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);

};