Straight to the point: AsyncLock

Swift’s actors ensure isolation of shared resources through automatic synchronization, but AsyncLock serves a distinct purpose: providing explicit control in specific scenarios. This includes preserving legacy code (e.g., classes using NSOperation), coordinating granular locks (useful in complex workflows or nested locks), and enabling classes to remain shareable without actor confinement (using @unchecked Sendable).

While actors abstract low-level concurrency, AsyncLock complements them by allowing manual locking for complex flows (e.g., releasing resources only after multiple conditions) and compatibility with older patterns like NSOperation, which relies on explicit state management.

The AsyncOperation class, used internally by AsyncLock, can be shared across actors while controlling state transitions (idle → scheduled → cancelled). Actors alone cannot replace this pattern in cases requiring conditional unlocking (e.g., waiting for external events) or integration with legacy frameworks like OperationQueue. AsyncLock thus acts as a bridge between modern Swift concurrency and legacy systems demanding refined control.

• lock()/unlock() : Explicit access control, unlike actors’ implicit isolation.
• withLock/withLockVoid : Simplify critical sections with async/await syntax.

AsyncOperation manages states via an enum (idle, scheduled, cancelled), using locks to prevent race conditions during scheduling/resumption. The dispose() method ensures pending tasks are safely terminated, propagating cancellation errors when needed. This combination of state management and thread safety allows AsyncLock to coordinate asynchronous operations predictably, even in complex environments.

When to Choose AsyncLock Over Actors?

While actors excel for new code benefiting from automatic isolation, AsyncLock is better suited for legacy systems relying on patterns like NSOperation or requiring multifaceted coordination (e.g., multi-step workflows).

Actors simplify common concurrency cases, but AsyncLock offers flexibility for conditional/nested locking—something unfeasible with standard actor isolation. This duality lets developers maintain legacy code while gradually adopting modern Swift concurrency.

Conclusion

AsyncLock demonstrates how Swift’s concurrency tools can coexist with legacy patterns. By combining @unchecked Sendable in classes and manual locking with AsyncLock , it addresses scenarios in async/await contexts where actors fall short—such as integrating NSOperation-based systems or managing complex state transitions.

For more information, explore Apple’s official documentation on Concurrency and try implementing AsyncLock in your own projects.

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