jQuery Monitor Plugin

<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.2.min.js"></script>
<script>
/**
* @Name: jQuery Monitor plugin
* @Version: 1.0.0
* @License: MIT
* @Copyright: Andrei Razumkou
* @Date: 07 Aug 2011
* @Author: Andrei Razumkou nix.d3v(at)gmail.com
* @Description: Simple generic monitoring plugin. Created to track changes in values
* returned by any user defined callback function.
*/

(function(factory) {
    if (typeof define === 'function' && define.amd) {
        // AMD. Register as an anonymous module.
        define(['jquery'], factory);
    } else {
        // Browser globals
        factory(jQuery);
    }
}( function( $ ) {
  var interval = 100;//milliseconds
var callbacks = [];
var started = false;
var stop = false;
var startMainLoop = function(){
(function() {
/*
* Checking if value has changed.
*/
var len = callbacks.length;
for(var i=0;i<len;i++){
var newVal = callbacks[i][1]();
if (callbacks[i][0] !== newVal) {
callbacks[i][0] = newVal;
callbacks[i][2]();
}
}

/*
* Starting the loop automatically if not yet started.
*/
if(!stop) {
setTimeout(arguments.callee, interval);
} else {
started = false;
}
})();

started = true;
}

var methods = {

/*
* @param execCallback - function to execute when valCallback changes
* @param valCallback - function that return a value to be compared to the previous value
*/
add : function(valCallback, execCallback) {
var valueNow = valCallback();
var newElem = [valueNow, valCallback, execCallback]
callbacks.push(newElem);
if(started === false){
stop=false;
startMainLoop();
}
},

/*
* Stort polling (if was stopped before for some reason)
*/
start: function() {
stop = false;
if(started === false){
startMainLoop();
}
},

/*
* Clear the callbacks and stops the loop
*/
clear: function(){
callbacks = [];
stop=true;
},

/*
* Stop polling
*/
stop : function() {
stop = true;
}
};


/*
* Entry point - generic functions
*/
$.monitor = function( method ) {
if ( methods[method] ) {
return methods[method].apply( this, Array.prototype.slice.call( arguments, 1 ));
} else {
$.error( 'Method ' +  method + ' does not exist on jQuery.monitor' );
}
};
}));
</script>
<script>
    $.monitor('add', function(){return $("#site-logo").val()}, function(){
                 
        alert("i change");

    });                                            
</script>

<input type="input" name="site-logo" id="site-logo" placeholder="Browse your image" />                          
<div id="uploaded-img-preveiw"><img style="max-width:400px" id="logo-preview" src="<?php echo "test" ?>" alt="image preview"/></div>