Directive scope variable is not getting update in angularjs

I have declared a controller in directive. I am performing add and remove operation on the list of data. But when i add the data in list the scope global variable is not getting update. I am using the service to get data. My service is // an angular storage service

var storageService = angular.module('storageService', [])
    .service('storage', function(){

    var todoKey = "todo_data";

    this.getTodos = function(){
        todolist = (localStorage.getItem(todoKey)!==null)?JSON.parse(localStorage.getItem(todoKey)) : [];
        return todolist     
        }
    this.addTodo = function(taskName){
        console.log("storage"+ taskName.text);
        todoList = this.getTodos()
        console.log(todoList);
        todoList.push(taskName);
        console.log(todoList);
        localStorage.setItem(todoKey, JSON.stringify(todoList));
      }
   this.removeTodo = function(taskName){
    var todoList = this.getTodos();
    todoList.splice($.inArray(taskName, todoList), 1);
    localStorage.setItem(todoKey, JSON.stringify(todoList));
     }

  this.completeTodo = function(task){
      task.completed = true;
  } 
});

I am calling this service through angular directive controller.My directive is

app.directive("todoList", function(){
    return{
       restrict: 'E',
       replace: 'true',
       templateUrl: 'partial_template/todolist.html',
       controller: function($scope, $element, storage){
       $scope.todos = storage.getTodos();
       $scope.addTodo = function(taskName) {
            task = new TODO.task(taskName);
            storage.addTodo(task);
            // create new object
            $scope.taskName = "";
        };

        $scope.removeTodo = function(taskName){
            // remove the task from the todolist
            storage.removeTodo(taskName);
        };
        $scope.completeTodo = function(taskName){
            // change the status of the task
            storage.completeTodo(task)
        };
    }
};

});

When i add todo item it doesn't not reflect on $scope.todos. If we update it inside the function then it is getting update.But i think it should be reflect the change outside the function.