migration-patterns_skill

Overview

This skill provides practical migration guides for moving between Apple framework generations: CoreData to SwiftData, UIKit to SwiftUI, ObservableObject to @Observable, and XCTest to Swift Testing. It focuses on concrete before/after mappings, coexistence patterns, and common pitfalls so teams can plan safe, incremental migrations. Use it to assess risk, pick a strategy, and get migration-ready checklists.

How this skill works

Select the migration type and follow the decision tree that maps legacy APIs to modern equivalents. Each guide details minimum OS/Xcode requirements, coexistence strategies, and code-level patterns (e.g., NSManagedObject -> @Model, UIViewController -> UIHostingController/Representable). The skill inspects surface area (models, screens, tests), flags risky areas, and recommends full, incremental, or no-migration strategies.

When to use it

• You’re upgrading a data layer from CoreData to SwiftData
• You want to adopt SwiftUI but retain parts of UIKit
• You’re moving from ObservableObject/Combine to @Observable/AsyncSequence
• You need to convert XCTest suites to Swift Testing
• You must run old and new frameworks side-by-side during rollout
• You’re deciding if the migration ROI justifies the effort

Best practices

• Migrate tests first to provide a safety net for app migrations
• Perform one migration per PR to keep reviews focused and revertible
• Keep both old and new code compiling during incremental adoption
• Feature-flag large changes so you can rollback without a revert
• Write migration tests for data-layer changes to verify roundtrips

Example use cases

• Convert a single CoreData entity to @Model and validate coexistence with the remaining NSManagedObjects
• Wrap an existing UIViewController with UIHostingController and migrate screens incrementally
• Replace ObservableObject-based models with @Observable and switch Combine publishers to AsyncSequence stepwise
• Port XCTest assertions to Swift Testing on Xcode 16 while keeping legacy tests running
• Assess whether a small app should stay on CoreData because of missing SwiftData features