Watch class change with angular

http://angularjs.org/

HTML

<script src="//netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/css/bootstrap-combined.min.css">
<div ng-controller="MyCtrl">
    <div class="btn-group" data-toggle="buttons-checkbox">
    <button type="button" class="btn btn-primary" data-on-class-change="">Left</button>
    <button type="button" class="btn btn-primary">Middle</button>
    <button type="button" class="btn btn-primary">Right</button>
    </div>
</div>

JavaScript

var myApp = angular.module('myApp',[]);

myApp.directive('onClassChange', function() {
    return {
        link: function (scope, element, attrs) {
            attrs.$observe("class", function(value) {
                alert(value);
            });
        }
    };

});
//myApp.factory('myService', function() {});

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