如何在下载文件时打印字节？-golang

Question

我想知道是否可以计算和打印下载文件时下载的字节数.

out, err := os.Create("file.txt")
    defer out.Close()
    if err != nil {
        fmt.Println(fmt.Sprint(err) )

        panic(err)
    }
    resp, err := http.Get("http://example.com/zip")
    defer resp.Body.Close()
    if err != nil {
        fmt.Println(fmt.Sprint(err) )
        panic(err)
    }

    n, er := io.Copy(out, resp.Body)
    if er != nil {
        fmt.Println(fmt.Sprint(err) )
    }
    fmt.Println(n, "bytes ")

Answer 1

如果我理解正确,您希望在传输数据时显示读取的字节数.大概要保持某种进步条或什么的.在这种情况下,您可以使用Go的组合数据结构将读取器或编写器包装在自定义io.Reader或io.Writer实现中.

它只是转发相应的Read或Write调用底层流,同时使用(int, error)它们返回的值进行一些额外的工作.以下是您可以在Go操场上运行的示例.

package main

import (
    "bytes"
    "fmt"
    "io"
    "os"
    "strings"
)

// PassThru wraps an existing io.Reader.
//
// It simply forwards the Read() call, while displaying
// the results from individual calls to it.
type PassThru struct {
    io.Reader
    total int64 // Total # of bytes transferred
}

// Read 'overrides' the underlying io.Reader's Read method.
// This is the one that will be called by io.Copy(). We simply
// use it to keep track of byte counts and then forward the call.
func (pt *PassThru) Read(p []byte) (int, error) {
    n, err := pt.Reader.Read(p)
    pt.total += int64(n)

    if err == nil {
        fmt.Println("Read", n, "bytes for a total of", pt.total)
    }

    return n, err
}

func main() {
    var src io.Reader    // Source file/url/etc
    var dst bytes.Buffer // Destination file/buffer/etc

    // Create some random input data.
    src = bytes.NewBufferString(strings.Repeat("Some random input data", 1000))

    // Wrap it with our custom io.Reader.
    src = &PassThru{Reader: src}

    count, err := io.Copy(&dst, src)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }

    fmt.Println("Transferred", count, "bytes")
}

它生成的输出是这样的:

Read 512 bytes for a total of 512
Read 1024 bytes for a total of 1536
Read 2048 bytes for a total of 3584
Read 4096 bytes for a total of 7680
Read 8192 bytes for a total of 15872
Read 6128 bytes for a total of 22000
Transferred 22000 bytes

Answer 2

stdlib现在提供类似jimt的东西PassThru:io.TeeReader.它有助于简化一些事情:

// WriteCounter counts the number of bytes written to it.
type WriteCounter struct {
    Total int64 // Total # of bytes transferred
}

// Write implements the io.Writer interface.  
// 
// Always completes and never returns an error.
func (wc *WriteCounter) Write(p []byte) (int, error) {
    n := len(p)
    wc.Total += int64(n)
    fmt.Printf("Read %d bytes for a total of %d\n", n, wc.Total)
    return n, nil
}

func main() {

    // ...    

    // Wrap it with our custom io.Reader.
    src = io.TeeReader(src, &WriteCounter{})

    // ...
}

操场