filename是下载的文件名称，downloaddata是接口返回的xml字符串数据
以下是方法内容：
downloadProcess = (filename, downloaddata) => {
const data = downloaddata;
var urlObject = window.URL || window.webkitURL || window;
var export_blob = new Blob([data]);
var save_link = document.createElement("a");
save_link.href = urlObject.createObjectURL(export_blob);
save_link.download = `${filename}.bpmn`;
document.body.appendChild(save_link);
save_link.click();
document.body.removeChild(save_link);
};
