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 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
|
+-- Testing framework
    +-- XCTest --> Swift Testing
        See xctest-to-swift-testing.md
        Min: Xcode 16 / Swift 6.0

Quick Reference

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
• 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

1. 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.

2. 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.

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

4. 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.

5. 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