A deep dive into designing Swift protocols with concurrency in mind, focusing on the Swift 6 language mode with 'Approachable Concurrency' features enabled. Covers how default actor isolation settings (MainActor, nonisolated) profoundly affect protocol design, the common protocol conformance isolation mismatch error, and multiple strategies for resolving it: using nonisolated defaults, explicit nonisolated protocols, requiring Sendable conformance, and making protocol requirements asynchronous. Highlights that synchronous protocol requirements are inflexible across isolation boundaries, while async requirements open many more conformance possibilities. Emphasizes that there is no single right solution — the best approach depends on how conforming types will be used.
