Add callback to 'end' event handler

Currently, it’s not possible to perform asynchronous operations within the end event and manipulate the resulting CSS based on these operations. This causes a lot of problems since node.js and its ecosystem is primarily asynchronous. To mitigate this problem, I propose that a callback should be added like we once had in versions prior to 0.40.0, e.g.:

stylus.on('end', function (css, callback) {
    someAsynchronousOperation(function (err, result) {
        if (err) callback(err);
        callback(null, result); 
    });
});

In case of multiple listeners, it could move to the next listener after the callback has been triggered to not cause any issues with asynchronous writes to the CSS.

[FezVrasta]

Any update on this? it’s quite important to make PostCSS work with Stylus 😕

[sugarshin]

👍