yeoman generator: copy or template doesn't work from inside async callback

Question

Inside a yeoman generator I am trying to do a conditional copy depending on the state of an external network resource. My problem is that the yeoman copy command (src.copy and template too for that matter) does not seem to do anything when invoked inside of an async callback, such as one from a http request.

Example code, inside of the yeoman.generators.NamedBase.extend block:

main: function(){  
//-> here this.copy('inlocation','outlocation') works as expected
var that = this;

var appName = ...
var url = ...

var req = http.request(url, function(res){
//-> here that.copy('inlocation','outlocation') DOES NOT work
    res.on('data', function (data) {
        //console.log('Response received, onData event');
        //-> here that.copy('inlocation','outlocation') DOES NOT work
    });
    //-> here that.copy('inlocation','outlocation') DOES NOT work
});
req.on('error',function(error){
    //...
});
req.end();
//-> here this.copy('inlocation','outlocation') works as expected, once again

Note the locations marked by '//-->' comments for points of reference - when it works, it works as expected. When it doesn't, there's no output on console whatsoever (so that.copy seems to exist as a function, in fact I can assert that typeof that.copy === 'function' !), no error messages, just no file created (the usual file create message is missing too which is a characteristic of the properly working command).

Using call or apply to pass an explicit this reference to the functions didnt change the behaviour, nor did binding this to the async functions.

What is the explanation to this behaviour, and how can I make copy calls in this async manner?

Answer 1

As per Eric MORAND's comment, I'll post the solution I found as a separate answer, instead of an edit to the original post, hopefully it'll be easier to find:

I've found a solution, using the async() function of the yeoman RunContext. (see the api docs here) The following line at the beginning of the async code:

var done = this.async();

then a call to done() right before I wanted to run copy made it behave as originally expected.