Callback function in Node.js addon

Question

I am wrapping a CAN Bus manager API in a node.js extension. Everything works well, this would be the final result:

// list interfaces
var interfaces = addon.listDevices();
console.log(interfaces);

// Allocating a new port
var port = new addon.CANPort();
port.setCallback(function(msg) {
    // I would like to receive all CAN datagrams
    console.log(msg);
});

// Connect to the 1st port
port.connect(0);
// Send a datagram
port.sendStruct(127508, 6, Array(0, 70, 5, 50, 0, 131, 114, 0));

Every function is OK but the setCallback function. Actually, I don't know what to do. I have a c++ callback of this this type :

void CANPort::SetReceiveCallback(functionCallback func);

which will call the function when it receives a new datagram.

I found in http://nikhilm.github.io/uvbook/threads.html some help, but I can't find the right way to do the same as the last example. I know I should use uv, but I don't know how...

Moreover, I'm using the v0.11.14-pre of Node.js, so a lot of documentation is out of date...

Thanks, Nicolas.

You cannot do this inline: function(msg) { // I would like to receive all CAN datagrams console.log(msg); } declare the function outside somewhere and call port.setCallback(function); — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jun 20, 2014 at 11:15
Why is it different ? The type is identical, right ? Maybe a difference between named functions and lambda functions ? — npielawski
– npielawski, Commented Jun 20, 2014 at 11:23
'Why is it different ?' Because that's not valid c++ syntax dude! Even lambda should be declared first IMHO, but you don't have lambda declaration/definition syntax there. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jun 20, 2014 at 11:26
what is your functionCallback ? i hope you have written some valid v8 bindings in cpp side; — Ashish Negi
– Ashish Negi, Commented Jun 20, 2014 at 11:27

aspoon · Accepted Answer · 2015-01-27 05:04:20Z

Although this is kind of old, but I used a hybrid javascript/C++ mechanism to integrate external callbacks. Basically call uv_queue_work() in your callback with uv_work_t that performs nothing, and use its after_work_cb to call your javascript function:

// C++

void nop() {}

void jscallback(uv_work_t* handle, int status)
{
  // extract some_data from handle and call node::MakeCallback() here
  // I make it call emit('event', some_data);
  delete handle;
}

void called_by_third_party_callback(some_type some_data)
{
  uv_work_t uv = new uv_work_t();
  if (!uv) {
    // error
  }
  // ensure some_data is accessible via uv->data
  if (uv_queue_work(uv_default_loop(), uv, nop, jscallback) != 0) {
    delete uv;
  }
}

On my javascript side I used a string-to-function map to implement a simple Event Emitter mechanism on top of my Engine object:

// Javascript

exports.Engine.prototype.eventMap = [];
exports.Engine.prototype.on = function (name, func) {
  if (typeof (func) === "function") {
    this.eventMap[name] = func;
}

exports.Engine.prototype.emit = function (name, value) {
  var func = this.eventMap[name];
  if (typeof (func) === "function") {
    func(value);
  }
}

This is how I use it:

// Javascript

var obj = new require('myaddon').Engine();

obj.on('event', function (v) {
  console.log('event: ' + v);
});

Sorry for using comments excessively. My code is too intertwined to be shown as snipplets.

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