Edit in JSFiddle

// Smart Observable
ko.smartObservable = function (initialValue, functions) {
    var _temp = initialValue;
    var _value = ko.observable(initialValue);

    var result = ko.computed({
        read: function () { return _value(); },
        write: function (newValue) {
            var shouldUpdate = true;
            _temp = newValue;
            if (functions) {
                for (var i = 0; i < functions.length; i++) {
                    if (shouldUpdate && typeof (functions[i]) == "function")
                        shouldUpdate = functions[i](newValue, this.parent);
                    if (!shouldUpdate) break;
                }
            }
            if (shouldUpdate) {
                if (_temp !== _value()) _value(_temp);
            }
            else {
                _value.valueHasMutated();
                _temp = _value();
            }
        }
    });
    return result;
};

//model from the KnockoutJS tutorial
function AppViewModel() {
    this.firstName = ko.smartObservable("Bert",[validateName,confirmChange]);
    this.lastName = ko.observable("Bertington");
    this.fullName = ko.computed(function() {
        return this.firstName() + " " + this.lastName();    
    }, this);
}

// Activates knockout.js
ko.applyBindings(new AppViewModel());


function validateName(value){
    if (value.length==0){
        alert("Name cannot be empty");
        return false;
    }
    return true;
}

function confirmChange(value){
    return confirm("Do you want to proceed with change?");
}

<!-- This is a *view* - HTML markup that defines the appearance of your UI -->
Enter an new value in the First Name field and get alert if string is empty or confirmation request otherwise.


<p>First name: <input data-bind="value: firstName" /></p>
<p>Last name: <input data-bind="value: lastName" /></p>

<p>Full name: <strong data-bind="text: fullName"></strong></p>