Making Old Async Code Work with Swift's New Async/Await Feature

A continuation in Swift is a bridge that lets you use older async code easily with the new async/await feature. You do this using withCheckedContinuation and withCheckedThrowingContinuation functions to make the old code fit smoothly into new async workflows.

• withCheckedContinuation is used when the asynchronous operation does not throw errors.
• withCheckedThrowingContinuation is used for asynchronous operations that can throw errors.

Here’s a basic example of how you might use withCheckedContinuation in a scenario where you have an asynchronous function that fetches data from a network resource but does not support async/await:

// Example of a non-async function
func fetchData(completion: @escaping (Data?, Error?) -> Void) {
    DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
        let data = "Example data".data(using: .utf8) // Simulated data
        completion(data, nil)
    }
}

// Using withCheckedContinuation to wrap the fetchData call
func fetchDataAsync() async throws -> Data {
    try await withCheckedThrowingContinuation { continuation in
        fetchData { data, error in
            if let error = error {
                continuation.resume(throwing: error)
            } else if let data = data {
                continuation.resume(returning: data)
            } else {
                continuation.resume(throwing: NSError(domain: "DataError", code: 0, userInfo: nil))
            }
        }
    }
}

// Async function to call fetchDataAsync
func exampleUsage() async {
    do {
        let data = try await fetchDataAsync()
        print("Fetched data: \(data)")
    } catch {
        print("An error occurred: \(error)")
    }
}

// Run the async function
Task {
    await exampleUsage()
}        

In this example - The fetchData function simulates an asynchronous network call using a completion handler. 

- The fetchDataAsync function then wraps this call in a withCheckedThrowingContinuation to allow it to be used with Swift's async/await pattern. 

- The exampleUsage function demonstrates how you would call fetchDataAsync asynchronously and handle any potential errors.