Hello,
I am currently studying the stream api.
In the below example from the api docs "unshift" is used to reparse chunk
which belongs to the body and not the header.
This actually works but I do not understand how.
1. Does unshift recall _read with the passed chunk?
2. How can I imagine the read queue? I currently only know that push will
recall _read and emit the given data.
// A parser for a simple data protocol.// The "header" is a JSON object,
followed by 2 \n characters, and// then a message body.//// Note: This can be
done more simply as a Transform stream. See below.
function SimpleProtocol(source, options) {
if (!(this instanceof SimpleProtocol))
return new SimpleProtocol(options);
Readable.call(this, options);
this._inBody = false;
this._sawFirstCr = false;
// source is a readable stream, such as a socket or file
this._source = source;
var self = this;
source.on('end', function() {
self.push(null);
});
// give it a kick whenever the source is readable
// read(0) will not consume any bytes
source.on('readable', function() {
self.read(0);
});
this._rawHeader = [];
this.header = null;}
SimpleProtocol.prototype = Object.create(
Readable.prototype, { constructor: { value: SimpleProtocol }});
SimpleProtocol.prototype._read = function(n) {
if (!this._inBody) {
var chunk = this._source.read();
// if the source doesn't have data, we don't have data yet.
if (chunk === null)
return this.push('');
// check if the chunk has a \n\n
var split = -1;
for (var i = 0; i < chunk.length; i++) {
if (chunk[i] === 10) { // '\n'
if (this._sawFirstCr) {
split = i;
break;
} else {
this._sawFirstCr = true;
}
} else {
this._sawFirstCr = false;
}
}
if (split === -1) {
// still waiting for the \n\n
// stash the chunk, and try again.
this._rawHeader.push(chunk);
this.push('');
} else {
this._inBody = true;
var h = chunk.slice(0, split);
this._rawHeader.push(h);
var header = Buffer.concat(this._rawHeader).toString();
try {
this.header = JSON.parse(header);
} catch (er) {
this.emit('error', new Error('invalid simple protocol data'));
return;
}
// now, because we got some extra data, unshift the rest
// back into the read queue so that our consumer will see it.
var b = chunk.slice(split);
this.unshift(b);
// and let them know that we are done parsing the header.
this.emit('header', this.header);
}
} else {
// from there on, just provide the data to our consumer.
// careful not to push(null), since that would indicate EOF.
var chunk = this._source.read();
if (chunk) this.push(chunk);
}};
// Usage:var parser = new SimpleProtocol(source);// Now parser is a readable
stream that will emit 'header'// with the parsed header data.
Best Regards,
Bodo
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