swift-concurrency

Swift Concurrency

Enterprise-grade skill for Swift Concurrency crash prevention, strict concurrency migration, and Swift 6.2 readiness. Opinionated: prescribes structured concurrency over unstructured, actors for shared mutable state, @MainActor for UI isolation, Mutex for synchronous critical sections, compile-time isolation over runtime hops, and withCheckedContinuation over withUnsafeContinuation. Every rule in this skill has broken a real production app.

Concurrency Layers

Application Layer    -> Structured concurrency: TaskGroup, async let, .task modifier
Actor Layer          -> actor for shared mutable state, @MainActor for UI state
Async/Await Layer    -> Cooperative, non-blocking async functions on the cooperative pool
Sendable Layer       -> Compile-time data-race safety at isolation boundary crossings
Compiler Layer       -> SWIFT_STRICT_CONCURRENCY=complete, Swift 6 language mode, TSan

Quick Decision Trees

"What isolation does this type need?"

Does this type own mutable state shared across concurrency domains?
+-- YES -> Does it need to update UI?
|   +-- YES -> @MainActor class/struct
|   +-- NO  -> Is the state complex or involves await points?
|       +-- YES -> actor
|       +-- NO  -> Mutex<State> (Swift 6+, iOS 18+) or NSLock wrapper
+-- NO  -> Is it a value type with only Sendable stored properties?
    +-- YES -> Implicitly Sendable (internal types only; public needs explicit conformance)
    +-- NO  -> Can it be redesigned as a value type?
        +-- YES -> Refactor to struct/enum
        +-- NO  -> Keep non-Sendable, contain within single isolation domain

"async let vs TaskGroup vs Task {}?"

How many concurrent operations?
+-- Known at compile time (2-5) -> async let (simplest, heterogeneous return types OK)
+-- Dynamic count (N items)     -> TaskGroup
|   +-- Need results?
|   |   +-- YES -> withThrowingTaskGroup (iterate with for await)
|   |   +-- NO  -> withDiscardingTaskGroup (no result accumulation leak)
|   +-- Unbounded input? -> Throttle: limit addTask to activeProcessorCount
+-- Fire-and-forget from sync context?
    +-- Need caller's isolation -> Task { } (inherits actor context)
    +-- Need background, no isolation -> Task.detached { }
       WARNING: detached strips priority, task-locals, cancellation propagation. Rarely correct.