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Core Data Persistence

Core Data is Apple's mature persistence framework, available on all iOS versions. While SwiftData (iOS 17+) is simpler, Core Data remains necessary for apps supporting iOS 16 and below. This entry covers the full Core Data stack with SwiftUI integration.

Key Facts

  • Core Data uses NSManagedObject subclasses auto-generated from .xcdatamodeld files
  • @FetchRequest in views replaces manual fetch calls - updates automatically when data changes
  • NSPredicate handles filtering with format strings (case-insensitive, compound predicates)
  • viewContext.save() must be called explicitly after every change
  • In-memory store (/dev/null URL) is used for previews
  • Extensions on NSManagedObject add computed properties without modifying auto-generated code
  • PersistenceController singleton pattern manages the Core Data stack

Patterns

Project Setup

  1. New project > Check "Use Core Data"
  2. Generated: Persistence.swift + YourApp.xcdatamodeld
  3. In .xcdatamodeld: add Entity, add Attributes (name, type)
  4. Xcode auto-generates NSManagedObject subclasses on build

Core Data Stack (Persistence.swift)

struct PersistenceController {
    static let shared = PersistenceController()

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        let samplePokemon = Pokemon(context: viewContext)
        samplePokemon.id = 1
        samplePokemon.name = "bulbasaur"
        try? viewContext.save()
        return result
    }()

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "Pokedex")
        if inMemory {
            container.persistentStoreDescriptions.first!.url =
                URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores { _, error in
            if let error { fatalError("Core Data error: \(error)") }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
    }
}

App Entry Point

@main
struct PokedexApp: App {
    let persistenceController = PersistenceController.shared

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext,
                    persistenceController.container.viewContext)
        }
    }
}

@FetchRequest in Views

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest<Pokemon>(
        sortDescriptors: [SortDescriptor(\Pokemon.id)],
        animation: .default
    ) private var pokedex: FetchedResults<Pokemon>

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

@FetchRequest with Static Predicate

@FetchRequest<Pokemon>(
    sortDescriptors: [SortDescriptor(\Pokemon.id)],
    predicate: NSPredicate(format: "favorite == true")
) private var favorites: FetchedResults<Pokemon>

Dynamic Predicate (onChange Pattern)

@FetchRequest<Pokemon>(sortDescriptors: [SortDescriptor(\Pokemon.id)])
private var pokedex: FetchedResults<Pokemon>

private var dynamicPredicate: NSPredicate {
    var predicates: [NSPredicate] = []

    if !searchText.isEmpty {
        predicates.append(
            NSPredicate(format: "name CONTAINS[c] %@", searchText)
        )
    }
    if filterByFavorites {
        predicates.append(
            NSPredicate(format: "favorite == %d", true)
        )
    }

    return NSCompoundPredicate(andPredicateWithSubpredicates: predicates)
}

var body: some View {
    List(pokedex) { ... }
    .searchable(text: $searchText)
    .onChange(of: searchText) { pokedex.nsPredicate = dynamicPredicate }
    .onChange(of: filterByFavorites) { pokedex.nsPredicate = dynamicPredicate }
}

pokedex.nsPredicate = newPredicate updates the fetch request live.

NSPredicate Format Strings

Format Purpose
"name CONTAINS[c] %@" Case-insensitive contains (string)
"favorite == %d" Boolean/int comparison
"id == %d" Exact match
NSCompoundPredicate(andPredicateWithSubpredicates:) Combine multiple

CRUD Operations

// Create
let newPokemon = Pokemon(context: viewContext)
newPokemon.id = 1
newPokemon.name = "bulbasaur"

// Save (required after every change)
do {
    try viewContext.save()
} catch {
    print("Save error: \(error)")
}

// Update
pokemon.favorite.toggle()
try? viewContext.save()

// Delete
viewContext.delete(pokemon)
try? viewContext.save()

Fetching a Single Object (for Previews)

func fetchPokemon(id: Int, context: NSManagedObjectContext) -> Pokemon {
    let fetchRequest: NSFetchRequest<Pokemon> = Pokemon.fetchRequest()
    fetchRequest.fetchLimit = 1
    fetchRequest.predicate = NSPredicate(format: "id == %d", id)
    let results = try! context.fetch(fetchRequest)
    return results.first!
}

Extending NSManagedObject Subclasses

extension Pokemon {
    var background: ImageResource {
        switch types?.first ?? "" {
        case "fire", "dragon": return .fireDragon
        case "water": return .water
        default: return .normalGrassElectricPoisonFairy
        }
    }

    var typeColor: Color {
        Color((types?.first ?? "normal").capitalized)
    }

    var stats: [Stat] {
        [
            Stat(id: 1, name: "HP", value: hp),
            Stat(id: 2, name: "Attack", value: attack),
            // ...
        ]
    }
}

Rule: no stored properties in extensions - only computed properties and functions.

Swift Charts with Core Data

import Charts

Chart(pokemon.stats) { stat in
    BarMark(
        x: .value("Value", stat.value),
        y: .value("Stat", stat.name)
    )
    .annotation(position: .trailing) {
        Text("\(stat.value)")
    }
}
.frame(height: 200)
.foregroundStyle(pokemon.typeColor)
.chartXScale(domain: 0...(pokemon.highestStat.value + 10))

List(pokedex) { pokemon in
    NavigationLink(value: pokemon) {
        PokemonRow(pokemon: pokemon)
    }
}
.navigationDestination(for: Pokemon.self) { pokemon in
    PokemonDetail(pokemon: pokemon)
}

Gotchas

  • Core Data changes are NOT persisted until viewContext.save() is called - forgetting this loses data
  • Auto-generated NSManagedObject properties are optionals (name: String?) - handle with nil coalescing
  • fallbackToDestructiveMigration() deletes all data on schema change - for production, use addMigrations() with ALTER TABLE
  • automaticallyMergesChangesFromParent = true is needed for background context changes to appear in UI
  • NSPredicate format strings are not type-safe - runtime crashes on format errors
  • When fetching from API and storing in Core Data, check for duplicates before inserting

See Also