如何使用SlimerJS获取websocket请求的响应正文

Question

我正在尝试使用SlimerJS获取response.body for websocket请求。找不到提及的api页面/博客。有什么指针吗？

var page = require("webpage").create();
var url = "https://****"; 
page.settings.userAgent = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:46.0) Gecko/20100101 Firefox/46.0"; 
page.captureContent = [ /text\/plain/ ]; // web socket req's mime type. 
var mimeTypes = {}; 
var timer = 0; 

page.onResourceReceived = function(response) {

    if(response.stage == "end") {

        if(mimeTypes[response.contentType] === undefined) {
            mimeTypes[response.contentType] = 1; 
        } else {
            mimeTypes[response.contentType] = ++mimeTypes[response.contentType]; 
        }
            // ** no response body is displayed ** 
            if(response.url.indexOf("websocket-requri.ws") !== -1){
                console.log(response.id, response.url, response.contentType, response.body.substring(0,50)); // no response is displayed
            }


    }
};

page.open(url, function(status){

})