为什么函数在调用时返回空字符串？

Question

我创建静态var然后发布JSON并用JSON填充它，然后我发布JSON参数，对于param字符串我使用创建的静态var但是当我打印它时它是空的，为什么？

public class SecurityOptions {

    static var _TokenID:String! = ""

}

然后我发布Json：

class Security {

    func Security () -> String {
        let url :NSURL = WebService.tokenWebAddress
        let session = NSURLSession.sharedSession()
        let request = NSMutableURLRequest(URL: url)
        request.HTTPMethod = "POST"
        let paramString = "data=Hello"
        request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData
        request.HTTPBody = paramString.dataUsingEncoding(NSUTF8StringEncoding)
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        session.dataTaskWithRequest(request) { (data:NSData?, response:NSURLResponse?, error:NSError?) -> Void in
            if let responsedata = data {
                do{
                    let json = try NSJSONSerialization.JSONObjectWithData(responsedata, options: NSJSONReadingOptions.AllowFragments)

                    if  let TokenDetail = json as? Dictionary <String,String>
                    {
                        SecurityOptions._TokenID = TokenDetail["TokenID"] as String!

                    }else{
                        print("KHATA")
                    }

                } catch{
                    print("not Serialized")
                }
            }

        }.resume()

        return SecurityOptions._TokenID
    }
}

之后：

 let a = Security()
 let url :NSURL = WebService.listWebAddress
 let session = NSURLSession.sharedSession()
 let request = NSMutableURLRequest(URL: url)
 let paramString = ["_TokenID":a.Security()] as Dictionary<String, AnyObject>

 request.HTTPMethod = "POST"
 let err: NSError?
 request.HTTPBody = try! NSJSONSerialization.dataWithJSONObject(paramString, options: [])
 request.addValue("application/json", forHTTPHeaderField: "Content-Type")
 request.addValue("application/json", forHTTPHeaderField: "Accept")
 let task = session.dataTaskWithRequest(request) { data, response, error in
     guard data != nil else {
         print("no data found: \(error)")
         return
     }

     var index:Int = 0;
     do {
         var json = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.AllowFragments)

         // Okay, the `json` is here, let's get the value for 'success' out of it
         if  let adsList = json as? Dictionary <String,AnyObject> {

             print("success")
         } else {
             let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)    // No error thrown, but not NSDictionary
             print("Error could not parse JSONs: \(jsonStr)")
         }
     } catch let parseError {
         print(parseError)                                                                      // Log the error thrown by `JSONObjectWithData`
         let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
         print("Error could not parse JSON: '\(jsonStr)'")
     }
}.resume()

当我将参数

用于参数时，我的参数字符串为空