Swift persistence

Swift Persistence

Instructions

Step 1: Choose the framework

Ask if not already specified:

SwiftData or CoreData?

	SwiftData	CoreData
Min iOS	iOS 17+	iOS 8+
Syntax	@Model macro, Swift-native	NSManagedObject, Objective-C roots
Recommended for	New projects on iOS 17+	Legacy projects, complex migration needs

Then load the matching reference:

• SwiftData → references/swiftdata.md
• CoreData → references/coredata.md

Step 2: Define the models

Ask: what entities are needed? For each entity:

• Name (singular, UpperCamelCase: Task, UserProfile, Transaction)
• Properties and their types
• Relationships to other entities (one-to-one, one-to-many, many-to-many)
• Any computed properties needed

Step 3: Generate the model layer

Models go in Core/Models/ — they are plain data types, framework-agnostic where possible.

Persistence infrastructure goes in Services/Storage/:

• PersistenceController.swift — container setup, in-memory preview/test variant
• [Entity]Repository.swift — CRUD operations for each entity
• [Entity]RepositoryProtocol.swift — protocol for testability

Rule: ViewModels inject a repository protocol. They never touch ModelContext or NSManagedObjectContext directly.

Services/Storage/
├── PersistenceController.swift
├── TaskRepository.swift
└── TaskRepositoryProtocol.swift

Step 4: Generate the repository protocol

Always define a protocol first so ViewModels can be tested with a mock:

// Services/Storage/TaskRepositoryProtocol.swift
protocol TaskRepositoryProtocol {
    func fetchAll() throws -> [Task]
    func save(_ task: Task) throws
    func delete(_ task: Task) throws
}

Step 5: Wire into the ViewModel

ViewModels receive the repository via init:

@MainActor
@Observable
final class TaskListViewModel {
    private(set) var tasks: [Task] = []
    private let repository: TaskRepositoryProtocol

    init(repository: TaskRepositoryProtocol = TaskRepository()) {
        self.repository = repository
    }

    func loadTasks() {
        tasks = (try? repository.fetchAll()) ?? []
    }
}

Step 6: Generate mock for tests

final class MockTaskRepository: TaskRepositoryProtocol {
    var stubbedTasks: [Task] = []
    var saveCallCount = 0
    var deleteCallCount = 0

    func fetchAll() throws -> [Task] { stubbedTasks }
    func save(_ task: Task) throws { saveCallCount += 1 }
    func delete(_ task: Task) throws { deleteCallCount += 1 }
}

Step 7: Quality checklist

• ModelContext / NSManagedObjectContext is never used in a ViewModel or View
• Repository protocol exists for every entity accessed by a ViewModel
• In-memory container/store exists for tests and previews
• Background context used for heavy writes (batch imports, migrations)
• Models in Core/Models/ have no direct framework import (SwiftData or CoreData) in the ViewModel layer
• Migrations are handled explicitly — no silent schema changes

Examples

Example 1: SwiftData task app

User says: "Add SwiftData to persist tasks with title, completion status, and due date"

Actions:

1. Load references/swiftdata.md
2. Generate Task model with @Model
3. Generate PersistenceController with .inMemory preview variant
4. Generate TaskRepository + TaskRepositoryProtocol
5. Update TaskListViewModel to inject repository
6. Generate MockTaskRepository for tests

Example 2: CoreData in existing UIKit app

User says: "I need to add CoreData to my existing UIKit project to cache user profiles"

Actions:

1. Load references/coredata.md
2. Generate UserProfile NSManagedObject subclass
3. Generate PersistenceController with in-memory option
4. Generate UserProfileRepository + protocol
5. Wire into existing ProfileViewModel

Troubleshooting

SwiftData: "Context is not available" — ModelContext is being accessed outside a SwiftData-aware environment. Ensure the ModelContainer is set up at the App level and injected via .modelContainer() modifier. Never create contexts manually in ViewModels.

CoreData: merge conflicts — You are writing on the main context from a background thread. Use performBackgroundTask or a dedicated background context. Always call context.save() on the correct thread.

Tests failing because model can't be found — The in-memory store is not configured. Use PersistenceController.preview (SwiftData) or an in-memory NSPersistentContainer in your test setup.