ko.extenders = {
    'throttle': function(target, timeout) {
        // Throttling means two things:

        // (1) For dependent observables, we throttle *evaluations* so that, no matter how fast its dependencies
        //     notify updates, the target doesn't re-evaluate (and hence doesn't notify) faster than a certain rate
        target['throttleEvaluation'] = timeout;

        // (2) For writable targets (observables, or writable dependent observables), we throttle *writes*
        //     so the target cannot change value synchronously or faster than a certain rate
        var writeTimeoutInstance = null;
        return ko.dependentObservable({
            'read': target,
            'write': function(value) {
                clearTimeout(writeTimeoutInstance);
                writeTimeoutInstance = setTimeout(function() {
                    target(value);
                }, timeout);
            }
        });
    },

    'notify': function(target, notifyWhen) {
        target["equalityComparer"] = notifyWhen == "always" ?
            null :  // null equalityComparer means to always notify
            valuesArePrimitiveAndEqual;
    }
};

var primitiveTypes = { 'undefined':1, 'boolean':1, 'number':1, 'string':1 };
function valuesArePrimitiveAndEqual(a, b) {
    var oldValueIsPrimitive = (a === null) || (typeof(a) in primitiveTypes);
    return oldValueIsPrimitive ? (a === b) : false;
}

function applyExtenders(requestedExtenders) {
    var target = this;
    if (requestedExtenders) {
        ko.utils.objectForEach(requestedExtenders, function(key, value) {
            var extenderHandler = ko.extenders[key];
            if (typeof extenderHandler == 'function') {
                target = extenderHandler(target, value) || target;
            }
        });
    }
    return target;
}

ko.exportSymbol('extenders', ko.extenders);
