Most existing Swift codebases were not written with async/await. They rely on completion handlers, nested callbacks, and GCD. Migrating legacy async code is unavoidable when moving to Swift 6.
This article focuses on strategies to migrate from completion handlers to async/await safely, incrementally, and in alignment with the new concurrency model.
Problems with traditional completion handlers
Completion handlers fragment execution flow, making code harder to follow and more prone to callback hell. Passing state through closures also increases the risk of data races.
In Swift 6, these issues become more visible when strict concurrency checking is enabled.
How async/await addresses these issues
Async/await transforms asynchronous execution into a linear flow that is easier for both developers and the compiler to reason about.
Structured concurrency ensures that task lifecycles are tightly controlled.
Incremental migration strategy
Rewriting an entire codebase at once is risky. An effective strategy is to wrap legacy APIs with async interfaces first.
Higher-level callers can then be gradually migrated to async/await.
Continuations and bridging legacy code
Swift provides withCheckedContinuation and withCheckedThrowingContinuation to safely bridge completion handlers into async/await.
The compiler can verify that continuations are resumed correctly, preventing subtle logic errors.
Swift 6 does not make migration heavier – it makes it disciplined
Migrating to async/await in Swift 6 may require upfront effort, but it results in clearer, more testable, and safer code.
A disciplined migration prepares the codebase for the future of Swift concurrency.
Conclusion
Migrating from completion handlers to async/await is essential to fully leverage Swift 6. A step-by-step approach leads to a successful transition.
