When would I use JQuery.Callbacks?

Question

I was looking through new stuff added to jQuery 1.7 and I saw they now have jQuery.Callbacks() http://api.jquery.com/jQuery.Callbacks/.

The documentation shows you how

Answer 1

An (almost) out of the box jQuery Pub/Sub system

This is IMHO the most interesting application, and since it was not clearly stated in the answers (although some do make allusion to the usage), I am adding it to this relatively old post.

NB: The usage is clearly indicated, with an example, in the jQuery docs, but I guess it was added after the question was posted.

Pub/sub, aka the observer pattern, is a pattern which promotes loose coupling and single responsibility in an application. Rather than having objects calling directly the methods of other objects, objects instead subscribe to a specific task or activity and are notified when it occurs. For a more detailed explanation of the benefits of using the Pub/Sub pattern, you can check Why would one use the Publish/Subscribe pattern (in JS/jQuery)?.

Sure, this was possible with custom events using trigger, .on() and .off(), but I find jQuery.Callbacks to be a lot more suitable to the task, producing cleaner code.

Here is the example snippet from the jQuery documentation:

var topics = {};
 
jQuery.Topic = function( id ) {
    var callbacks,
        method,
        topic = id && topics[ id ];
 
    if ( !topic ) {
        callbacks = jQuery.Callbacks();
        topic = {
            publish: callbacks.fire,
            subscribe: callbacks.add,
            unsubscribe: callbacks.remove
        };
        if ( id ) {
            topics[ id ] = topic;
        }
    }
    return topic;
};

And a usage example:

// Subscribers
$.Topic( "mailArrived" ).subscribe( fn1 );
$.Topic( "mailArrived" ).subscribe( fn2 );
$.Topic( "mailSent" ).subscribe( fn1 );
 
// Publisher
$.Topic( "mailArrived" ).publish( "hello world!" );
$.Topic( "mailSent" ).publish( "woo! mail!" );
 
// Here, "hello world!" gets pushed to fn1 and fn2
// when the "mailArrived" notification is published
// with "woo! mail!" also being pushed to fn1 when
// the "mailSent" notification is published.
 
/*
output:
hello world!
fn2 says: hello world!
woo! mail!
*/