ios-concurrency

Replace serial DispatchQueues protecting state with actors, convert completion handlers to async throws functions using withCheckedThrowingContinuation, and replace DispatchGroup with async let or TaskGroup. Migrate leaf operations first, then their callers.

Actor reentrancy means another task can run on the same actor while a suspended task waits at an await, so state can change across suspension points. Fix it by completing state mutations before awaiting or re-checking invariants after each await.

For new code on iOS 15+, prefer Swift Concurrency with async/await. Keep Combine for existing pipelines, complex operator chains like debounce and zip, and SwiftUI's @Published bindings, and bridge between them using publisher.values.

Calling sync on a serial queue from within that same queue blocks the queue waiting for itself, causing a deadlock. The same happens when calling sync on the main queue from the main thread, or when two queues synchronously wait on each other.

Enable Thread Sanitizer in Xcode's scheme diagnostics to catch races at runtime, and turn on strict concurrency checking in build settings to surface Sendable and isolation violations at compile time. Swift 6 makes data races compile errors by default.

Use OperationQueue when you need dependency graphs between operations, KVO-observable state, or integration with existing NSOperation-based code. For new code, TaskGroup with structured concurrency is preferred for parallel work with cancellation propagation.