如何根据用户选择更新我的税款

Question

I'm trying to figure out how to update my tax percentage when user selects a particular state. Here's my code (stripped of unrelated data):

<select data-bind="event: {change: checkTax }">
    <option value="PA">PA</option>
    <option value="FL">FL</option>
</select>

<span data-bind="text: taxedItems"></span>

var MainViewModel = function() {
    var self = this;
    var response = arrayFromJson;
    // ...

    self.checkTax = function (viewModel, event) {
        if (event.target.value === "FL") {
            userTax = "this should appear";
        }
    };

    self.taxedItems = ko.pureComputed(function() {
        taxcost = userTax;
        return taxcost;
    });

    self.CartItems($.map(response, function(item) {
        return new AppViewModel(item);
    }));
};

function AppViewModel(data) {
    var self = this;
    // ...
}

ko.applyBindings(new MainViewModel());

At first taxedItems should be null, then after the user changes it to FL , taxedItems should say "this should appear" but it doesn't it just bypasses updating taxedItems altogether. Does anyone know why this is?

Answer 1

if you want to have taxedItems in your mainViewModel you can do this :
VM :

 var MainViewModel = function () {
    var self = this;
    self.CartItems = ko.observableArray([]);
    self.States = ko.observableArray([{Value:"CA",Name:"CA"},
                                      {Value:"FL",Name:"FL"},
                                      {Value:"NY",Name:"NY"},
                                      {Value:"VG",Name:"VG"},
                                      {Value:"TX",Name:"TX"}
                                    ]);

    var response = [{ }];
    self.CartItems($.map(response, function (item) {
      return new AppViewModel(item);
    }));


    self.TaxedItems = ko.observable();
    self.SelectedState = ko.observable(0);

    self.SelectedState.subscribe(function (StateValue) {
        if(StateValue ==='FL'){
            self.TaxedItems('this should appear');
        }else{
           self.TaxedItems('');
        }
    })
  }

  function AppViewModel(data) {
    var self = this;
    // ...
}

  ko.applyBindings(new MainViewModel());

View :

<select data-bind="foreach: States ,value:SelectedState">
<option data-bind="visible:$index()==0" value="0" disabled>Select Tax State..</option>
  <option data-bind="value: Value,text:Name"></option>
</select>
<br>

<span data-bind="text: TaxedItems"></span>

Example : http://jsfiddle.net/GSvnh/5095/