knockout.js computed array not firing on change

Question

I am trying to create an editable grid with a computed observableArray in MVC 4 and Knockout.js. The computed() method gets called when the array is first loaded but not when any data changes on the grid via user editing.

The ViewModel:

function CarsVm() {
var self = this;

self.url = 'http://localhost/FederatedWcf/Services/RestCars.svc/json/cars';

self.cars = ko.observableArray([]);

self.car = ko.computed(function () {
    if ( this.cars().length > 0)
        return this.cars()[this.cars().length-1].Make;
    else {
        return "";
    }
}, this);

self.GetCars = function () {
    var count = 0;
    $.getJSON(self.url, function (allData) {
        var mappedCars = $.map(allData.JsonCarsResult, function (item) {
            console.log(count);
            console.log(item);
            count = count + 1;
            return new Cars(item);
        });
        self.cars(mappedCars);
        console.log(self.cars());
    });
    $.ajaxSetup({ cache: false });
};

}

And the html fragment:

<table>
<thead>
    <tr>
        <th></th>
        <th>Year</th>
        <th>Make</th>
        <th>Model</th>
        <th></th>
    </tr>
</thead>
<tbody data-bind="foreach: cars">
    <tr>
        <td><span style="display:none" data-bind="text:Id"></span></td>
        <td><input data-bind="value:Year"></input></td>
        <td><input data-bind="value:Make"></input></td>
        <td><input data-bind="value:Model"></input></td>
        <td><button data-bind="click: $root.removeCar">-</button></td>
    </tr>
</tbody>
</table>
<span data-bind="text:car"></span>

If I edit the last Make in the grid, I expected the data bound car element to be updated but it is not.

How do I detect changes on a grid, like during an onblur event, in a knockout observerableArray?

Answer 1

Make sure to mark each property in Cars as observable or knockout won't notice they changed.

function Cars(data) {
   var self = this;
   self.Id = data.Id;
   self.Year = ko.observable(data.Year);
   self.Make = ko.observable(data.Make);
   self.Model = ko.observable(data.Model);
}