migration-patterns

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