Get previous value of an observable in subscribe of same observable

ko.subscribable.fn.subscribeChanged = function (callback) {
    var oldValue;
    this.subscribe(function (_oldValue) {
        oldValue = _oldValue;
    }, this, 'beforeChange');

    this.subscribe(function (newValue) {
        callback(newValue, oldValue);
    });
};

Use the above like this:

MyViewModel.MyObservableProperty.subscribeChanged(function (newValue, oldValue) {

});