Swift Concurrency Expert Guide

Swift 6 migration requires adopting the Sendable protocol and actor isolation to resolve data race safety issues. This guide provides architectural strategies to refactor legacy applications into modern, safe concurrent systems incrementally.

Cooperative multitasking in Swift relies on tasks voluntarily yielding execution time to the cooperative thread pool, unlike preemptive multitasking where the system interrupts threads. This model reduces context switching overhead and thread explosion.

Actors isolate mutable state to prevent concurrent access, while the Sendable protocol guarantees types can be safely passed across concurrency boundaries. Together they form the architectural foundation for race-free Swift applications.

CheckedContinuation bridges callback-based APIs into async/await state machines, while OSAllocatedUnfairLock provides low-level synchronization primitives. Both are supported for advanced synchronization and integrating legacy locking mechanisms with structured concurrency.

Structured concurrency using TaskGroups ensures child tasks are scoped and automatically canceled if the parent fails, creating predictable execution graphs. Unstructured tasks are suitable for detached fire-and-forget operations where lifecycle management is manual.

Actor reentrancy allows an actor to process new requests while awaiting an asynchronous operation, which can lead to intermediate state inconsistencies if not carefully managed. Developers must design actor methods to be logically independent or use synchronous execution paths.