let container = migrationManager.container else { return }
    let context = container.newBackgroundContext()
    try context.performAndWait {
        // Transform data between schema versions
        let request = NSFetchRequest<NSManagedObject>(entityName: "Person")
        let people = try context.fetch(request)
        for person in people {
            let fullName = person.value(forKey: "name") as? String ?? ""
            let parts = fullName.split(separator: " ", maxSplits: 1)
            person.setValue(String(parts.first ?? ""), forKey: "firstName")
            person.setValue(parts.count > 1 ? String(parts.last!) : "", forKey: "lastName")
        }
        try context.save()
    }
}

// Apply to the persistent store
let manager = NSStagedMigrationManager([stage1to2, stage2to3])
let description = NSPersistentStoreDescription()
description.setOption(manager,
    forKey: NSPersistentStoreStagedMigrationManagerOptionKey)
container.persistentStoreDescriptions = [description]
container.loadPersistentStores { _, error in
    if let error { fatalError("Migration failed: \(error)") }
}

For apps targeting below iOS 17, use lightweight migration (

NSInferMappingModelAutomaticallyOption

) or mapping models.

NSLightweightMigrationStage

takes version checksums (

[String]

), not human-readable model names.

Composite Attributes

iOS 17+ supports composite attributes: groups of sub-attributes on an entity that act as a single logical unit. Define them in the model editor by adding a Composite type attribute and nesting sub-attributes beneath it.

Docs: NSCompositeAttributeDescription

Composite attributes map to

Codable

structs in SwiftData coexistence scenarios.

Testing

In-Memory Store for Tests

swift

import CoreData
import Testing

struct CoreDataTests {
    func makeTestContainer() throws -> NSPersistentContainer {
        let container = NSPersistentContainer(name: "MyAppModel")
        let description = NSPersistentStoreDescription()
        description.type = NSInMemoryStoreType
        container.persistentStoreDescriptions = [description]

        var loadError: Error?
        container.loadPersistentStores { _, error in loadError = error }
        if let loadError { throw loadError }
        return container
    }

    @Test func createAndFetchTrip() throws {
        let container = try makeTestContainer()
        let context = container.viewContext

        let trip = CDTrip(context: context)
        trip.name = "Test Trip"
        trip.startDate = .now
        try context.save()

        let request: NSFetchRequest<CDTrip> = CDTrip.fetchRequest()
        let trips = try context.fetch(request)
        #expect(trips.count == 1)
        #expect(trips.first?.name == "Test Trip")
    }
}

Tips:

Share the
```
NSManagedObjectModel
```
instance across tests to avoid "duplicate entity" warnings.
Use a single shared model loaded once:

swift

private let sharedModel: NSManagedObjectModel = {
    let url = Bundle.main.url(forResource: "MyAppModel", withExtension: "momd")!
    return NSManagedObjectModel(contentsOf: url)!
}()

func makeTestContainer() throws -> NSPersistentContainer {
    let container = NSPersistentContainer(name: "MyAppModel",
                                          managedObjectModel: sharedModel)
    // ... configure in-memory store
}

Common Mistakes

Mistake	Fix
Passing `NSManagedObject` across threads	Pass `objectID` and re-fetch in the target context
Forgetting to merge batch operation results	Call `mergeChanges(fromRemoteContextSave:into:)`
Calling `save()` without checking `hasChanges`	Guard with `context.hasChanges` first
Using deprecated `init(concurrencyType:)` confinement type	Use `.privateQueueConcurrencyType` or `.mainQueueConcurrencyType`
Not setting `mergePolicy` on `viewContext`	Set `NSMergeByPropertyObjectTrumpMergePolicy` to avoid conflict crashes
Modifying fetch request on live `NSFetchedResultsController` without deleting cache	Call `deleteCache(withName:)` first or use `cacheName: nil`
Batch delete ignoring Deny delete rule	Batch delete bypasses delete rules; validate manually
Marking `NSManagedObject` as `@unchecked Sendable`	Do not. Pass `objectID` instead

Review Checklist

```
NSPersistentContainer
```
is initialized once and shared
```
viewContext
```
used only on main queue; background contexts for writes
```
perform(_:)
```
or
```
performAndWait(_:)
```
wraps all off-queue context access

automaticallyMergesChangesFromParent

set on

viewContext

```
mergePolicy
```
set on
```
viewContext
```
to prevent conflict crashes
Batch operation results merged into relevant contexts
```
NSFetchedResultsController
```
fetch requests have sort descriptors
Persistent history tracking enabled for multi-target apps
Tests use in-memory stores with shared
```
NSManagedObjectModel
```
No
```
NSManagedObject
```
instances cross thread boundaries

References

Apple docs: Core Data | NSPersistentContainer | NSFetchedResultsController | NSStagedMigrationManager

core-data

NPX Install

Tags

SKILL.md Content

Core Data

Contents

Stack Setup

Concurrency and Threading

Swift Concurrency Integration

NSFetchedResultsController

Batch Operations

NSBatchInsertRequest (iOS 13+)

NSBatchDeleteRequest (iOS 9+)

NSBatchUpdateRequest (iOS 8+)

Persistent History Tracking

Enable History Tracking

Observe, Fetch, Merge, and Purge

Staged Migration

Composite Attributes

Testing

In-Memory Store for Tests

Common Mistakes

Review Checklist

References