How do I subclass the jQuery ajax object or reuse a common callback?

I'm sorry for ambiguous title, but this question is hard for me to put into words, I can only seem to describe it with code.

I have the following function

function initRPC(procedureName, parameters)
{
    var parcel = //some code that parcels up the procedure name and params

    $.get(
        'url'     : 'rpcserver.php',
        'data'    : parcel,
        'success' : function(returnXML) {
            unparcel = //some code to unwrap the returned data

            /**
             * now instance specific code
             */
        }
    );
}

Where it says now instance specific code is where I want to specific code, and to reuse the rest as initRPC will be called frequently.

For example I'd like to initiate the initRPC like this:

function getUsername()
{
    alert('Your name is: ' + initRPC('getUsername', { id: someVar }));
}

or like this (in this example the signature of the initRPC command would extend 开发者_JAVA技巧to include the passed function reference, but I don't know how to pass that reference to the $.post callback:

function getUsername()
{
    initRPC('getUsername', {id: someVar}, function todoAfterUnWrap(data) {
        alert('Your name is: ' + data);
    });
}

I can't see a way of doing what I hoped, it feels like I will always have to repeat the $.ajax() call with unwrapping and wrapping in each function (like getUsername()).

Rephrased: How do I encapsulate data preparation code, the AJAX request, and unwrapping code together and then allow for instance specific code to run after those. Asynchronously.

[edit]The idea coming to me now is the typical way of reusing code in this situation is as follows:

function getUsername()
{
    var request = prepareRequest('getUsername', { id: someVar });

    $.get(
        'url'     : 'rpcserver.php',
        'data'    : request
        'success' : function(response) {
            response = decodeResponse(response);
            /* instance specific code */
        }
    );
}

Perhaps this is the only way, it just seems to me to require repeating much of the same code in each function like getUsername()

If the idea is to post a callback function, that's fairly easy. You could keep your code as provided in this example:

function getUsername()
{
    initRPC('getUsername', {id: someVar}, function todoAfterUnWrap(data) {
        alert('Your name is: ' + data);
    });
}

And write initRPC like this:

function initRPC(procedureName, parameters, callbackFn)
{
    var parcel = //some code that parcels up the procedure name and params

    $.get(
        'url'     : 'rpcserver.php',
        'data'    : parcel,
        'success' : function(returnXML) {
            unparcel = //some code to unwrap the returned data
            callbackFn(unparcel);
    });
}