practice of await/async

await_async.swift

struct Promise<T, Error: Swift.Error> {
    let handler: (Result<T, Error>) -> Void
    
    
}

extension Promise where T == Payload {
    func fire() -> Result<T, Error> {
        var result: Result<T, Error>!
        let semaphore = DispatchSemaphore(value: 0)
        DispatchQueue.global().async {
            defer { semaphore.signal() }
            sleep(10)
            result = Result.success(Payload(name: "Terry"))
            self.handler(result)
            
        }
        semaphore.wait(timeout: .distantFuture)
        let payload = try! result.get()
        print(payload, result!, Date())
//        sleep(5)
        return result
    }
}

struct Payload {
    let name: String
}

let promise = Promise<Payload, Never> { (result) in
    print(result, Date())
}

print("before call await function", Date())
let result = promise.fire()
print(result, Date())