分片下载后端的文件

exportSlidesRequest(id,want_db,filename){
            return new Promise((r)=>{
              axios({
                  headers: {'Content-Type': ' application/x-www-form-urlencoded'},
                  method: 'post',
                  url: `/aipath/api/files/download`,
                  data: qs.stringify({caseid:id,want_db,companyid:userInfo().company}),
                  responseType:'blob',
                  getResponse:true
              })
              .then(async (res)=> {
                  let contentType = res.headers['content-type'];
                  if(contentType==='application/json') r(0)
                  else{

                    let blob = new Blob([res.data]);
                    let fileSize = blob.size;
                    filename = filename?filename:res.headers['content-disposition'].split('=')[1];
                    let chunkSize = 20*1024*1024;
                    var writeStream1 = new fs.createWriteStream(this.folder+'\\'+filename);
                    console.log('下载完成:'+filename)
                    writeStream1
                      .on('error', (err) => {
                        console.log('error')
                        r('error')
                      })
                      .on('finish',async () => {
                        console.log('下载完成:'+filename)
                        r('done')
                      });
                    for (let start = 0; start < fileSize; start += chunkSize) {
                      let chunk = blob.slice(start, start + chunkSize);
                      if((start + chunkSize)>fileSize){
                        chunk = blob.slice(start, fileSize + 1);
                      }
                      let curBuff = await getBuffer(chunk);
                      var passThrough1 = new stream.PassThrough().end(toBuffer(curBuff)); 
                      if((start + chunkSize)>fileSize){
                        passThrough1.pipe(writeStream1)
                      }
                      else{
                        passThrough1.pipe(writeStream1,{end:false})
                      }
                    }
                    
                    function toBuffer(ab) {
                        var buf = new Buffer(ab.byteLength);
                        var view = new Uint8Array(ab);
                        for (var i = 0; i < buf.length; ++i) {
                            buf[i] = view[i];
                        }
                        return buf;
                    }
                    function getBuffer(chunk){
                      return new Promise(r=>{
                        let reader  = new FileReader();
                        reader.readAsArrayBuffer(chunk);
                        reader.onload = (e)=>{
                          r(e.target.result)
                        }
                      })
                    }
                    
                  }
              })
              .catch((e)=> {
                r('error')
              });

            })            
          },