Programming Tutorials

  You’ve migrated from Core Data to SwiftData, enticed by the promise of pure Swift syntax and macro magic. The preview works perfectly. But the moment you introduce a background   Task   to fetch API data or perform a batch update, your app terminates with a cryptic   EXC_BAD_ACCESS   or a "context invalid" error. This is the most common hurdle in modern iOS development:  SwiftData concurrency violations. While SwiftData abstracts away the  NSManagedObjectContext  boilerplate, it does not remove the underlying threading rules of the persistence engine. This guide dissects why these crashes occur, explains the strict thread confinement rules, and provides a robust  ModelActor  pattern to fix them permanently. The Root Cause: Thread Confinement & The Abstraction Leak To debug this, you must understand what SwiftData is hiding. Under the hood, SwiftData is still powered by Core Data. A  ModelContext  wraps an  NSManagedO...

  You are likely reading this because your iOS app is crashing on launch. You modified a   @Model   class—perhaps you renamed a property, changed a data type, or added a non-optional field—and now Xcode is throwing a   fatalError   inside the SwiftData container initialization. Common error signatures include: "The model used to open the store is incompatible with the one used to create the store" NSMigrationMissingSourceModelError Cannot use staged migration This crash occurs because the shape of your data in code (the Model) no longer matches the shape of the data stored on the device (the SQLite schema). SwiftData attempts "lightweight" migrations automatically, but when changes are ambiguous or complex, it fails to prevent data corruption. This guide provides a production-grade implementation of  VersionedSchema  and  SchemaMigrationPlan  to resolve these crashes and handle data evolution safely. The Root Cause: Why SwiftData Crashes Unde...