1ko.subscribable.fn.subscribeChanged = function (callback) {
2    var savedValue = this.peek();
3    return this.subscribe(function (latestValue) {
4        var oldValue = savedValue;
5        savedValue = latestValue;
6        callback(latestValue, oldValue);
7    });
8};
9
10//example use
11function Mvvm() {
12	var self = this;
13	self.Input = ko.observable();
14  
15    self.Input.subscribeChanged(function (newValue, oldValue) {
16        console.log("new value: " + newValue);
17        console.log("old value: " + oldValue);
18    });
19}
20
21