write-swift

Work down a prioritized list: first stop sharing the object by giving each concurrent job its own instance, then make it a Sendable value type, then isolate it to an actor, and only as a last resort use Mutex, Atomics, or @unchecked Sendable. Most real errors are fixed by the first step.

In Swift 6.2, marking a function async no longer moves it off the current actor — it runs where it was called. Use @concurrent to move CPU-heavy work to the background thread pool and nonisolated for library APIs so callers decide where work runs.

Use an actor only to move state off the main actor when too much main-actor state forces constant task hopping. Start single-threaded, add async/await for latency, then @concurrent for CPU work, and actors last — actors guarantee mutual exclusion, not transactions or FIFO ordering.

Write some P by default since it preserves associated-type relationships and lets the compiler specialize. Switch to any P only when you need heterogeneous storage or optionality of the underlying type, accepting type erasure and opaque dispatch costs.

Use Swift Testing for all new tests — it supports @Test on any function, #expect with ordinary expressions, parameterized arguments, and parallel execution by default. XCTest remains required only for UI automation, performance metrics, and Objective-C exception tests.

Retain cycles come from strong reference loops between objects. Prefer restructuring over weak references: factor shared data into a third type both sides reference, turning the cycle into a tree, since weak reads after the owner's last use can silently return nil.