How to call async method from Meteor own callbacks?

I've just spent a few hours reading SO with answers such as Meteor: Calling an asynchronous function inside a Meteor.method and returning the result

Unfortunately, I still didn't manage to user fibers, or futures for that matter.

I'm trying to do something fairly simple (I think!).

When creating a user, add a variable to the user object, based on the result of an asynchronous method. So imagine if you will my async method is called on a 3rd party db server called BANK, which could take several seconds to return.

Accounts.onCreateUser(function(options,user){

var Fiber = Npm.require("fibers");

Fiber(function() { 
    BANK.getBalance(function(err, theBalance) {

        if (err) return console.log(err);

        _.extend(user,{
            balance: theBalance;
        });

    });
}).run();

return user;

});

So what happens in the above is that the BANK method is called, but by the time it returns the code has already moved on and _.extend is never invoked.

I tried placing the return call inside the Fiber, that only made things worse: it never return user. Well it did, but 3 seconds too late so by then everything downstream was bailing out.

Thank you for any help!

Solution

Answering my own question which hopefully will help some people in the future. This is based on the excellent advice of Avital Oliver and David Glasser to have a look at Mike Bannister's meteor-async.md. You can read it here: https://gist.github.com/possibilities/3443021

Accounts.onCreateUser(function(options,user){
    _.extend(user,{
        balance: getBalance(),
    });
  return user;
});


function getBalance() {
  var Future = Npm.require("fibers/future");
  var fut = new Future();
  BANK.getBalance(function(err, bal) {
    if (err) return console.log(err);
    fut.return(bal);
  });
  return fut.wait();
}

I believe there's an even better way to handle this, which is directly by wrapping the BANK API in Futures within the npm package itself, as per this example (from Avital Oliver): https://github.com/avital/meteor-xml2js-npm-demo/blob/master/xml2js-demo.js

I hope it helps!