AngularJS: add attribute based on watch -- From http://stackoverflow.com/questions/14886174/angular-equivalent-of-ngclass-for-custom-attributes

attribute.js

app.directive('bubble', function() {
    return function(scope, element, attrs) {
        // Whatever the data-bubble attr is set to will
        // be the expression we watch.
        var expr = attrs.bubble;

        scope.$watch(expr, function(val) {
            if (val > 0) {
                element.attr('data-bubble', val);
            } else {
                element.removeAttr('data-bubble');
            }
        });

    };
});