Navigating Swift’s Continuations: Safety vs. Control
In this post, I want to talk about the differences between withCheckedContinuation and withUnsafeContinuation.
Swift’s concurrency features offer a powerful way to write asynchronous code. A key concept in this model is continuations, which allow you to pause a function’s execution and resume it later with a result. But with two options available, withCheckedContinuation and withUnsafeContinuation, programmers might be unsure which to choose.
What is Continuations?
Imagine you’re writing a function that fetches data from a server. Traditionally, this might involve a callback function that gets executed once the data is retrieved. Continuations provide a more structured way to handle this. With withCheckedContinuation or withUnsafeContinuation, you create a closure that acts like a placeholder for the final result. Inside this closure, you can perform your asynchronous operation and then use the continuation to signal completion.
What is withCheckedContinuation?
This function is your go-to choice for safety and clarity. It enforces a contract: you must call the continuation exactly once, either with the successful result or an error. This prevents common mistakes like forgetting to resume the suspended function or resuming it multiple times. The compiler also performs checks to ensure you’re using the continuation correctly, adding an extra layer of confidence in your code.
What is withUnsafeContinuation?
While withCheckedContinuation provides safety, withUnsafeContinuation offers more control. It bypasses the compiler checks, allowing you to call the continuation multiple times or not at all. This flexibility can be beneficial for advanced use cases, but it comes with a caveat: you're responsible for ensuring proper continuation usage. Forgetting to resume the function or resuming it incorrectly can lead to bugs and crashes.
Use Cases
- Bridging Async and Existing Code: When working with asynchronous functions that don’t natively support
async/await,withCheckedContinuationallows you to wrap them in anasyncfunction, making them feel more synchronous. - Writing Clear and Maintainable Code: The enforced usage pattern of
withCheckedContinuationleads to cleaner and easier-to-understand code, especially when dealing with complex asynchronous operations.
Choosing the Right Tool
When in doubt, prefer withCheckedContinuation. Its safety checks make it ideal for most scenarios. However, if you have a specific need for more control and are confident in your ability to manage the continuation correctly, withUnsafeContinuation might be a suitable choice. Remember, with great power comes great responsibility!
Conclusion
Both withCheckedContinuation and withUnsafeContinuation are valuable tools in Swift's concurrency arsenal. By understanding their strengths and weaknesses, you can write robust and efficient asynchronous code. For most developers, withCheckedContinuation will be the safe and recommended option. But if you need the extra control and understand the risks involved, withUnsafeContinuation can be a powerful tool in your belt.
