web文件流下载文件

    loadDown (val) { // 点击下载
      this.fileLinkToStreamDownload(this.url, this.fileName)
    },
    fileLinkToStreamDownload (url, fileName) {
      const xhr = new XMLHttpRequest()
      xhr.open('get', url, true)
      xhr.setRequestHeader('Content-Type', `application/pdf`)
      xhr.responseType = 'blob'
      const that = this
      xhr.onload = function () {
        if (this.status === 200) {
          // 接受二进制文件流
          var blob = this.response
          that.downloadExportFile(blob, fileName)
        }
      }
      xhr.send()
    },
    downloadExportFile (blob, tagFileName) {
      const downloadElement = document.createElement('a')
      let href = blob
      if (typeof blob === 'string') {
        downloadElement.target = '_blank'
      } else {
        href = window.URL.createObjectURL(blob) // 创建下载的链接
      }
      downloadElement.href = href
      downloadElement.download = tagFileName
      document.body.appendChild(downloadElement)
      downloadElement.click() // 点击下载
      document.body.removeChild(downloadElement) // 下载完成移除元素
      if (typeof blob !== 'string') {
        window.URL.revokeObjectURL(href) // 释放掉blob对象
      }
    },