Migration Patterns Skill for Claude

Migration Patterns

Comprehensive guides for migrating between Apple framework generations. Each guide covers the full before/after mapping, coexistence strategies, and common pitfalls.

When This Skill Activates

• User asks how to migrate from CoreData to SwiftData
• User is moving UIKit code to SwiftUI (or embedding one in the other)
• User needs to update from ObservableObject/Combine to @Observable/AsyncSequence
• User is converting XCTest tests to Swift Testing
• User asks about coexistence strategies (running old and new frameworks side by side)
• User wants to know whether a migration is worth doing for their situation
• User is migrating Objective-C code to Swift (bridging headers, incremental migration)
• User is migrating from StoreKit 1 to StoreKit 2 (in-app purchases, subscriptions)
• User encounters errors during a framework migration

Decision Tree

What are you migrating?
|
+-- Data persistence layer
|   +-- CoreData --> SwiftData
|   |   See coredata-to-swiftdata.md
|   |   Min: iOS 17 / macOS 14
|   |
|   +-- Still need CoreData features SwiftData lacks?
|       Stay on CoreData or use coexistence mode
|
+-- UI framework
|   +-- UIKit --> SwiftUI
|   |   See uikit-to-swiftui.md
|   |   Min: iOS 13 (basic), iOS 16+ (modern navigation)
|   |
|   +-- Full rewrite or incremental?
|       Incremental is almost always better -- adopt screen by screen
|
+-- State management / observation
|   +-- ObservableObject --> @Observable
|   |   See observable-migration.md
|   |   Min: iOS 17 / macOS 14
|   |
|   +-- Combine publishers --> AsyncSequence
|       Also covered in observable-migration.md
|
+-- Programming language
|   +-- Objective-C --> Swift
|   |   See objc-to-swift.md
|   |   Incremental: migrate leaves first, trunks last
|   |
|   +-- Mixed-language project?
|       Both languages coexist via bridging headers
|
+-- In-app purchases
|   +-- StoreKit 1 --> StoreKit 2
|   |   See storekit-migration.md
|   |   Min: iOS 15
|   |
|   +-- Using a third-party SDK (RevenueCat, etc.)?
|       Check if SDK already supports StoreKit 2 internally
|
+-- Testing framework
    +-- XCTest --> Swift Testing
        See xctest-to-swift-testing.md
        Min: Xcode 16 / Swift 6.0

Quick Reference

| Migration | Reference File | Minimum OS | Risk Level |

|-----------|---------------|------------|------------|

| CoreData to SwiftData | coredata-to-swiftdata.md | iOS 17 / macOS 14 | High (data layer) |

| UIKit to SwiftUI | uikit-to-swiftui.md | iOS 13+ | Medium (incremental) |

| ObservableObject to @Observable | observable-migration.md | iOS 17 / macOS 14 | Low-Medium |

| Objective-C to Swift | objc-to-swift.md | Any | Medium (incremental) |

| StoreKit 1 to StoreKit 2 | storekit-migration.md | iOS 15 | Medium-High (payments) |

| XCTest to Swift Testing | xctest-to-swift-testing.md | Xcode 16 | Low |

Process

1. Assess the Migration

Before starting, determine:

• What is the minimum deployment target? Many migrations require iOS 17+.
• How large is the surface area? (number of models, screens, test files)
• Can you adopt incrementally or is it all-or-nothing?
• Are there third-party dependencies that assume the old framework?

2. Load Relevant Reference File

Based on the migration type, read from this directory:

• coredata-to-swiftdata.md -- NSManagedObject to @Model, migration stages, coexistence
• uikit-to-swiftui.md -- UIHostingController, Representable, incremental adoption
• observable-migration.md -- @Observable macro, @Environment injection, AsyncSequence
• objc-to-swift.md -- Bridging headers, @objc, incremental file-by-file migration
• storekit-migration.md -- StoreKit 2 async/await purchases, Transaction.currentEntitlements, JWS
• xctest-to-swift-testing.md -- @Test, #expect, #require, parameterized tests

3. Review the User's Code

Scan for old-framework patterns and map each to its modern equivalent using the reference file. Check for:

• [ ] Deprecated API usage that has a direct modern replacement
• [ ] Custom workarounds that are no longer needed with the new framework
• [ ] Third-party dependencies that may conflict with the migration
• [ ] Coexistence requirements (both frameworks running simultaneously)

4. Recommend a Migration Strategy

For each migration, decide between:

• Full migration: Replace all old-framework code at once. Best for small codebases or when old framework is causing problems.
• Incremental migration: Migrate piece by piece while both frameworks coexist. Best for large codebases, production apps, or when timeline is flexible.
• No migration: The old framework is still appropriate. See "When NOT to Migrate" in each reference file.

General Migration Principles

• Migrate tests first. If you are migrating both app code and tests, migrate tests to Swift Testing first. This gives you a safety net for the app-code migration.

• One migration at a time. Do not migrate CoreData to SwiftData and ObservableObject to @Observable in the same PR. Each migration should be independently reviewable and revertible.

• Keep the old code compiling. During incremental migration, both old and new code must compile and run. Use coexistence patterns from each reference file.

• Feature-flag large migrations. For production apps, consider gating new-framework code behind a feature flag so you can roll back without a code revert.

• Write migration tests. For data-layer migrations (CoreData to SwiftData), write tests that verify data roundtrips correctly through both stacks.

References

• Migrating from CoreData to SwiftData
• Migrating to new navigation types
• Migrating from ObservableObject to Observable
• Migrating a test from XCTest
• Migrating an app from StoreKit 1