Waiting on multiple asynchronous calls to complete before continuing

So, I have a page that loads and through jquery.get makes several requests to populate drop downs with their values.

$(function() {
    LoadCategories($('#Category'));
    LoadPositions($('#Position'));
    LoadDepartments($('#Department'));

    LoadContact();
};

It then calls LoadContact(); Which does another call, and when it returns it populates all the fields on the form. The problem is that often, the dropdowns aren't all populated, and thus, it can't set them to the correct value.

What I need to be able to do, is somehow have LoadContact only execute once the other methods are complete and callbacks done executing.

But, I don't want to have to put a bunch of flags in the end of the drop down population callbacks, that I then check, and have to have a recursive setTimeout call checking, prior to calling LoadContact();

Is there something in jQuery that allows me to say, "Execute this, when all of these are done."?

More Info I am thinking something along these lines

$().executeAfter(
    function () {   // When these are done
        LoadCategories($('#Category'));
        LoadPositions($('#Position'));
        LoadDepartments($('#Department'));
    },
    LoadContact // Do this
);

...it would need to keep track of the ajax calls that happen during the execution of the methods, and when they are all complete, call LoadContact;

If I knew how to intercept ajax that are being made in that function, I could probably write a jQuery extension to do this.

My Solution

;(function($) {
    $.fn.executeAfter = function(methods, callback) {

        var stack = [];

        var trackAjaxSend = function(event, XMLHttpRequest, ajaxOptions) {
            var url = ajaxOptions.url;

            stack.push(url);
        }

        var trackAjaxComplete = function(event, XMLHttpRequest, ajaxOptions) {
            var url = ajaxOptions.url;

            var index = jQuery.inArray(url, stack);

            if (index >= 0) {
                stack.splice(index, 1);
            }

            if (stack.length == 0) {
                callback();
                $this.unbind("ajaxComplete");
            }
        }

        var $this = $(this);

        $this.ajaxSend(trackAjaxSend)
        $this.ajaxComplete(trackAjaxComplete)

        methods();
        $this.unbind("ajaxSend");
    };
})(jQuery);

This binds to the ajaxSend event while the methods are being called and keeps a list of urls (need a better unique id though) that are called. It then unbinds from ajaxSend so only the requests we care about are tracked. It also binds to ajaxComplete and removes items from the stack as they return. When the stack reaches zero, it executes our callback, and unbinds the ajaxComplete event.

You can use .ajaxStop() like this:

$(function() {
  $(document).ajaxStop(function() {
    $(this).unbind("ajaxStop"); //prevent running again when other calls finish
    LoadContact();
  });
  LoadCategories($('#Category'));
  LoadPositions($('#Position'));
  LoadDepartments($('#Department'));
});

This will run when all current requests are finished then unbind itself so it doesn't run if future ajax calls in the page execute. Also, make sure to put it before your ajax calls, so it gets bound early enough, it's more important with .ajaxStart(), but best practice to do it with both.

Expanding on Tom Lianza's answer, $.when() is now a much better way to accomplish this than using .ajaxStop().

The only caveat is that you need to be sure the asynchronous methods you need to wait on return a Deferred object. Luckily jQuery ajax calls already do this by default. So to implement the scenario from the question, the methods that need to be waited on would look something like this:

function LoadCategories(argument){
    var deferred = $.ajax({
       // ajax setup
    }).then(function(response){ 
       // optional callback to handle this response 
    });
    return deferred;
}

Then to call LoadContact() after all three ajax calls have returned and optionally executed their own individual callbacks:

// setting variables to emphasize that the functions must return deferred objects
var deferred1 = LoadCategories($('#Category'));
var deferred2 = LoadPositions($('#Position'));
var deferred3 = LoadDepartments($('#Department'));

$.when(deferred1, deferred2, deferred3).then(LoadContact);

If you're on Jquery 1.5 or later, I suspect the Deferred object is your best bet: http://api.jquery.com/category/deferred-object/

The helper method, when, is also quite nice: http://api.jquery.com/jQuery.when/