In Angular, we want to keep our controllers lightweight and specific to the view that it is controlling. To accomplish this, we are going to refactor our categories collection into a CategoriesModel. A recurring theme in Angular 2 is that everything is "just a class". When we create our controllers and services in an Angular 2 style, they are also "just a class". We will see this in action as we convert our controller almost verbatim into a service.