迅速完成处理问题

我试图弄清楚这个完成处理程序，但是我无法理解。我有这个程序，我试图下载一个json文件并解析它。一切正常，但是我希望用户界面在文件下载和解析后进行更新。我必须按两次按钮才能使其正常工作。我知道为什么会这样。该代码是异步样式，只是不进行任何更新即可更新UI。我如何获取它以等待json完成解析和下载。感谢您的帮助和提示

这是我到目前为止的内容：

 @IBAction func testerClick(sender: AnyObject) {


    print("Button pushed")

    print("Downloading the json file")
    downlaodPromoData(promoUrl, myUser: myUserName, myPass: myPassword)



}

func parsePromoJson(json : String)
{
    //parse the json file
    if let data = downloadJson.dataUsingEncoding(NSUTF8StringEncoding) {
        let json = JSON(data: data)
        for item in json[].arrayValue {
            pictures.append(item["picture"].stringValue)
        }
        for item in json[].arrayValue{
            path.append(item["path"].stringValue)
        }
        for item in json[].arrayValue{
            label.append(item["label"].stringValue)
        }
    }
    // Loop over the array with a for-loop.
    for i in 0 ..< pictures.count {
        uiResultsTextField.text.appendContentsOf("Picture: " + pictures[i] + "\n" + "Path: " + path[i] + "\n" + "Label: " + label[i] + "\n")
    }

}

func downlaodPromoData(myUrl : String, myUser : String, myPass : String)
{
    Alamofire.request(.GET, myUrl)
        .authenticate(user: myUser, password: myPass)
        .validate(statusCode: 200..<300)
        .responseString { response in
            print("Success: \(response.result.isSuccess)")
            print("Response String: \(response.result.value)")

            self.downloadJson = response.result.value!

            /*
             .response {(request, response, data, error) in
             print(response)
             print(data)
             print(request)
             */
        }.responseJSON { response in
            print("Response JSON: \(response.result.value)")
    }

    print("Calling parser")
    parsePromoJson(downloadJson)
}