Angularjs directive with prism code highlight

ng-prism.js

/**
 * Usage 
 * 
 * <ng-prism class="language-css">
 *      body {
 *         color: red; 
 *      }
 * </ng-prism>
 * 
 * the files prismjs and prism css must be included in HTML, use class "language-XXX" to specify language
 * */
app.directive('ngPrism',['$interpolate', function ($interpolate) {
        "use strict";
        return {
            restrict: 'E',
            template: '<pre><code ng-transclude></code></pre>',
            replace: true,
            transclude: true,
            link: function (scope, elm) {
                var tmp = $interpolate(elm.find('code').text())(scope);
                elm.find('code').html(Prism.highlightElement(tmp).value);
            }
        };
    }]);