Change observable value on dropdown change Knockout Js

Question

I have this dropdown which have options if vehicle is new or used.

<select name="VehicleType" id="vehicleTypeDropdown" data-bind="value: VehicleType">    
    <option value="New" selected="selected">Nuevo</option>
    <option value="Used">Usado</option>
</select> `

And this input:

<input type="text" name="Mileage" data-bind="disable: VehicleType() === 'New',value:  Mileage" class="input-large"> `

If the value in the dropdown selected is New the input must be disabled, and if used the input should be enabled, but if I enter a value the observable will grab this value and if I change the dropdown value to new the observable must become zero.

Solution 1

A manual subscription in your view model is a good way to handle something like this. The subscription might look like:

this.VehicleType.subscribe(function(newValue) {
    if (newValue === "New") {
        this.Mileage(0);
    }
}, this);

Here is a sample using it: http://jsfiddle.net/rniemeyer/h4cKC/

The HTML:

<select name="VehicleType" id="vehicleTypeDropdown" data-bind="value: VehicleType">
    <option value="New" selected="selected">Nuevo</option> 
    <option value="Used">Usado</option> 
</select>

<input type="text" name="Mileage" data-bind="disable: VehicleType() === 'New', value: Mileage" class="input-large">
<hr/>

<pre data-bind="text: ko.toJSON($root, null, 2)"></pre>

The JS:

var ViewModel = function() {
    this.VehicleType = ko.observable();
    this.Mileage = ko.observable();

    this.VehicleType.subscribe(function(newValue) {
        if (newValue === "New") {
            this.Mileage(0);
        }
    }, this);
};

ko.applyBindings(new ViewModel());

Solution 2

If you're using the ko mapping plugin you can intercept the creation of a new object and set up the subscription there. If you have a whole list of items and you want to perform an action when something changes.

This is a view model for a shopping cart line item (properties such as qty, sku, description, price).

// View Model for an item in my shopping cart
var CartItemViewModel = function(data) 
{
    // map all the properties
    // we could map manually if we want, but that's the whole point of
    // the mapping plugin that we don't need to
    ko.mapping.fromJS(data, {}, this);

    // subscribe to qty change
    this.qty.subscribe(function (newValue)
    {
        alert('qty now ' + this.qty());
        // UpdateCart()

    }, this);    

    // add computed observables if needed
    // ....
}

When you update (or create) your model using the mapping plugin you specify that for a property called 'items' each element in the array should be created with the 'create' function you specify.

    var mapping =
    {
        'items':
        {
            // map cart item 
            create: function (options)
            {
                return new CartItemViewModel(options.data);
            }
        }
    };

    var data = ......  // get your data from somewhere

    // update the CartViewModel using the mapping rules
    ko.mapping.fromJS(data, mapping, rrvm.CartViewModel);