ios – 了解swift Alamofire completionHandler

func authenticateUser(completionHandler: (responSEObject: String?,error: NSError?) -> ()) {
        makeAuthenticateUserCall(completionHandler)
    }

    func makeAuthenticateUserCall(completionHandler: (responSEObject: String?,error: NSError?) -> ()) {
        Alamofire.request(.GET,loginUrlString)
            .authenticate(user: "a",password: "b")
            .responseString { request,response,responseString,responseError in
                completionHandler(responSEObject: responseString as String!,error: responseError)
        }
    }

然后在另一个类中,我使用以下代码来访问数据：

API().authenticateUser{ (responSEObject,error) in
    println(responSEObject)
}

代码正在运行,但我完全不理解.

> func authenticateUser具有参数completionHandler