Persistence and app data

Persistence means data survives after the app closes. Choose storage based on what the data means and how it will be queried.

UserDefaults

UserDefaults stores small preferences:

UserDefaults.standard.set(true, forKey: "notificationsEnabled")

let enabled = UserDefaults.standard.bool(forKey: "notificationsEnabled")

Use it for settings, flags, simple preferences, and lightweight values. Do not use it as a database.

Wrap keys so strings do not spread through the app:

enum DefaultsKey {
    static let notificationsEnabled = "notificationsEnabled"
}

Files

Files are useful for documents, cached JSON, exports, and user-created content:

let data = try JSONEncoder().encode(draft)
try data.write(to: fileURL)

Reading and writing files can throw, so handle errors. File storage is explicit and flexible, but you must design paths, formats, and migration carefully.

Databases

Use a database when data is structured, related, searchable, or large. On Apple platforms, common choices include SwiftData, Core Data, SQLite-based libraries, and cloud-backed storage.

This course does not go deep into databases because database choice depends heavily on app needs. The modeling habits still apply: define clear types, validate external data, and keep persistence details out of views.

Cache vs source of truth

A cache stores data you can recreate or fetch again. A source of truth stores data the app relies on as authoritative.

Treat them differently. It is safe to delete a cache. It is not safe to silently delete a user’s saved work.

What to carry forward

• persistence keeps data beyond app lifetime
• UserDefaults is for small preferences
• file storage works for explicit documents and cached data
• databases fit structured, queryable, or larger data
• separate persistence code from views
• know whether data is cache or source of truth

Next, you will bring the course ideas together in a small app.