[angularjs] Angular JS update input field after change

I'm trying to build a simple calculator in Angular in which I can override the total if I want. I have this part working but when I then go back to enter in a number in fields one or two the total isn't updated in the field.

Here is my jsfiddle http://jsfiddle.net/YUza7/2/

The form

<div ng-app>
  <h2>Calculate</h2>

  <div ng-controller="TodoCtrl">
    <form>
        <li>Number 1: <input type="text" ng-model="one">  
        <li>Number 2: <input type="text" ng-model="two">
        <li>Total <input type="text" value="{{total()}}">       
        {{total()}}
    </form>
  </div>
</div>

The javascript

function TodoCtrl($scope) {
    $scope.total = function(){
        return $scope.one * $scope.two;
    };
}

You just need to correct the format of your html

<form>
    <li>Number 1: <input type="text" ng-model="one"/> </li>
    <li>Number 2: <input type="text" ng-model="two"/> </li>
        <li>Total <input type="text" value="{{total()}}"/>  </li>      
    {{total()}}

</form>

http://jsfiddle.net/YUza7/105/

I'm guessing that when you enter a value into the totals field that value expression somehow gets overwritten.

However, you can take an alternative approach: Create a field for the total value and when either one or two changes update that field.

<li>Total <input type="text" ng-model="total">{{total}}</li>

And change the javascript:

function TodoCtrl($scope) {
    $scope.$watch('one * two', function (value) {
        $scope.total = value;
    });
}

Example fiddle here.

I wrote a directive you can use to bind an ng-model to any expression you want. Whenever the expression changes the model is set to the new value.

 module.directive('boundModel', function() {
      return {
        require: 'ngModel',
        link: function(scope, elem, attrs, ngModel) {
          var boundModel$watcher = scope.$watch(attrs.boundModel, function(newValue, oldValue) {
            if(newValue != oldValue) {
              ngModel.$setViewValue(newValue);
              ngModel.$render();
            }
          });

          // When $destroy is fired stop watching the change.
          // If you don't, and you come back on your state
          // you'll have two watcher watching the same properties
          scope.$on('$destroy', function() {
              boundModel$watcher();
          });
        }
    });

You can use it in your templates like this:

 <li>Total<input type="text" ng-model="total" bound-model="one * two"></li>      

Create a directive and put a watch on it.

app.directive("myApp", function(){
link:function(scope){

    function:getTotal(){
    ..do your maths here

    }
    scope.$watch('one', getTotals());
    scope.$watch('two', getTotals());
   }

})