When using Swift concurrency, how can I call async code from a synchronous context and wait for the result?

To call async code from a synchronous context and wait for the result in Swift, you can use the withCheckedContinuation or withCheckedThrowingContinuation functions. These functions allow you to bridge between synchronous and asynchronous code by creating a continuation that you can resume once the asynchronous operation completes.

Here's a brief example of how you might use withCheckedContinuation:

func performAsyncOperation() async -> String {
    // Simulate an async operation
    return "Async result"
}

func callAsyncFromSync() -> String {
    var result: String = ""
    let semaphore = DispatchSemaphore(value: 0)
    
    Task {
        result = await performAsyncOperation()
        semaphore.signal()
    }
    
    semaphore.wait()
    return result
}

In this example, performAsyncOperation is an asynchronous function. The callAsyncFromSync function creates a DispatchSemaphore to wait for the asynchronous operation to complete. Inside a Task, it calls the async function and signals the semaphore once the result is obtained. The synchronous function then waits for the semaphore to be signaled before returning the result.

For more detailed information on using continuations in Swift, you can refer to the session Go further with Swift Testing (24:51), which discusses techniques for waiting on asynchronous conditions with Swift testing, including the use of withCheckedContinuation and withCheckedThrowingContinuation.