Node.js将相同的可读流管道化为多个（可写）目标 [英] Node.js Piping the same readable stream into multiple (writable) targets

问题描述

我需要运行两个需要从同一个流中读取数据的命令。
在将流传输到另一个流后，缓冲区被清空，因此我无法再次从该流中读取数据，因此这不起作用：

I need to run two commands in series that need to read data from the same stream. After piping a stream into another the buffer is emptied so i can't read data from that stream again so this doesn't work:

var spawn = require('child_process').spawn;
var fs = require('fs');
var request = require('request');

var inputStream = request('http://placehold.it/640x360');
var identify = spawn('identify',['-']);

inputStream.pipe(identify.stdin);

var chunks = [];
identify.stdout.on('data',function(chunk) {
  chunks.push(chunk);
});

identify.stdout.on('end',function() {
  var size = getSize(Buffer.concat(chunks)); //width
  var convert = spawn('convert',['-','-scale',size * 0.5,'png:-']);
  inputStream.pipe(convert.stdin);
  convert.stdout.pipe(fs.createWriteStream('half.png'));
});

function getSize(buffer){
  return parseInt(buffer.toString().split(' ')[2].split('x')[0]);
}

请求抱怨此

Error: You cannot pipe after data has been emitted from the response.

并将 inputStream 更改为 fs.createWriteStream 当然会产生同样的问题。
我不想写入文件，但重用以某种方式请求生成的流（或其他任何内容）。

and changing the inputStream to fs.createWriteStream yields the same issue of course. I don't want to write into a file but reuse in some way the stream that request produces (or any other for that matter).

有没有办法在完成管道后重复使用可读流？
完成类似上述示例的最佳方法是什么？

Is there a way to reuse a readable stream once it finishes piping? What would be the best way to accomplish something like the above example?

推荐答案

你必须创建副本通过将它流到两个流来流动。您可以使用PassThrough流创建一个简单的流，它只是将输入传递给输出。

You have to create duplicate of the stream by piping it to two streams. You can create a simple stream with a PassThrough stream, it simply passes the input to the output.

const spawn = require('child_process').spawn;
const PassThrough = require('stream').PassThrough;

const a = spawn('echo', ['hi user']);
const b = new PassThrough();
const c = new PassThrough();

a.stdout.pipe(b);
a.stdout.pipe(c);

let count = 0;
b.on('data', function (chunk) {
  count += chunk.length;
});
b.on('end', function () {
  console.log(count);
  c.pipe(process.stdout);
});

输出：

8
hi user

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