Change observable value on dropdown change Knockout Js

Answer 1

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);