I am refactoring a legacy iOS app and I want to move away from deeply nested completion handlers (callback hell) to the new Swift Concurrency model. What is the safest way to wrap existing escaping closures into async functions? I am specifically looking for examples using 'withCheckedContinuation' and how to handle potential thread-safety issues during the transition.
3 answers
Switching to async/await is a game changer for readability. To wrap old code, you use withCheckedContinuation which suspends the current task until the closure calls the resume method. One major thing I learned in 2023 while updating a social media app was that you must ensure the continuation is called exactly once. If you call it twice, your app crashes; if you never call it, the task leaks and stays suspended forever. Also, remember to mark your UI-updating classes with @MainActor to ensure that even though the background work is concurrent, the final display happens on the main thread.
Have you noticed any significant performance improvements in terms of CPU usage since moving to the cooperative thread pool in Swift Concurrency compared to DispatchQueues?
Start by converting your lowest-level network calls first. Once those are async, you can propagate the changes upward through your ViewModel and ViewController layers quite easily.
That bottom-up approach is exactly what we did. It prevents you from having to use 'Task { }' blocks everywhere in your UI code right at the start of the migration process.
Steven, the main benefit isn't necessarily raw speed, but rather efficiency. DispatchQueues can lead to "thread explosion" where the system creates hundreds of threads, causing context-switching overhead. Swift's cooperative pool limits the number of threads to the number of CPU cores. In our tests, this led to much smoother scrolling in our app because the system wasn't struggling with thread management while the user was interacting with the UI.