how to properly use async func inside a completion-based method?

To properly use an async function inside a completion-based method in Swift, you can take advantage of Swift's concurrency features. If the code you're working with uses a completion handler and an async overload is not available, you can convert it to an expression that can be awaited using withCheckedContinuation or withCheckedThrowingContinuation. This allows you to bridge the gap between completion handlers and async/await syntax.

Here's a brief overview of how you can achieve this:

Check for Async Overloads: First, see if Swift provides an async overload for the completion handler-based method. If it does, you can directly use the async version.
Use Continuations: If no async overload is available, use withCheckedContinuation or withCheckedThrowingContinuation to wrap the completion handler. This will allow you to await the result of the asynchronous operation.
Handle Multiple Callbacks: If the callback may fire more than once, such as in an event handler, ensure that your code safely handles concurrency. Swift 6 introduces stricter concurrency checks to prevent data races, so be mindful of how you manage shared state.

For more detailed information, you can refer to the session Go further with Swift Testing (24:51) which discusses handling asynchronous conditions in testing, and the session A Swift Tour: Explore Swift’s features and design (18:33) which covers concurrency basics in Swift.