NodeJS中的流

Question

I need some help to understand how stream work in NodeJS

I explain, i need to write a module which call a UNIX process (with spawn ) and I want to redirect the stdout of this process to a Readable Stream.

I want this behavior to exports the Readable Stream and allow another module to read them.

To do this, I have write a little piece of code :

var spawn = require('child_process').spawn
var Duplex = require('stream').Duplex;
var stream = new Duplex;


var start = function() {
    ps = spawn('mycmd', [/*... args ...*/]);
    ps.stdout.pipe(stream);
};


exports.stream = stream;
exports.start = start;

But if I use this module I throw an exception which say that the stream doesn't implement the _read method.

Can you help me with this problem ?

Thanks in advance.

[ EDIT ] I have try the solution of creating a Stream object, but that's doesnt work, here is the code:

var spawn = require('child_process').spawn;
var Stream = require('stream');
var ps = null;

var audio = new Stream;
audio.readable = audio.writable = true;

var start = function() {
    if(ps == null) {
        ps = spawn('mycmd', []);

        ps.stdout.pipe(stream);
    }
};

var stop = function() {
    if(ps) {
        ps.kill();
        ps = null;
    }
};


exports.stream = stream;
exports.start = start;
exports.stop = stop;

But when I try to listen the stream, I encount an new error :

_stream_readable.js:583
    var written = dest.write(chunk);
                       ^
TypeError: Object #<Stream> has no method 'write'

Answer 1

Most of Node's Stream classes aren't meant to be used directly, but as the base of a custom type:

Note that stream.Duplex is an abstract class designed to be extended with an underlying implementation of the _read(size) and _write(chunk, encoding, callback) methods as you would with a Readable or Writable stream class.

One notable exception is stream.PassThrough , which is a simple echo stream implementation.

var PassThrough = require('stream').PassThrough;
var stream = new PassThrough;

---

Also note that ps will be a global , making it directly accessible in all other modules.

Answer 2

If you simply want to use stream then you should do :

var stream = new Stream;
stream.readable = stream.writable = true;

Duplex is meant for developers. Some methods like _read and _write need to be implemented for it.

[ Update ]

OK, you have data source, from the stdout. You will need write function, use this :

stream.write = function(data){this.emit('data', data);};