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Zhongwei Li
527
skills/expertise/iphone-apps/references/data-persistence.md
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527
skills/expertise/iphone-apps/references/data-persistence.md
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# Data Persistence
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SwiftData, Core Data, and file-based storage for iOS apps.
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## SwiftData (iOS 17+)
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### Model Definition
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```swift
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import SwiftData
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@Model
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class Item {
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var name: String
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var createdAt: Date
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var isCompleted: Bool
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var priority: Int
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@Relationship(deleteRule: .cascade)
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var tasks: [Task]
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@Relationship(inverse: \Category.items)
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var category: Category?
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init(name: String, priority: Int = 0) {
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self.name = name
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self.createdAt = Date()
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self.isCompleted = false
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self.priority = priority
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self.tasks = []
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}
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}
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@Model
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class Task {
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var title: String
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var isCompleted: Bool
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init(title: String) {
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self.title = title
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self.isCompleted = false
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}
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}
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@Model
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class Category {
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var name: String
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var items: [Item]
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init(name: String) {
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self.name = name
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self.items = []
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}
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}
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```
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### Container Setup
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```swift
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@main
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struct MyApp: App {
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var body: some Scene {
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WindowGroup {
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ContentView()
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}
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.modelContainer(for: [Item.self, Category.self])
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}
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}
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```
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### Querying Data
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```swift
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struct ItemList: View {
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// Basic query
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@Query private var items: [Item]
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// Sorted query
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@Query(sort: \Item.createdAt, order: .reverse)
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private var sortedItems: [Item]
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// Filtered query
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@Query(filter: #Predicate<Item> { $0.isCompleted == false })
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private var incompleteItems: [Item]
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// Complex query
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@Query(
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filter: #Predicate<Item> { !$0.isCompleted && $0.priority > 5 },
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sort: [
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SortDescriptor(\Item.priority, order: .reverse),
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SortDescriptor(\Item.createdAt)
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]
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)
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private var highPriorityItems: [Item]
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var body: some View {
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List(items) { item in
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ItemRow(item: item)
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}
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}
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}
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```
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### CRUD Operations
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```swift
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struct ItemList: View {
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@Query private var items: [Item]
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@Environment(\.modelContext) private var context
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var body: some View {
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List {
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ForEach(items) { item in
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ItemRow(item: item)
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}
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.onDelete(perform: delete)
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}
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.toolbar {
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Button("Add", action: addItem)
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}
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}
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private func addItem() {
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let item = Item(name: "New Item")
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context.insert(item)
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// Auto-saves
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}
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private func delete(at offsets: IndexSet) {
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for index in offsets {
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context.delete(items[index])
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}
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}
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}
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```
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### Custom Container Configuration
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```swift
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@main
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struct MyApp: App {
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let container: ModelContainer
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init() {
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let schema = Schema([Item.self, Category.self])
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let config = ModelConfiguration(
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schema: schema,
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isStoredInMemoryOnly: false,
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allowsSave: true,
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groupContainer: .identifier("group.com.yourcompany.app")
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)
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do {
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container = try ModelContainer(for: schema, configurations: config)
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} catch {
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fatalError("Failed to configure SwiftData container: \(error)")
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}
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}
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var body: some Scene {
