core-data

core-data is a focused engineering reference for building, reviewing, and improving Core Data persistence in iOS apps that have not adopted SwiftData. It addresses the recurring difficulty of getting Core Data right — correct stack setup, safe concurrency, efficient list UIs, and non-trivial migrations — by consolidating current best practice with runnable Swift examples and links to Apple documentation. It explicitly scopes itself to Core Data and routes Core Data + SwiftData coexistence or migration questions to a separate swiftdata skill.

The SKILL.md covers stack setup with NSPersistentContainer (and NSPersistentCloudKitContainer for CloudKit sync), concurrency and threading rules (using perform/performAndWait, never passing NSManagedObject across boundaries, passing NSManagedObjectID instead), Swift Concurrency integration via the async perform overload, and NSFetchedResultsController driving table/collection views with diffable snapshots. Additional sections and reference files address batch operations (NSBatchInsert/Delete/UpdateRequest), persistent history tracking, staged migration (NSStagedMigrationManager, iOS 17+), composite attributes (iOS 17+), testing, common mistakes, and a review checklist. Bundled references include staged-migration.md and persistent-history.md, plus an evals file.

It is aimed at iOS/Swift developers maintaining or extending established Core Data codebases, and at reviewers checking persistence code for correctness. Use cases include setting up a fresh stack, integrating Core Data with Swift Concurrency, wiring list-driven UI with change tracking, performing bulk writes, and executing staged schema migrations on iOS 17+.