Javascript MAVLink implementation

This code generates npm modules that can be used with Node.js. As with the other implementations in Python and C, the MAVLink protocol is specified in XML manifests which can be modified to add custom messages.

See the gotcha's and todo's section below for some important caveats. This implementation should be considered pre-beta: it creates a working MAVLink parser, but there's plenty of rough edges in terms of API.

Generating the JS implementation

Folders in the implementations/ directory are npm modules, automatically generated from XML manifests that are in the mavlink/mavlink project. If you wish to generate custom MAVLink packets, you would need to follow the directions there.

You need to have Node.js and npm installed to build.

To build the Javascript implementations:

cd mavlink/pymavlink/generator && make -f javascriptMakefile

Usage in Node.js

The generated modules emit events when valid MAVLink messages are encountered. The name of the event is the same as the name of the message: HEARTBEAT, FETCH_PARAM_LIST, REQUEST_DATA_STREAM, etc. In addition, a generic message event is emitted whenever a message is successfully decoded.

The below code is a rough sketch of how to use the generated module in Node.js. A somewhat more complete (though early, early alpha) example can be found here.

Generating the parser

After running the generator, copy the version of the MAVLink protocol you need into your project's node_modules folder, then enter that directory and install its dependencies using npm install:

cp -R javascript/implementations/mavlink_ardupilotmega_v1.0 /path/to/my/project/node_modules/
cd /path/to/my/project/node_modules/mavlink_ardupilotmega_v1.0 && npm install

Then, you can use the MAVLink module, as sketched below.

Initializing the parser

In your server.js script, you need to include the generated parser and instantiate it; you also need some kind of binary stream library that can read/write binary data and emit an event when new data is ready to be parsed (TCP, UDP, serial port all have appropriate libraries in the npm-o-sphere). The connection's "data is ready" event is bound to invoke the MAVLink parser to try and extract a valid message.

// requires Underscore.js, can use Winston for logging ,
// see package.json for dependencies for the implementation
var mavlink = require('mavlink_ardupilotmega_v1.0'), 
	net = require('net');

// Instantiate the parser
mavlinkParser = new MAVLink();

// Create a connection -- can be anything that can receive/send binary
connection = net.createConnection(5760, '127.0.0.1');

// When the connection issues a "got data" event, try and parse it
connection.on('data', function(data) {
	mavlinkParser.parseBuffer(data);
});

Receiving MAVLink messages

If the serial buffer has a valid MAVLink message, the message is removed from the buffer and parsed. Upon parsing a valid message, the MAVLink implementation emits two events: message (for any message) and the specific message name that was parsed, so you can listen for specific messages and handle them.

// Attach an event handler for any valid MAVLink message
mavlinkParser.on('message', function(message) {
	console.log('Got a message of any type!');
	console.log(message);
});

// Attach an event handler for a specific MAVLink message
mavlinkParser.on('HEARTBEAT', function(message) {
	console.log('Got a heartbeat message!');
	console.log(message); // message is a HEARTBEAT message
});

Sending MAVLink messages

See the gotcha's and todo's section below for some important caveats. The below code is preliminary and will change to be more direct. At this point, the MAVLink parser doesn't manage any state information about the UAV or the connection itself, so a few fields need to be fudged, as indicated below.

Sending a MAVLink message is done by creating the message object, populating its fields, and packing/sending it across the wire. Messages are defined in the generated code, and you can look up the parameter list/docs for each message type there. For example, the message REQUEST_DATA_STREAM has this signature:

mavlink.messages.request_data_stream = function(target_system, target_component, req_stream_id, req_message_rate, start_stop) //...

Creating the message is done like this:

request = new mavlink.messages.request_data_stream(1, 1, mavlink.MAV_DATA_STREAM_ALL, 1, 1);

// Hack alert: we tack on a few extra fields that could/should be inserted automagically by the MAVLink
// connection itself.
_.extend(request, {
	  srcSystem: 255,
	  srcComponent: 0,
	  seq: 1
});

// Create a buffer consisting of the packed message, and send it across the wire.
// Hack alert: again, the MAVLink connection could/should encapsulate this.
p = new Buffer(request.pack());
connection.write(p);

Gotchas and todo's

The JS library that is implementing the pack/unpack functions (jspack) doesn't match Python's struct library identically. For example, it doesn't support the q or Q flags representing int64_t and uint64_t, respectively. Those have been replaced with d and double, for the moment.

This code isn't great idiomatic Javascript (yet!), instead, it's more of a line-by-line translation from Python as much as possible.

The Python MAVLink code manages some information about the connection status (system/component attached, bad packets, durations/times, etc), and that work isn't present in this code yet.

Code to create/send MAVLink messages to a client is very clumsy at this point in time and will change to make it more direct.

Development

Unit tests cover basic packing/unpacking functionality against mock binary buffers representing valid MAVlink generated by the Python implementation. You need to have mocha installed to run the unit tests.

To run tests, use the makefile in the parent directory (mavlink/pymavlink/generator/):

make -f javascriptMakefile test

Specific instructions for generating Jenkins-friendly output is done through the makefile as well:

make -f javascriptMakefile ci