ngValueDirective = function() {
  return {
    priority: 100,
    compile: function(tpl, tplAttr) {
      if (CONSTANT_VALUE_REGEXP.test(tplAttr.ngValue)) {
        return function ngValueConstantLink(scope, elm, attr) {
          attr.$set('value', scope.$eval(attr.ngValue));
        };
      } else {
        return function ngValueLink(scope, elm, attr) {
          scope.$watch(attr.ngValue, function valueWatchAction(value) {
            attr.$set('value', value);
          });
        };
      }
    }
  };
}
@ngdoc directive
@name ng.directive:ngValue

@description
Binds the given expression to the value of `input[select]` or `input[radio]`, so
that when the element is selected, the `ngModel` of that element is set to the
bound value.

`ngValue` is useful when dynamically generating lists of radio buttons using `ng-repeat`, as
shown below.

@element input
@param {string=} ngValue angular expression, whose value will be bound to the `value` attribute
  of the `input` element

@example
    <doc:example>
      <doc:source>
       <script>
          function Ctrl($scope) {
            $scope.names = ['pizza', 'unicorns', 'robots'];
            $scope.my = { favorite: 'unicorns' };
          }
       </script>
        <form ng-controller="Ctrl">
          <h2>Which is your favorite?</h2>
            <label ng-repeat="name in names" for="{{name}}">
              {{name}}
              <input type="radio"
                     ng-model="my.favorite"
                     ng-value="name"
                     id="{{name}}"
                     name="favorite">
            </label>
          <div>You chose {{my.favorite}}</div>
        </form>
      </doc:source>
      <doc:protractor>
        var favorite = element(by.binding('my.favorite'));

        it('should initialize to model', function() {
          expect(favorite.getText()).toContain('unicorns');
        });
        it('should bind the values to the inputs', function() {
          element.all(by.model('my.favorite')).get(0).click();
          expect(favorite.getText()).toContain('pizza');
        });
      </doc:protractor>
    </doc:example>