javascript - 如何在没有 Duplexer 包的情况下解决 NodeSchool 的 Duplexer 挑战

Question

问题(可从 nodeschool.io/#stream-adventure 下载)

Write a program that exports a function that spawns a process from a cmd string and an args array and returns a single duplex stream joining together the stdin and stdout of the spawned process. There is a very handy module you can use here: duplexer. The duplexer module exports a single function duplexer(writable, readable) that joins together a writable stream and readable stream into a single, readable/writable duplex stream.

官方解决方案

var spawn = require('child_process').spawn;
var duplexer = require('duplexer');

module.exports = function (cmd, args) {
    var ps = spawn(cmd, args);
    return duplexer(ps.stdin, ps.stdout);
}; 

此代码主要执行以下操作:

var spawn = require("child_process").spawn,
    Stream = require("stream");

module.exports = function (cmd, args) {
    var ps = spawn(cmd, args),
        stream = new Stream();
    stream.write = function (chunk, enc, cb) { ps.stdin.write(chunk, enc, cb); }
    stream.end = function (chunk, enc, cb) { ps.stdin.end(chunk, enc, cb); }
    ps.stdout.on("data", function (chunk) { stream.emit("data", chunk); });
    ps.stdout.on("end", function () { stream.emit("end"); });
    return stream;
}; 

没有双工器包的解决方案

我试图通过继承 Duplex 来解决问题类:

var spawn = require("child_process").spawn,
    Stream = require("stream");

require("util").inherits(Dx, Stream.Duplex);
function Dx(stdin, stdout) {
    Stream.Duplex.call(this);
    this.stdin = stdin;
    this.stdout = stdout;
}
Dx.prototype._write = function (chunk, enc, cb) { this.stdin.write(chunk, enc, cb); };
Dx.prototype._read = function (size) { return this.stdout.read(size); };

module.exports = function (cmd, args) {
    var ps = spawn(cmd, args),
        stream = new Dx(ps.stdin, ps.stdout);
    return stream;
};

当我测试这个 (stream-adventure verify program.js) 时，测试程序打印出 ACTUAL vs. EXPECTED 表(看起来正确)，但在那之后它被卡住了。我的代码有什么问题？感谢您的帮助。

更新

添加这一行解决了问题:

Dx.prototype.end = function (chunk, enc, cb) { this.stdin.end(chunk, enc, cb); }

那么问题来了，为什么实现_read和_write方法还不够呢？

Answer 1

process.stdin 在完成所有其他操作后仍然存在，这就是该过程不会结束的原因。要让进程结束，通常是方法process.stdin.pause()已使用，因为您可以在此之后恢复它。

您可以尝试覆盖 .end()方法，但这不是一个好的做法，更好的做法是:

#1 如果您知道要为此流写入的结束消息，您应该添加 .resume() ._write() 中的方法实现:

dstream._write = function (chunk, encoding, callback) {

    this.stdin.write(chunk, encoding, callback);

    if (someConditionToEnd) {
        this.stdin.pause();
    }
};

#2 收听结束事件:

/*
inside the stream prototype constructor
*/

this.on('end', function () {
    this.stdin.pause();
});