XMLHttpRequest下载文件方法中添加处理服务器返回json格式的错误提示信息

       //为了更清晰的说明问题，此处把其它代码都去掉了，只保留返回错误情况的代码
 public IActionResult GetPBbackDtoPrintListForPB(GetWorkTimeArrangeInput args)
        {

                Response.StatusCode = 206;
                string msg = "没有数据！请确保筛选条件下有数据！";
                return Json(new BizResult(){ Success = false, Message = msg });
            }

            if (this.status === 206) {//处理错误提示
                var result = this.response;
                if (result.type === ‘application/json‘) {
                    var reader = new FileReader();
                    reader.readAsText(result, ‘utf-8‘);
                    reader.onload = (e) => {
                        var jsonData = JSON.parse(reader.result);
                        if (!jsonData.success) {
                            $.messager.alert("警告", jsonData.message);
                        }
                    }
                }

    /**
     * 异步下载文件 依赖easyui的进度条
     *
     */
    downloadAsync: function (method, url, data,contentType, displayfileName, callback) {
        if (!displayfileName)
            displayfileName = ‘‘;
        if (!method)
            method = "get";
        method = method.toLowerCase();
        if (!url)
            throw "url mast has value";

        if (method != "post" && method != "get")
            method = "get";
        var xhr = new XMLHttpRequest();

        var sendData = null;
        if (data && (typeof (data) == "object" || typeof (data) == "string")) {
            if (contentType.toLowerCase() ==="application/json") {
                sendData = JSON.stringify(data);
            } else if (method.toLowerCase() === "get") {
                //参数追加到url上
                var params;
                if (typeof (data) == "string")
                    params = data;
                else
                    params = $.param(data);
                var ls = url.indexOf(url.length);
                switch (ls) {
                    case "?":
                    case "&":
                        url += params
                        break;
                    default:
                        if (url.indexOf(‘?‘) < 0) {
                            url += "?" + params;
                        } else {
                            url += "&" + params;
                        }
                        break;
                }
            } else {
                sendData = $.param(data);
            }
        }
        xhr.open(method, url, true);    // 也可以使用POST方式，根据接口       
        xhr.setRequestHeader(‘content-type‘, contentType);
        xhr.responseType = "blob";  // 返回类型blob

        $.messager.progress({ msg: ‘下载中。。。‘, interval: 0 });
        var bar = $.messager.progress(‘bar‘);

        //进度事件
        xhr.onprogress = function (ev) {
            if (ev.lengthComputable) {
                progress = ev.loaded / ev.total;
                //更新进度条
                bar.progressbar(‘setValue‘, parseFloat(progress, 2) * 100);
            }
        };

        // 定义请求完成的处理函数
        xhr.onload = function () {
            // 请求完成
            if (this.status === 200) {
                // 返回200
                var blob = this.response;
                var reader = new FileReader();
                reader.readAsDataURL(blob);  // 转换为base64，可以直接放入a表情href
                reader.onload = function (e) {
                    // 转换完成，创建一个a标签用于下载
                    var a = document.createElement(‘a‘);
                    if (!displayfileName || typeof (displayfileName) != ‘string‘)
                        displayfileName = new Date().getTime + ‘.txt‘;
                    console.log(displayfileName);
                    a.download = displayfileName;
                    a.href = e.target.result;
                    $("body").append(a);  // 修复firefox中无法触发click
                    a.click();
                    $(a).remove();
                    if (typeof (callback) == "function")
                        callback();
                };
            }
            else if (this.status === 206) {//处理错误提示
                var result = this.response;
                if (result.type === ‘application/json‘) {
                    var reader = new FileReader();
                    reader.readAsText(result, ‘utf-8‘);
                    reader.onload = (e) => {
                        var jsonData = JSON.parse(reader.result);
                        if (!jsonData.success) {
                            $.messager.alert("警告", jsonData.message);
                        }
                    }
                }
            }
            else {
                errortitle = "错误";
                error = "导出错误";
                switch (this.status) {
                    case 404:
                        error += " 页面找不到";
                        break;
                    case 400:
                        error += " 导出参数不对";
                        break;
                    case 500:
                        error += " 导出数据失败";
                        break;
                    default:
                        break;
                }
                $.messager.alert(errortitle, error);
            }
            $.messager.progress(‘close‘);
        };
        xhr.onreadystatechange = function (ev) {
        }
        // 发送ajax请求
        xhr.send(sendData);
    }