Which persistence framework?

├─ Targeting iOS 17+ only?
│  ├─ Simple data model? → SwiftData (recommended)
│  ├─ Need public CloudKit database? → Core Data (SwiftData is private-only)
│  ├─ Need custom migration logic? → Core Data (more control)
│  └─ Existing Core Data app? → Keep Core Data or migrate gradually
│
├─ Targeting iOS 16 or earlier?
│  └─ Core Data (SwiftData unavailable)
│
└─ Need both? → Use Core Data with SwiftData wrapper (advanced)

import CoreData

class CoreDataStack {
    static let shared = CoreDataStack()

    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: "Model")

        // Configure for CloudKit if needed
        // container.persistentStoreDescriptions.first?.cloudKitContainerOptions =
        //     NSPersistentCloudKitContainerOptions(containerIdentifier: "iCloud.com.app")

        container.loadPersistentStores { description, error in
            if let error = error {
                // Handle appropriately for production
                fatalError("Failed to load store: \(error)")
            }
        }

        // Enable automatic merging
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

        return container
    }()

    var viewContext: NSManagedObjectContext {
        persistentContainer.viewContext
    }

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

import CoreData

class CloudKitStack {
    lazy var container: NSPersistentCloudKitContainer = {
        let container = NSPersistentCloudKitContainer(name: "Model")

        guard let description = container.persistentStoreDescriptions.first else {
            fatalError("No store description")
        }

        // Enable CloudKit sync
        description.cloudKitContainerOptions = NSPersistentCloudKitContainerOptions(
            containerIdentifier: "iCloud.com.yourapp"
        )

        // Enable history tracking for sync
        description.setOption(true as NSNumber,
                             forKey: NSPersistentHistoryTrackingKey)
        description.setOption(true as NSNumber,
                             forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)

        container.loadPersistentStores { _, error in
            if let error = error {
                fatalError("CloudKit store failed: \(error)")
            }
        }

        container.viewContext.automaticallyMergesChangesFromParent = true

        return container
    }()
}

// ❌ WRONG: Passing object across threads
let user = viewContext.fetch(...)  // Main thread
Task.detached {
    print(user.name)  // CRASH: Wrong thread
}

// ✅ CORRECT: Pass objectID, fetch on target context
let userID = user.objectID
Task.detached {
    let bgContext = CoreDataStack.shared.newBackgroundContext()
    let user = bgContext.object(with: userID) as! User
    print(user.name)  // Safe
}

// ✅ CORRECT: Background context for heavy work
func importData(_ items: [ImportItem]) async throws {
    let context = CoreDataStack.shared.newBackgroundContext()

    try await context.perform {
        for item in items {
            let entity = Entity(context: context)
            entity.configure(from: item)
        }

        try context.save()
    }
}

// Changes automatically merge to viewContext if configured

// Modern async context operations
func fetchUsers() async throws -> [User] {
    let context = CoreDataStack.shared.viewContext

    return try await context.perform {
        let request = User.fetchRequest()
        request.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
        return try context.fetch(request)
    }
}

// In User entity
@NSManaged var posts: NSSet?

// Convenience accessors
extension User {
    var postsArray: [Post] {
        (posts?.allObjects as? [Post]) ?? []
    }

    func addPost(_ post: Post) {
        mutableSetValue(forKey: "posts").add(post)
    }
}

// Both sides have NSSet
// User.tags <-> Tag.users

extension User {
    func addTag(_ tag: Tag) {
        mutableSetValue(forKey: "tags").add(tag)
        // Core Data automatically adds to tag.users
    }
}

struct UserList: View {
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \User.name, ascending: true)],
        predicate: NSPredicate(format: "isActive == YES"),
        animation: .default
    )
    private var users: FetchedResults<User>

    var body: some View {
        List(users) { user in
            Text(user.name ?? "Unknown")
        }
    }
}

// Dynamic predicates
struct FilteredList: View {
    @FetchRequest var items: FetchedResults<Item>

    init(category: String) {
        _items = FetchRequest(
            sortDescriptors: [NSSortDescriptor(keyPath: \Item.date, ascending: false)],
            predicate: NSPredicate(format: "category == %@", category)
        )
    }
}

// ❌ WRONG: N+1 queries
let users = try context.fetch(User.fetchRequest())
for user in users {
    print(user.posts?.count ?? 0)  // Fault fired for each user
}

// ✅ CORRECT: Prefetch relationships
let request = User.fetchRequest()
request.relationshipKeyPathsForPrefetching = ["posts"]
let users = try context.fetch(request)
for user in users {
    print(user.posts?.count ?? 0)  // Already loaded
}

let request = User.fetchRequest()
request.fetchBatchSize = 20  // Load 20 at a time as needed
request.returnsObjectsAsFaults = true  // Default, memory efficient

let description = NSPersistentStoreDescription()
description.shouldMigrateStoreAutomatically = true
description.shouldInferMappingModelAutomatically = true

// Create mapping model in Xcode:
// File → New → Mapping Model
// Select source and destination models

// ❌ WRONG: One context for all operations
class DataManager {
    let context = CoreDataStack.shared.viewContext

    func importInBackground() {
        // Using main context on background = crash
        for item in largeDataset {
            let entity = Entity(context: context)
        }
    }
}

// ✅ CORRECT: Context per operation type
func importInBackground() {
    let bgContext = CoreDataStack.shared.newBackgroundContext()
    bgContext.perform {
        // Safe background work
    }
}

// ❌ WRONG: Fetch on every render
var body: some View {
    let users = try? context.fetch(User.fetchRequest())  // Called repeatedly!
    List(users ?? []) { ... }
}

// ✅ CORRECT: Use @FetchRequest
@FetchRequest(sortDescriptors: [])
var users: FetchedResults<User>

var body: some View {
    List(users) { ... }  // Automatic updates
}

// ❌ WRONG: No merge policy (conflicts crash)
let context = container.viewContext

// ✅ CORRECT: Define merge behavior
context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
context.automaticallyMergesChangesFromParent = true