What is MVC, in relation to the Zend framework?

Question

I'm learning the Zend Framework and it uses a MVC model.

I still have not got my head around what MVC Model, View, Controller is.

What are the three different areas for and what would the program flow look like?

Answer 1

M - Models - are often the biggest source of confusion. These are the parts of your application that do all the 'heavy lifting' - they handle database access, perform the complex application-specific logic and are responsible for most of what your application 'does'. Unlike Views and Controllers, the Zend Framework doesn't have a base class for Models - this is because there's no real consistency in what they do. Some frameworks (like Ruby on Rails) try to present some sort of database-wrapper as a base for Model, but there are many cases (3rd party feed/API, static files, non-persistent calculations, concepts that span multiple tables...) for which this is, at best, a misleading practice. The Model is the part of the application where you're still forced to program and the framework can't really save you from it.

V - Views - are the simplest components here. They should be simple PHP/HTML templates. They're given view objects, arrays, strings, etc. which they then put into a page. There shouldn't be much (if any) complex logic here - loop over these, display this (if defined), zebra stripe this table and whatnot. There's some magic happening with View Helpers (such as the helper that magically renders a Zend_Form), but that's not essential to understanding the overall system.

C - Controllers - In the broadest sense, the controller is responsible for taking user requests, sending them off to Model objects and preparing Models to be handed to the Views. It's the glue that holds everything together. If you're using Zend MVC, you're concerned with 2 controllers - Zend_Controller_Front and Zend_Controller_Action.

Zend_Controller_Front (which you get 'for free' if you use Zend_Layout::startMVC()) is a single point of entry for your application - it handles the raw user requests and translates URLs into an Action to call. There are various places to 'plug-in' to this to handle things like authentication and access restrictions, but, at the core it's just the 'traffic cop' at the front gate directing incoming requests.

Zend_Controller_Action is the base class for actions - essentially an Action represents something your application does (log in, list blog entries, launch an ICBM, order a pizza...), but is not directly responsible for actually doing it. The Action Controllers are pretty boring - they pull values out of forms and URLs, call a few methods on Model classes to actually perform the action and push the results out into the view. As has been said before, they're the 'glue' that holds Models and Views together.

A rough test to see if you're splitting things along the right lines is to envision a major change to your site. A visual redesign would almost entirely be handled in the Views. Moving all your URLs would change your Controllers. Converting from a web app to a GUI app would replace the views and controllers, but your model would still be mostly unchanged. If you rewrite your models, you have a whole new application.

Answer 2

There are several other questions on Stackoverflow that give an explanation of the MVC concept:

• What are MVP and MVC and what is the difference?
• What is MVC (Model View Controller)?

A very good explanation of the concept can be found on Wikipedia:

Model-view-controller (MVC) is an architectural pattern used in software engineering. Successful use of the pattern isolates business logic from user interface considerations, resulting in an application where it is easier to modify either the visual appearance of the application or the underlying business rules without affecting the other. In MVC, the model represents the information (the data) of the application; the view corresponds to elements of the user interface such as text, checkbox items, and so forth; and the controller manages the communication of data and the business rules used to manipulate the data to and from the model.

In relation to Zend Framework:

• models are generally presented by extensions of the Zend_Db_Table class
• views are presented as *.phtml files in your defined scripts folder, which are handled by the Zend_View class.
• controllers are defined by extensions of the Zend_Controller_Action class.