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WindowGroup {
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ContentView()
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}
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.modelContainer(container)
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}
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}
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```
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### iCloud Sync
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SwiftData syncs automatically with iCloud when:
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1. App has iCloud capability
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2. User is signed into iCloud
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3. Container uses CloudKit
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```swift
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let config = ModelConfiguration(
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cloudKitDatabase: .automatic
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)
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```
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## Core Data (All iOS Versions)
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### Stack Setup
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```swift
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class CoreDataStack {
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static let shared = CoreDataStack()
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lazy var persistentContainer: NSPersistentContainer = {
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let container = NSPersistentContainer(name: "MyApp")
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// Enable cloud sync
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guard let description = container.persistentStoreDescriptions.first else {
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fatalError("No persistent store description")
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}
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description.cloudKitContainerOptions = NSPersistentCloudKitContainerOptions(
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containerIdentifier: "iCloud.com.yourcompany.app"
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)
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container.loadPersistentStores { description, error in
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if let error = error {
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fatalError("Core Data failed to load: \(error)")
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}
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}
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container.viewContext.automaticallyMergesChangesFromParent = true
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container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
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return container
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}()
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var viewContext: NSManagedObjectContext {
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persistentContainer.viewContext
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}
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func saveContext() {
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let context = viewContext
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if context.hasChanges {
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do {
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try context.save()
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} catch {
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print("Failed to save context: \(error)")
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}
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}
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}
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}
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```
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### With SwiftUI
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```swift
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@main
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struct MyApp: App {
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let coreDataStack = CoreDataStack.shared
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var body: some Scene {
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WindowGroup {
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ContentView()
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.environment(\.managedObjectContext, coreDataStack.viewContext)
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}
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}
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}
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struct ItemList: View {
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@FetchRequest(
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sortDescriptors: [NSSortDescriptor(keyPath: \Item.createdAt, ascending: false)],
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predicate: NSPredicate(format: "isCompleted == NO")
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)
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private var items: FetchedResults<Item>
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@Environment(\.managedObjectContext) private var context
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var body: some View {
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List(items) { item in
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ItemRow(item: item)
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}
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}
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}
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```
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## File-Based Storage
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### Codable Models
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```swift
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struct UserSettings: Codable {
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var theme: Theme
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var fontSize: Int
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var notificationsEnabled: Bool
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enum Theme: String, Codable {
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case light, dark, system
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}
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}
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class SettingsStore {
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private let fileURL: URL
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init() {
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let documentsDirectory = FileManager.default.urls(
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for: .documentDirectory,
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in: .userDomainMask
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).first!
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fileURL = documentsDirectory.appendingPathComponent("settings.json")
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}
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func load() -> UserSettings {
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guard let data = try? Data(contentsOf: fileURL),
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let settings = try? JSONDecoder().decode(UserSettings.self, from: data) else {
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return UserSettings(theme: .system, fontSize: 16, notificationsEnabled: true)
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}
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return settings
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}
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func save(_ settings: UserSettings) throws {
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let data = try JSONEncoder().encode(settings)
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try data.write(to: fileURL)
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}
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}
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```
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### Document Directory Paths
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```swift
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extension FileManager {
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var documentsDirectory: URL {
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urls(for: .documentDirectory, in: .userDomainMask).first!
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}
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var cachesDirectory: URL {
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urls(for: .cachesDirectory, in: .userDomainMask).first!
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}
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var applicationSupportDirectory: URL {
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let url = urls(for: .applicationSupportDirectory, in: .userDomainMask).first!
