我正在按照Braintree的指南将dropIn功能添加到iOS应用程序。为了显示dropIn，我使用以下方法:funcshowDropIn(clientTokenOrTokenizationKey:String){letrequest=BTDropInRequest()letdropIn=BTDropInController(authorization:clientTokenOrTokenizationKey,request:request){(controller,result,error)inif(error!=nil){print("ERROR")}elseif(result?