MVC architecture of Yii Framework

As mentioned previously, Yii is an MVC framework and it provides an explicit
folder structure for each piece of model, view, and controller code. Before we start
building our first Yii application, we need to define a few key terms, and look at
how Yii implements and enforces this MVC architecture.

The model
Typically in an MVC architecture, the model is responsible for maintaining state.
Thus, it should encapsulate the business rules that apply to the data that defines
this state. A model in Yii is any instance of the framework class CModel or its child
class. A model class typically comprises data attributes that can have separate labels
(something user-friendly for the purpose of display), and can be validated against a
set of rules defined in the model. The data that makes up the attributes in the model
class could come from a row of a database table or from the fields in a user input form.
Yii implements two kinds of models: The form model (CFormModel class) and the
active record model (CActiveRecord class). They both extend from the same base
class CModel. CFormModel represents a data model that collects inputs in HTML
form. It encapsulates all the logic for form field validation and any other business
logic that may need to be applied to the form field data. It can then store this data
in memory, or with the help of an active record model, store data in a database.
Active Record (AR) is a design pattern used to abstract database access in an
object-oriented fashion. Each AR object in Yii is an instance of CActiveRecord or
its child class that wraps a single row in a database table or view, encapsulates all
the logic and details around database access, and houses much of the business logic
that is required to be applied to that data. The data field values for each column in
the table row are represented as properties of the AR object. AR is described in more
detail a little later.

The view
Typically, the view is responsible for rendering the user interface, based on the
data in the model. A view in Yii is a PHP script that contains user interface related
elements, often built using HTML, but can also contain PHP statements. Usually any
PHP statements within the view are very simple conditional or looping statements,
or refer to other Yii UI-related elements such as HTML helper class methods or
prebuilt widgets. More sophisticated logic should be separated from the view and
placed appropriately in either the model (if dealing directly with the data), or in the
controller for a more general business logic.


The controller
The controller is our main director of a routed request and is responsible for taking
user input, interacting with the model, and instructing the view to update and
display appropriately. A controller in Yii is an instance of CController or its child.
When a controller runs, it performs the requested action, which then interacts with
needed models and renders an appropriate view. An action, in its simplest form, is
a controller class method whose name starts with the word action.