String Interpolation Won't Work when Setting Attribute Values on a Custom Directive

Question

Should I be able to pass attributes from the $scope into a custom directive like this?

        <div ng-repeat="ratable in ratables">
            <div>How much do you like {{ratable.name}}?</div>

            <!-- CONFUSED - First element does not work, second element does -->
            <rating currentRatingValue="{{ratable.currentvalue}}" maxRatingValue="10"></rating>
        </div>

The hardwired "10" passes through into the directive just fine, but the string interpolation when I try to pass {{ratable.currentvalue}} never seems to occur. Am I doing something obviously wrong?

http://jsfiddle.net/ADukg/2168/

var myApp = angular.module('myApp',[])
    .directive("rating", function () {
        return {
            restrict: "E",
            scope: {},
            template: "<div class='rating'>Current rating: {{currentratingvalue}} out of {{maxratingvalue}}</div>",
                link: function (scope, element, attributes) {
                    console.log(attributes.currentratingvalue);    // Does not work but seems like it should
                    console.log(attributes.maxratingvalue);        // Does work
                }
            };    
        });

function MyCtrl($scope) {
    $scope.name = 'Superhero';

    $scope.ratables = [
        { name: "sledding", currentvalue: 3, maxvalue: 5 },
        { name: "water skiing", currentvalue: 7, maxvalue: 10 },
        { name: "whitewater rafting", currentvalue: null, maxvalue: 10 }
    ];
}

<div>
    <div ng-controller="MyCtrl">
      Hello, {{name}}!

        <div ng-repeat="ratable in ratables">
            <div>How much do you like {{ratable.name}}?</div>

            <!-- CONFUSED - First element does not work, second element does -->
            <rating currentRatingValue="{{ratable.currentvalue}}" maxRatingValue="10"></rating>
        </div>
    </div>
</div>

Answer 1

A few things:

• directive attributes in HTML need to use kebab-case
• you don't need an isolate scope (in fact it is causing problems); instead use scope: true
• you need to set local scope properties so your template can pick them up
• $observe must be used to get the value of interpolated attributes (i.e., attributes that use {{}}s)

HTML:

<rating current-rating-value="{{ratable.currentvalue}}" max-rating-value="10"></rating>

Directive:

link: function (scope, element, attributes) {
   attributes.$observe('currentRatingValue', function(newValue) {
      console.log('newValue=',newValue);
      scope.currentRatingValue = newValue
   })
   scope.maxRatingValue = attributes.maxRatingValue;
   console.log('mrv=',attributes.maxRatingValue);
}

Fiddle

---

Here is a version that uses an isolate scope:

.directive("rating", function () {
   return {
      restrict: "E",
      scope: { currentRatingValue: '@',
               maxRatingValue:     '@' },
      template: "<div class='rating'>Current rating: {{currentRatingValue}}"
              + " out of {{maxRatingValue}}</div>",
  };    
});

Fiddle

If you want to see the value of an isolate scope property in your link function, you'll need to use $observe or $watch because we used '@'. If you use '=' (for two-way databinding), you don't need to use $observe or $watch.