Migration Patterns Skill

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.

Agent Skills: Migration Patterns

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