从io.Writer写的内容中读取内容

本文介绍了从io.Writer写的内容中读取内容的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

有一个导出文件的库，但我想捕获文件的内容。我想将一个编写器传递给库，并能够读取编写器写入文件的内容。最终我想扩充库以跳过编写此文件。
这可以用io.Copy或io.Pipe吗？

There's a library that exports a file but I'd like to capture the contents of the file. I'd like to pass a writer to the library and be able to read what the writer wrote to the file. Eventually i want to augment the library to skip writing this file. Is this possible with io.Copy or io.Pipe?

库代码创建一个*文件并将此句柄用作io.Writer。
我尝试使用io.Copy，但只读取了0个字节。

The library code creates a *File and uses this handle as an io.Writer.I tried using io.Copy but only 0 bytes were read.

func TestFileCopy(t *testing.T) {

codeFile, err := os.Create("test.txt")
if err != nil {
    t.Error(err)
}
defer codeFile.Close()
codeFile.WriteString("Hello World")
n, err := io.Copy(os.Stdout, codeFile)
if err != nil {
    t.Error(err)
}
log.Println(n, "bytes")
}

推荐答案

如果要在写入时捕获字节，请使用， bytes.Buffer 作为第二作者。

If you want to capture the bytes as they are written, use an io.MultiWriter with a bytes.Buffer as the second writer.

var buf bytes.Buffer
w := io.MultiWriter(codeFile, &buf)

或者在stdout上查看文件：

or to see the file on stdout as it's written:

w := io.MultiWriter(codeFile, os.Stdout)

否则，如果你想要重新读取同一个文件，你需要在写完后回头找：

Otherwise, if you want to re-read the same file, you need to seek back to the start after writing:

codeFile.Seek(0, 0)

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