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()
    }
}

// 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()
    }
}

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
}

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()
    }
}

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)
    }
}

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]
            )
        }
    }
}

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]
            )
        }
    }
}

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]
            )
        }
    }
}

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

// 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)
    }
}

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)") }
}

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")
    }
}

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
}