// 阅读图书
  readBook:function(e){
    var that = this
    // 获取当前点击图书的ID
    let id = e.currentTarget.dataset.id
   // 获取当前点击图书的Url
    let fileUrl = e.currentTarget.dataset.file
    // 查看本地缓存
    let path =wx.getStorageSync(id)
    // 未曾下载过
    if(path ==''){
    // 切换到下载时的蒙层
      that.setData({
        isDownloading:true
      })
      // 先下载图书
      const downloadTask = wx.downloadFile({
        url:fileUrl,
        success:function(res){
          // 关闭下载时的蒙层
          that.setData({
            isDownloading:false
          })

          // 下载成功
          if (res.statusCode == 200){
            // 获取地址
            path = res.tempFilePath
            // 保存并打开图书
            that.saveBook(id,path)
          }
          // 连上服务器，下载失败
          else{
            that.showTips('暂时无法下载')
          }
        },

        //请求失败
        fail:function(e){
          // 关闭下载时的蒙层
          that.setData({
            isDownloading:false
          })
          that.showTips('无法连接到服务器!')
        }
      })
      // 监听当前文件下载的下载速度
      downloadTask.onProgressUpdate((res) =>{
        let progress = res.progress
        that.setData({
          percentNum: progress
        })
      })
    }
    else{
      // 打开图书
      that.openBook(path)
    }
  },

当我点击图书下载前，无错误，但当我点击时，只会提示弹出‘无法连接到服务器’这个消息框，但是无法下载，并出错以下错误

thirdScriptError

Cannot read property ‘onProgressUpdate’ of undefined; [Component] Event Handler Error @ pages/index/index#bound readBook

TypeError: Cannot read property ‘onProgressUpdate’ of undefined

    at me.readBook (http://127.0.0.1:52924/appservice/pages/index/index.js:141:18)

    at Object.r.safeCallback (http://127.0.0.1:52924/appservice/__dev__/WAService.js:2:1434440)

    at http://127.0.0.1:52924/appservice/__dev__/WAService.js:2:1552253

    at s (http://127.0.0.1:52924/appservice/__dev__/WAService.js:2:1561021)

    at http://127.0.0.1:52924/appservice/__dev__/WAService.js:2:1552178

    at r (http://127.0.0.1:52924/appservice/__dev__/WAService.js:2:1505702)

当我点击图书时，只提示‘无法连接到服务器’这个选项，但是无法下载，

’‘