Question

I have a computed observable that uses ko.toJS() to post values (inner observables) from the view model. One of the inner observables is bound to a textbox. How can I prevent changes to the textbox from automatically triggering the computed observable (i.e. the postback)?

function viewModel() {
    var self = this;
    self.SearchParams = {
        OrderStatusID: ko.observable(),
        OrderNumber: ko.observable(), // I don't want this observable to trigger the postback
        OrderBy: ko.observable(),
        OrderByDirection: ko.observable(),
        PageIndex: ko.observable(),
        PageSize: ko.observable()
    };

    // This computed observable automatically subscribes to every
    // observable in self.SearchParams. How can I ignore changes to
    // self.SearchParams.OrderNumber?
    ko.computed(function () {
        NS.post({
            url: '/SomeUrl',
            data: ko.toJS(self.SearchParams)
        });
    }).extend({ throttle: 1 });
}

Answer 1

Knockout 2.2+ includes a peek function to access observables without subscribing. So you could do the following:

ko.computed(function () {
    var s = self.SearchParams;
    NS.post({
        url: '/SomeUrl',
        data: {
            OrderStatusID: s.OrderStatusID(), 
            OrderNumber: s.OrderNumber.peek(),
            OrderBy: s.OrderBy(),
            OrderByDirection: s.OrderByDirection(),
            PageIndex: s.PageIndex(),
            PageSize: s.PageSize()
        }
    });
}).extend({ throttle: 1 });

If you’re stuck on Knockout 2.1.0, you can extend observables manually to add peek like this (thanks Ryan Niemeyer):

var backdoorObservable = function(initialValue) {
    var _value = initialValue,
        result = ko.observable(initialValue);

    result.peek = function() { return _value; };

    result.subscribe(function(newValue) {
        _value = newValue;
    });

    return result;
};

And then use backdoorObservable for OrderNumber:

OrderNumber: backdoorObservable()