withCheckedThrowingContinuation in Swift

Swift’s concurrency model includes various tools to handle asynchronous operations more efficiently and intuitively. One such tool is the withCheckedThrowingContinuation function. This post will explain what withCheckedThrowingContinuation is, how it works, and provide examples to help you understand how to use it in your Swift projects.

1. What is withCheckedThrowingContinuation?

withCheckedThrowingContinuation is a function used to convert callback-based asynchronous code into code that uses Swift’s async and await syntax. This function ensures that the conversion is safe and correctly managed, including error handling.

2. How to Use withCheckedThrowingContinuation

To use withCheckedThrowingContinuation, you need to wrap your callback-based asynchronous function inside it. Here’s a basic example:

func fetchUserData(completion: @escaping (Result<String, Error>) -> Void) {
    // Simulate a delay
    DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
        if Bool.random() {
            completion(.success("Data fetched successfully"))
        } else {
            completion(.failure(NSError(domain: "", code: -1, userInfo: [NSLocalizedDescriptionKey: "Failed to fetch data"])))
        }
    }
}

func fetchUserData() async throws -> String {
    return try await withCheckedThrowingContinuation { continuation in
        fetchUserData { result in
            switch result {
            case .success(let data):
                continuation.resume(returning: data)
            case .failure(let error):
                continuation.resume(throwing: error)
            }
        }
    }
}        

In this example, fetchUserData is a callback-based function that simulates a network request. The fetchUserData async function wraps this callback-based function using withCheckedThrowingContinuation.

3. Real-World Example

Let’s look at a real-world scenario where you might use withCheckedThrowingContinuation to convert a legacy API into an async function.

import Foundation

// Legacy API
func loadUserData(completion: @escaping (Result<String, Error>) -> Void) {
    DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
        let success = Bool.random()
        if success {
            completion(.success("User data loaded"))
        } else {
            completion(.failure(NSError(domain: "com.example.error", code: 1, userInfo: [NSLocalizedDescriptionKey: "Failed to load user data"])))
        }
    }
}

// Async Function
func loadUserData() async throws -> String {
    return try await withCheckedThrowingContinuation { continuation in
        loadUserData { result in
            switch result {
            case .success(let data):
                continuation.resume(returning: data)
            case .failure(let error):
                continuation.resume(throwing: error)
            }
        }
    }
}

// Usage Example within an async context
func exampleUsage() async {
    do {
        let userData = try await loadUserData()
        print("Loaded data: \(userData)")
    } catch {
        print("Error: \(error)")
    }
}

// Calling the async function within an async context
func runExample() {
    Task {
        await exampleUsage()
    }
}

// Start the example
runExample()        

In this example:

• loadUserData(completion:) is the legacy API that uses a completion handler.
• loadUserData() is the new async function that wraps the legacy API using withCheckedThrowingContinuation.
• exampleUsage demonstrates calling the async function and handling its result or error within an async context.
• runExample starts the example by calling exampleUsage inside a Task to launch the async function.

4. Key Points to Remember

• Safety Checks: withCheckedThrowingContinuation includes safety checks to ensure the continuation is resumed exactly once, helping you avoid common pitfalls in asynchronous programming.
• Error Handling: This function makes it straightforward to handle errors in a way that integrates seamlessly with Swift’s async/await error-handling model.

5. Conclusion

withCheckedThrowingContinuation is a powerful tool for converting callback-based asynchronous code into Swift’s async/await syntax, making your code cleaner and more readable. By wrapping legacy APIs or complex asynchronous operations, you can leverage the benefits of Swift’s modern concurrency features while ensuring safety and correctness.

By understanding and using withCheckedThrowingContinuation, you can smoothly transition from callback-based asynchronous code to Swift’s async/await model, enhancing the readability and maintainability of your code.

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