Models, Views and Controllers

Back in 1979, Trygve Reenskaug came up with a new architecture for developing interactive applications. In his design, applications were broken into three types of components: models, views, and controllers.

Models

The model is responsible for maintaining the state of an application. Sometimes this state is transient, lasting for just a couple of interactions with the user. Sometimes the state is permanent and is stored outside the application, often in a database.

A model is more than data. It enforces all the business rules that apply to that data. For example, if a discount shouldn’t be applied to orders of less than $20, the model enforces that constraint. By putting the implementation of these business rules in the model, we make sure that nothing else in the application can make our data invalid. The model acts as both a gatekeeper and a data store.

Views

The view is responsible for generating a user interface, which is normally based on data in the model. For example, say an online store has a list of products to be displayed on a catalog screen. This list is accessible via the model, but it’s the view that formats the list for the end-user. Although ...