Swift：我可以将异步/等待语法与已经同步返回值的 function 一起使用吗？

Question

I know that I can use async/await to replace this:

func test(_ completion: @escaping (Int) -> Void) {
    // ...
    completion(foundValue)
}

with this:

func test async -> Int {
    let result = await calculate()
    return result
}

However, can I do something if the initial function is like this?

func test(_ completion: @escaping (Int) -> Void) -> Int {
    // ...
}

Thank you for your help

Answer 1

If your function doesn't need to wait for completion handler call then you can create a top level task and call your completion handler from the task after doing asynchronous work:

func test(_ completion: @escaping (Int) -> Void) -> Int {
    // synhronous calls
    Task {
        // async funtion calls
        completion(value)
    }
    return value
}

If you need to wait for all your asynchronous task completion before returning, you can use a semaphore for that:

func test(_ completion: @escaping (Int) -> Void) -> Int {
    let semaphore = DispatchSemaphore(value: 0)
    // synhronous calls
    Task {
        // async funtion calls
        completion(value)
        // signal async task completion
        semaphore.signal()
    }
    // wait for async task completion
    semaphore.wait()
    return value
}

Note that using semaphore.wait() blocks the current thread and blocking the thread can cause undefined runtime behaviors in modern concurrency. So proceed with caution when using such approach.