chrome-native-messaging/index.js

Transform streams for writing Chrome App native messaging hosts in Node.js

// chrome-native-messaging module
//
// Defines three Transform streams:
//
// - Input - transform native messages to JavaScript objects
// - Output - transform JavaScript objects to native messages
// - Transform - transform message objects to reply objects
// - Debug - transform JavaScript objects to lines of JSON (for debugging, obviously)

var stream = require('stream');
var util = require('util');

function Input() {
    stream.Transform.call(this);

    // Transform bytes...
    this._writableState.objectMode = false;
    // ...into objects.
    this._readableState.objectMode = true;

    // Unparsed data.
    this.buf = new Buffer(0);
    // Parsed length.
    this.len = null;
}

util.inherits(Input, stream.Transform);

Input.prototype._transform = function(chunk, encoding, done) {
    // Save this chunk.
    this.buf = Buffer.concat([ this.buf, chunk ]);

    var self = this;

    function parseBuf() {
        // Do we have a length yet?
        if (typeof self.len !== 'number') {
            // Nope. Do we have enough bytes for the length?
            if (self.buf.length >= 4) {
                // Yep. Parse the bytes.
                self.len = self.buf.readUInt32LE(0);
                // Remove the length bytes from the buffer.
                self.buf = self.buf.slice(4);
            }
        }

        // Do we have a length yet? (We may have just parsed it.)
        if (typeof self.len === 'number') {
            // Yep. Do we have enough bytes for the message?
            if (self.buf.length >= self.len) {
                // Yep. Slice off the bytes we need.
                var message = self.buf.slice(0, self.len);
                // Remove the bytes for the message from the buffer.
                self.buf = self.buf.slice(self.len);
                // Clear the length so we know we need to parse it again.
                self.len = null;
                // Parse the message bytes.
                var obj = JSON.parse(message.toString());
                // Enqueue it for reading.
                self.push(obj);
                // We could have more messages in the buffer so check again.
                parseBuf();
            }
        }
    }

    // Check for a parsable buffer (both length and message).
    parseBuf();

    // We're done.
    done();
};

function Output() {
    stream.Transform.call(this);

    this._writableState.objectMode = true;
    this._readableState.objectMode = false;
}

util.inherits(Output, stream.Transform);

Output.prototype._transform = function(chunk, encoding, done) {
    var len = new Buffer(4);
    var buf = new Buffer(JSON.stringify(chunk));

    len.writeUInt32LE(buf.length, 0);

    this.push(len);
    this.push(buf);

    done();
};

function Transform(handler) {
    stream.Transform.call(this);

    this._writableState.objectMode = true;
    this._readableState.objectMode = true;

    this.handler = handler;
}

util.inherits(Transform, stream.Transform);

Transform.prototype._transform = function(msg, encoding, done) {
    this.handler(msg, this.push.bind(this), done);
};

function Debug() {
    stream.Transform.call(this);

    this._writableState.objectMode = true;
    this._readableState.objectMode = false;
}

util.inherits(Debug, stream.Transform);

Debug.prototype._transform = function(chunk, encoding, done) {
    this.push(JSON.stringify(chunk) + '\n');

    done();
};

exports.Input = Input;
exports.Output = Output;
exports.Transform = Transform;
exports.Debug = Debug;