XMLhttpRequest 如何在Native 情况下监听到 progress 回调

Creator 2.x
#1

var xhr = cc.loader.getXMLHttpRequest();

    xhr.responseType = 'arraybuffer';
    xhr.onreadystatechange = function () {
        cc.info("xhr.readyState  " + xhr.readyState);
        cc.info("xhr.status  " + xhr.status);
        if (xhr.readyState === 4) {
            if (xhr.status === 200) {
                saveFile(xhr.response);
            } else {
                //下载失败 返回 弹出提示框
            }
        }
    }
    //.bind(this); 
   // xhr.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
    xhr.setRequestHeader("Content-Type","application/octet-stream");
    xhr.open("GET", url, true);
    xhr.onprogress = function (event) {
        if (event.lengthComputable) {
           cc.info("xhr.onprogress  event.loaded " + event.loaded);
           cc.info("xhr.onprogress  event.total " + event.total);
        }    
    }
    xhr.send();

xhr.onprogress 在native 上没有回调
xhr.status 打印的状态也只有 4

#2

这个接口c++层没有实现

#3

C++层没有实现， 我们该怎样实现呢？