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try? createDirectory(at: url, withIntermediateDirectories: true)
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return url
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}
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}
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```
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## UserDefaults
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### Basic Usage
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```swift
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// Save
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UserDefaults.standard.set("value", forKey: "key")
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UserDefaults.standard.set(true, forKey: "hasCompletedOnboarding")
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// Load
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let value = UserDefaults.standard.string(forKey: "key")
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let hasCompletedOnboarding = UserDefaults.standard.bool(forKey: "hasCompletedOnboarding")
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```
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### @AppStorage
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```swift
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struct SettingsView: View {
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@AppStorage("fontSize") private var fontSize = 16
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@AppStorage("isDarkMode") private var isDarkMode = false
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@AppStorage("username") private var username = ""
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var body: some View {
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Form {
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Stepper("Font Size: \(fontSize)", value: $fontSize, in: 12...24)
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Toggle("Dark Mode", isOn: $isDarkMode)
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TextField("Username", text: $username)
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}
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}
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}
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```
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### Custom Codable Storage
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```swift
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extension UserDefaults {
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func set<T: Codable>(_ value: T, forKey key: String) {
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if let data = try? JSONEncoder().encode(value) {
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set(data, forKey: key)
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}
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}
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func get<T: Codable>(_ type: T.Type, forKey key: String) -> T? {
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guard let data = data(forKey: key) else { return nil }
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return try? JSONDecoder().decode(type, from: data)
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}
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}
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// Usage
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UserDefaults.standard.set(userProfile, forKey: "userProfile")
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let profile = UserDefaults.standard.get(UserProfile.self, forKey: "userProfile")
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```
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## Keychain (Sensitive Data)
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### Simple Wrapper
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```swift
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import Security
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class KeychainService {
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enum KeychainError: Error {
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case saveFailed(OSStatus)
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case loadFailed(OSStatus)
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case deleteFailed(OSStatus)
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case dataConversionError
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}
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func save(_ data: Data, for key: String) throws {
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// Delete existing
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try? delete(key)
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let query: [String: Any] = [
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kSecClass as String: kSecClassGenericPassword,
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kSecAttrAccount as String: key,
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kSecValueData as String: data,
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kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlocked
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]
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let status = SecItemAdd(query as CFDictionary, nil)
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guard status == errSecSuccess else {
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throw KeychainError.saveFailed(status)
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}
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}
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func load(_ key: String) throws -> Data {
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let query: [String: Any] = [
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kSecClass as String: kSecClassGenericPassword,
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kSecAttrAccount as String: key,
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kSecReturnData as String: true
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]
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var result: AnyObject?
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let status = SecItemCopyMatching(query as CFDictionary, &result)
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guard status == errSecSuccess else {
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throw KeychainError.loadFailed(status)
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}
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guard let data = result as? Data else {
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throw KeychainError.dataConversionError
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}
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return data
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}
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func delete(_ key: String) throws {
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let query: [String: Any] = [
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kSecClass as String: kSecClassGenericPassword,
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kSecAttrAccount as String: key
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]
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let status = SecItemDelete(query as CFDictionary)
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guard status == errSecSuccess || status == errSecItemNotFound else {
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throw KeychainError.deleteFailed(status)
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}
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}
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}
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// String convenience
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extension KeychainService {
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func saveString(_ value: String, for key: String) throws {
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guard let data = value.data(using: .utf8) else {
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throw KeychainError.dataConversionError
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}
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try save(data, for: key)
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}
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func loadString(_ key: String) throws -> String {
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let data = try load(key)
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guard let string = String(data: data, encoding: .utf8) else {
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throw KeychainError.dataConversionError
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}
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return string
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}
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}
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```
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### Usage
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```swift
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let keychain = KeychainService()
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// Save API token
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try keychain.saveString(token, for: "apiToken")
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// Load API token
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let token = try keychain.loadString("apiToken")
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// Delete on logout
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try keychain.delete("apiToken")
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```
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## Migration Strategies
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### SwiftData Migrations
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```swift
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enum SchemaV1: VersionedSchema {
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static var versionIdentifier = Schema.Version(1, 0, 0)
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static var models: [any PersistentModel.Type] {
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[Item.self]
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}
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@Model
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class Item {
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var name: String
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init(name: String) { self.name = name }
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}
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}
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enum SchemaV2: VersionedSchema {
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static var versionIdentifier = Schema.Version(2, 0, 0)
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static var models: [any PersistentModel.Type] {
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[Item.self]
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}
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@Model
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class Item {
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var name: String
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var createdAt: Date // New field
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init(name: String) {
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self.name = name
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self.createdAt = Date()
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}
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}
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}
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enum MigrationPlan: SchemaMigrationPlan {
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static var schemas: [any VersionedSchema.Type] {
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[SchemaV1.self, SchemaV2.self]
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}
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static var stages: [MigrationStage] {
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[migrateV1toV2]
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}
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static let migrateV1toV2 = MigrationStage.lightweight(
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fromVersion: SchemaV1.self,
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toVersion: SchemaV2.self
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)
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}
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```
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Reference in New Issue
Block a user