ViewChangeListeners not working in angular directive

Question

Basically, I need a directive that updates the input class whenever viewValue is not empty. Here is the code.

angular.module('app').directive('input', function() { return { restrict: 'E', require: '^?ngModel', link: function(scope, element, attrs, ngModelController) { if (ngModelController === null) return; var updateClass = function() { if (!(ngModelController.$isEmpty(ngModelController.$viewValue))) { element.addClass('input-has-value'); } else { element.removeClass('input-has-value'); } } ngModelController.$viewChangeListeners.push(function() { updateClass(); }); } }; }); 

But I have something missing, as it never runs the updateClass method. What am I doing wrong? It never runs the updateClass method.