请教各位大大，如何使用pb第三方库实现一个上传进度条，如下代码上传进度瞬间完成，不是真实的进度，请问如何改？

<pre><code> package main import ( "bytes" "fmt" "io" "io/ioutil" "mime/multipart" "net/http" "os" "time" "github.com/cheggaaa/pb" ) func uploadFile(fileName string, targetURL string) (err error) { buf := &bytes.Buffer{} bodyMultiWriter := multipart.NewWriter(buf) fileWriter, err := bodyMultiWriter.CreateFormFile("uploadFile", fileName) if err != nil { fmt.Println("writing to buf error.") return err } hFile, err := os.Open(fileName) if err != nil { fmt.Println("opening file error.") return err } //获取文件的大小 fileInfo, errFileInfo := os.Stat(fileName) if errFileInfo != nil { fmt.Println("file_info size error.") return errFileInfo } //显示进度条 bar := pb.New(int(fileInfo.Size())).SetUnits(pb.U_BYTES).SetRefreshRate(time.Millisecond * 10) bar.Start() bar.ShowSpeed = true bar.ShowFinalTime = true bar.SetMaxWidth(80) writer := io.MultiWriter(fileWriter, bar) _, err = io.Copy(writer, hFile) if err != nil { return err } bar.Finish() //关闭进度条 contentType := bodyMultiWriter.FormDataContentType() bodyMultiWriter.Close() resp, err := http.Post(targetURL, contentType, buf) if err != nil { return err } defer resp.Body.Close() _, err = ioutil.ReadAll(resp.Body) if err != nil { return err } return nil } func main() { targetURL := "http://127.0.0.1:9090/upload" fileName := `TEST.exe` err := uploadFile(fileName, targetURL) if err != nil { fmt.Println("upload error.") os.Exit(1) } var stop string fmt.Scanln(&stop) } </code></pre>