Bridging Callback APIs to async/await

Thu, 28 May 2026 00:00:00 +0000

New Swift projects are utilising Structured Concurrency. Long-living codebases on the other hand are full of older callback-based APIs that hand you an Operation and call you back with a Result. The gap between those two worlds looks trivial to close — until you think about cancellation.

The problem

Suppose you have a legacy API shaped like this:

// Hands back an `Operation` you can cancel, and calls `completion` when done.
func fetch(
 argument: String,
 completion: @escaping (Result<String, Error>) -> Void
) -> Operation

You want to call it like this instead:

Concurrency on blishtan

Bridging Callback APIs to async/await

The problem