错误：请求中的额外参数我使用Swift 4.为什么我必须传递[String

Alamofire.request(APPURL.GetAccounts,method: .post,parameters: transactionData,encoding: JSONEncoding.default,headers: nil).responseJSON { responseData in }

我的问题是限制我只能传递类型字典[String：Any]的原因是什么？

完整代码：

class func getAccounts( transactionData: [String:Any]?,withCompletionHandler: @escaping (_ response:AnyObject?)->(Void) ) {

    Alamofire.request(APPURL.GetAccounts,headers: nil).responseJSON { responseData in

        if((responseData.result.value) != nil) {
            let swiftyJsonVar = JSON(responseData.result.value!)
            if let resData = swiftyJsonVar["Accounts"].arrayObject {
                let resultData = resData as! [[String:AnyObject]]
                withCompletionHandler(resultData as AnyObject)
            }else{
                withCompletionHandler(nil)
            }
        }else{
            withCompletionHandler(nil)
        }
    }
}