What is MVVM?

MVVM - Model View ViewModel

Advantage : 

Better Separation of Concerns

In a project built with the Model-View-Controller pattern, you are often faced with the question which code goes where. Code that doesn’t fit or belong in the model or view layer is often put in the controller layer. This inevitably leads to fat controllers that are difficult to test and manage.

The MVVM pattern presents a better separation of concerns by adding view models to the mix. The view model translates the data of the model layer into something the view layer can use. The controller is no longer responsible for this task.

Improved Testability

View controllers are notoriously hard to test because of their relation to the view layer. By migrating data manipulation to the view model, testing becomes much easier. Testing view models is easy. Because a view model doesn’t have a reference to the object it is owned by, it easy to write unit tests for a view model.

Another, and often overlooked, benefit of MVVM is improved testability of view controllers. The view controller no longer depends on the model layer, which makes them easier to test.

Transparent Communication

The responsibilities of the view controller are reduced to controlling the interaction between the view layer and the model layer, glueing both layers together.

The view model provides a transparent interface to the view controller, which it uses to populate the view layer and interact with the model layer. This results in a transparent communication between the four layers of your application.

There are some rules you know when using MVVM

Rule1 : The view does not know about the controller it is owned by. Remember that views are supposed to be dumb. They only know how to present the data they are given to the user.

Rule 2 : The controller does not know about the model. This is what separates MVC from MVVM.

Rule 3 : The model does not know about the view model it is owned by.

Rule 4 : The view model owns the model. When using the Model-View-Controller pattern, the model is usually owned by the controller.

Rule 5 : The view controller owns the view. This relationship remains unchanged when using the Model-View-ViewModel pattern.

Rule 6 : And, finally, the controller owns the view model. It interacts with the model layer through one or more view models.