我正在学习Swift。我在一本书中看到了这些代码行。importUIKitvarerrorCodeString:String?errorCodeString="404"varerrorDescription:String?iflettheError=errorCodeString,leterrorCodeInteger=Int(theError),errorCodeInteger==404{errorDescription="\(errorCodeInteger+200):resourcewasnotfound."}varupCaseErrorDescription=errorDesc