大文件下载

带有进度条的大文件下载，将响应主体传递到其中io.Copy()，使用TeeReader则可以传递计数器来跟踪进度。在下载时，将文件另存为临时文件，因此在完全下载文件之前，不会覆盖有效文件。

package main
import (
    "fmt"
    "io"
    "net/http"
    "os"
    "strings"
    "github.com/dustin/go-humanize"
)
type WriteCounter struct {
    Total uint64
}
func (wc *WriteCounter) Write(p []byte) (int, error) {
    n := len(p)
    wc.Total += uint64(n)
    wc.PrintProgress()
    return n, nil
}
func (wc WriteCounter) PrintProgress() {
    fmt.Printf("\r%s", strings.Repeat(" ", 35))
    fmt.Printf("\rDownloading... %s complete", humanize.Bytes(wc.Total))
}
func main() {
    fmt.Println("Download Started")
    fileUrl := "http://xxx/static/1.png"
    err := DownloadFile("1.png", fileUrl)
    if err != nil {
        panic(err)
    }
    fmt.Println("Download Finished")
}
func DownloadFile(filepath string, url string) error {
    out, err := os.Create(filepath + ".tmp")
    if err != nil {
        return err
    }
    resp, err := http.Get(url)
    if err != nil {
        out.Close()
        return err
    }
    defer resp.Body.Close()
    counter := &WriteCounter{}
    if _, err = io.Copy(out, io.TeeReader(resp.Body, counter)); err != nil {
        out.Close()
        return err
    }
    fmt.Print("\n")
    out.Close()
    if err = os.Rename(filepath+".tmp", filepath); err != nil {
        return err
    }
    return nil
}