dpearson2699/core-data

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 and Threading
• NSFetchedResultsController
• Batch Operations
• Persistent History Tracking
• Staged Migration
• Composite Attributes
• SwiftData Boundary
• Testing
• Common Mistakes
• Review Checklist
• References

Stack Setup

NSPersistentContainer encapsulates the Core Data stack.

Docs: NSPersistentContainer

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.

// 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.

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:

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

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

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

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+)

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

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

// 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 in 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

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 checksumV3 = "<ModelV3 version checksum>"
let stage1to2 = NSLightweightMigrationStage([checksumV1, checksumV2])
stage1to2.label = "Add isFavorite property"

let modelV2 = NSManagedObjectModelReference(
    name: "ModelV2",
    in: Bundle.main,
    versionChecksum: checksumV2
)
let modelV3 = NSManagedObjectModelReference(
    name: "ModelV3",
    in: Bundle.main,
    versionChecksum: checksumV3
)
let stage2to3 = NSCustomMigrationStage(
    migratingFrom: modelV2,
    to: modelV3
)
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.

SwiftData Boundary

Use the swiftdata skill for Core Data + SwiftData coexistence or migration implementation. Before handing off, preserve these Core Data boundaries:

• SwiftData must point at the existing persistent store URL when it is meant to share or migrate Core Data data.
• Shared persisted data must keep entity names, property names, types, and schema compatible across the Core Data model and SwiftData @Model classes.
• Map renamed persisted properties with SwiftData @Attribute(originalName:).

Testing

In-Memory Store for Tests

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:

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 | | Pointing SwiftData at a fresh store during coexistence | Use the existing store URL and compatible schema when SwiftData should share or migrate Core Data data |

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
• [ ] Core Data + SwiftData handoff preserves store URL, schema compatibility, entity/property names, and rename mappings
• [ ] Tests use in-memory stores with shared NSManagedObjectModel
• [ ] No NSManagedObject instances cross thread boundaries

References

• Apple docs: Core Data | NSPersistentContainer | NSFetchedResultsController | NSStagedMigrationManager