Swift 6 Concurrency Migration Notes from a Real Codebase

Concurrency correctness is one of the most expensive forms of technical debt in iOS apps.

My migration focus this quarter has been reducing hidden race-risk before it turns into production instability.

What I prioritized first

• Explicit actor ownership for mutable shared state.
• Sendable conformance cleanup for core models.
• Cancellation-aware async APIs for user-triggered flows.

Things that produced immediate clarity

1. Defining ownership boundaries at module level.
2. Isolating side effects from pure transformations.
3. Avoiding convenience async wrappers that hide cancellation semantics.

Practical review checklist I now use

• Can this value cross concurrency boundaries safely?
• Is cancellation handled and observable?
• Are retries idempotent?
• Are UI updates guaranteed on main actor boundaries?

Migration work is rarely glamorous, but this has already reduced ambiguity in code review and improved confidence in releases.

The largest gain has been shared understanding across the team, not just compiler silence.