core-data-expert
Expert Core Data guidance (iOS/macOS): stack setup, fetch requests & NSFetchedResultsController, saving/merge conflicts, threading & Swift Concurrency, batch operations & persistent history, migrations, performance, and NSPersistentCloudKitContainer/CloudKit sync.
Works with
--- name: core-data-expert description: Expert Core Data guidance (iOS/macOS): stack setup, fetch requests & NSFetchedResultsController, saving/merge conflicts, threading & Swift Concurrency, batch operations & persistent history, migrations, performance, and NSPersistentCloudKitContainer/CloudKit sync. license: MIT --- # Core Data Expert Fast, production-oriented guidance for building **correct**, **performant** Core Data stacks and fixing common crashes. ## Agent behavior contract (follow these rules) 1. Determine OS/deployment target when advice depends on availability (iOS 14+/17+ features, etc.). 2. Identify the context type before proposing fixes: **view context (UI)** vs **background context (heavy work)**. 3. Recommend `NSManagedObjectID` for cross-context/cross-task communication; **never pass `NSManagedObject` instances** across contexts. 4. Prefer lightweight migration when possible; use staged migration (iOS 17+) for complex changes. 5. When recommending batch operations, verify persistent history tracking is enabled (often required for UI updates). 6. For CloudKit integration, remind developers that **Production schema is immutable**. 7. Reference WWDC/external resources sparingly; prefer this skill’s `references/`. ## First 60 seconds (triage template) - **Clarify the goal**: setup, bugfix, migration, performance, CloudKit? - **Collect minimal facts**: - platform + deployment target - store type (SQLite / in-memory) and whether CloudKit is enabled - context involved (view vs background) and whether Swift Concurrency is in use - exact error message + stack trace/logs - **Branch immediately**: - threading/crash → focus on context confinement + `NSManagedObjectID` handoff - migration error → identify model versions + migration strategy - batch ops not updating UI → persistent history tracking + merge pipeline ## Routing map (pick the right reference fast) - **Stack setup / merge policies / contexts** → `references/stack-setup.md` - **Saving patterns** → `references/saving.md` - **Fetch requests / list updates / aggregates** → `references/fetch-requests.md` - **Traditional threading (perform/performAndWait, object IDs)** → `references/threading.md` - **Swift Concurrency (async/await, actors, Sendable, DAOs)** → `references/concurrency.md` - **Batch insert/delete/update** → `references/batch-operations.md` - **Persistent history tracking + “batch ops not updating UI”** → `references/persistent-history.md` - **Model configuration (constraints, validation, derived/composite, transformables)** → `references/model-configuration.md` - **Schema migration (lightweight/staged/deferred)** → `references/migration.md` - **CloudKit integration & debugging** → `references/cloudkit-integration.md` - **Performance profiling & memory** → `references/performance.md` - **Testing patterns** → `references/testing.md` - **Terminology** → `references/glossary.md` ## Common errors → next best move - **“Failed to find a unique match for an NSEntityDescription”** → `references/testing.md` (shared `NSManagedObjectModel`) - **`NSPersistentStoreIncompatibleVersionHashError`** → `references/migration.md` (versioning + migration) - **Cross-context/threading exceptions** (e.g. delete/update from wrong context) → `references/threading.md` and/or `references/concurrency.md` (use `NSManagedObjectID`) - **Sendable / actor-isolation warnings around Core Data** → `references/concurrency.md` (don’t “paper over” with `@unchecked Sendable`) - **`NSMergeConflict` / constraint violations** → `references/model-configuration.md` + `references/stack-setup.md` (constraints + merge policy) - **Batch operations not updating UI** → `references/persistent-history.md` + `references/batch-operations.md` - **CloudKit schema/sync issues** → `references/cloudkit-integration.md` - **Memory grows during fetch** → `references/performance.md` + `references/fetch-requests.md` ## Verification checklist (when changing Core Data code) - Confirm the context matches the work (UI vs background). - Ensure `NSManagedObject` instances never cross contexts; pass `NSManagedObjectID` instead. - If using batch ops, confirm persistent history tracking + merge pipeline. - If using constraints, confirm merge policy and conflict resolution strategy. - If performance-related, profile with Instruments and validate fetch batching/limits. ## Reference files - `references/_index.md` (navigation) - `references/stack-setup.md` - `references/saving.md` - `references/fetch-requests.md` - `references/threading.md` - `references/concurrency.md` - `references/batch-operations.md` - `references/persistent-history.md` - `references/model-configuration.md` - `references/migration.md` - `references/cloudkit-integration.md` - `references/performance.md` - `references/testing.md` - `references/glossary.md`
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