Migrating to Swift 6

Turn on the Swift 6 language mode without stopping your team — and understand every error the compiler shows you.

Swift 6 makes data-race safety a compile-time guarantee. That is a big win, but for an existing app it often starts with hundreds of errors about Sendable, actor isolation, and the main actor. Most teams can't pause feature work for weeks to fix them, and many of the quick fixes found online silence the compiler without making the code any safer.

Migrating to Swift 6 shows you how to get there step by step. Every technical statement in this book was checked against its sources before it was written. Every one of its 117 Swift code examples was run through the Swift 6 compiler, including the examples that show an error, which are verified to fail with exactly the diagnostic described.

Across 15 chapters, you will learn how to:

Understand data-race safety and what the Swift 6 language mode actually checks
Make types Sendable and read sendability diagnostics with confidence
Protect state with actors, the main actor, and global actors
Use region-based isolation and sending to pass values safely without extra annotations
Structure concurrent work with async let, task groups, and cancellation, and know when unstructured and detached tasks are the right tool
Adopt Swift 6.2's approachable concurrency and default main-actor isolation
Bridge completion handlers, delegates, and Dispatch code with continuations and dynamic isolation
Consume Combine publishers from async code, and replace Futures and subjects with async functions and AsyncStream
Plan a module-by-module migration that keeps shipping
Use escape hatches — nonisolated(unsafe), @unchecked Sendable, and @preconcurrency — knowing exactly what each one turns off, and retire them afterwards
Reach for Mutex and Atomic when code can't be asynchronous
Test and debug concurrent code with Swift Testing, Thread Sanitizer, and Instruments

The book ends with a capstone that migrates a small SwiftUI app — a network client, an image cache, and legacy callback code — from Swift 5 to the Swift 6 language mode, one compiler diagnostic at a time.

Written for iOS and Swift engineers at the intermediate to senior level, this book replaces guesswork with changes you can explain and defend.