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Model-view-controller

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A simple diagram depicting the relationship between the Model, View, and Controller. Note: the solid lines indicate a direct association, and the dashed lines indicate an indirect association (e.g., observer pattern).

Model-view-controller (MVC) is an architectural pattern used in software engineering. In complex computer applications that present a large amount of data to the user, a developer often wishes to separate data (model) and user interface (view) concerns, so that changes to the user interface will not affect data handling, and that the data can be reorganized without changing the user interface. The model-view-controller solves this problem by decoupling data access and business logic from data presentation and user interaction, by introducing an intermediate component: the controller.

History

The pattern was first described in 1979^[1] by Trygve Reenskaug, then working on Smalltalk at Xerox research labs. The original implementation is described in depth in the influential paper Applications Programming in Smalltalk-80(TM):How to use Model-View-Controller^[2].

Pattern description

It is common to split an application into separate layers: presentation (UI), domain logic, and data access. In MVC the presentation layer is further separated into view and controller. MVC encompasses more of the architecture of an application than is typical for a design pattern.

MODEL definition: The domain-specific representation of the information that the application operates. Domain logic adds meaning to raw data (e.g., calculating whether today is the user's birthday, or the totals, taxes, and shipping charges for shopping cart items).; Many applications use a persistent storage mechanism (such as a database) to store data. MVC does not specifically mention the data access layer because it is understood to be underneath or encapsulated by the Model.
View: Renders the model into a form suitable for interaction, typically a user interface element. Multiple views can exist for a single model for different purposes.
Controller: Processes and responds to events, typically user actions, and may invoke changes on the model.

MVC is often seen in web applications, where the view is the actual HTML page, and the controller is the code that gathers dynamic data and generates the content within the HTML. Finally, the model is represented by the actual content, usually stored in a database or XML files. Though MVC comes in different flavors, control flow generally works as follows:

The user interacts with the user interface in some way (e.g., presses a button).
A controller handles the input event from the user interface, often via a registered handler or callback.
The controller accesses the model, possibly updating it in a way appropriate to the user's action (e.g., controller updates user's Shopping cart).^[3]
A view uses the model (indirectly) to generate an appropriate user interface (e.g., the view produces a screen listing the shopping cart contents). The view gets its own data from the model. The model has no direct knowledge of the view.
The user interface waits for further user interactions, which begins the cycle anew.

By decoupling models and views, MVC helps to reduce the complexity in architectural design, and to increase flexibility and reuse.

Exemplary MVC conceptions of selected frameworks

GUI frameworks

Java: Java Swing

Java Swing is different from the other frameworks, in that it supports two MVC patterns.

Model (Frame level): Like the other frameworks, the design of the real model is usually left to the developer.
Model (Control level): Swing also supports models on the control level. Unlike other frameworks, Swing exposes the internal storage of each control as a model.
View: The view is represented by a class that inherits from Component.
Controller: Java Swing doesn't necessarily use a single controller. Because its event model is based on interfaces, it is common to create an anonymous action class for each event. In fact, the real controller is in a separate thread. It catches and propagates the events to the view and model.

Web frameworks

Combined frameworks

Java: Java Enterprise Edition (Java EE)

Unlike the other frameworks, Java EE defines a pattern for model objects.

Model: The model is commonly represented by entity beans, although the model can be created by a servlet using a business object framework such as Spring.
View: The view in a Java EE application may be represented by a Java Server Page, which may be currently implemented using JavaServer Faces Technology (JSF). Alternatively, the code to generate the view may be part of a servlet.
Controller: The controller in a Java EE application may be represented by a servlet, which may be currently implemented using JavaServer Faces Technology (JSF).

Implementations of MVC as GUI frameworks

Smalltalk's MVC implementation inspired many other GUI frameworks, such as the following:

Core Data framework in Mac OS X, a modern addition to Cocoa.
GNUstep, based on these technologies, also use MVC.
JFace
Microsoft Foundation Classes (MFC) (called Document/View architecture here)
NeXTSTEP development environments encourage the use of MVC. Cocoa
OPENSTEP development environments encourage the use of MVC. Cocoa
Qt Toolkit since Qt4 Release.
Java Swing

Implementations of MVC as web-based frameworks

In the design of web applications, MVC is implemented by web template systems as "View for web" component. MVC is also known as a "Model 2" architecture in Sun parlance. Complex web applications continue to be more difficult to design than traditional applications, and MVC is being pushed as a potential solution to these difficulties.

.NET

Web Client Software Factory
Maverick.NET
Microsoft UIP
Monorail An ActionPack inspired MVC framework from the Castle Project
Igloo software (Experimental) MVC Framework for .Net 2.0

ActionScript

ARP Usable with AIR, Flex, and Flash
Cairngorm Usable with AIR, Flex, and Flash
PureMVC Suitable for use with AIR, Flex, Flash, and any platform running AS3 (no non-native class dependencies)

ColdFusion

Mach-II A framework that focuses on trying to ease software development and maintenance
Fusebox
Model-Glue
ColdBox

Erlang

ErlyWeb

Java

MVC web application frameworks:

JavaScript

AJILE Cross-browser compatible (ECMAScript-262, edition 3, JavaScript 1.3+, JScript 3.0+)
Archetype Javascript Framework for structuring and ease development of RIA.
JavaScript MVC An open-source JavaScript framework for developing web applications.

Informix 4GL

Informix 4GL MVC models to use for Informix 4GL report and form creation
EGL – IBM's EGL MVC Implementation

Perl

Catalyst An MVC-based avant-garde web framework.
CGI::Application A mature, lightweight, flexible MVC framework for web application development.
Gantry Framework A web application framework for Apache/mod_perl, CGI and Fast-CGI.
Jifty A full-stack application framework.
Maypole A Perl framework for MVC-oriented web applications, similar to Jakarta's Struts
OpenInteract2 is a web application server written in Perl. It features integrated data persistence, security, user and group management, plus an easy way to create and distribute fully database-independent applications.
PageKit A mod_perl based web application framework that uses a template system and XML.

PHP

Agavi an open-source, LGPL licensed MVC framework for creating applications written using PHP5.
Akelos PHP Framework a Ruby on Rails port to PHP4/5.
BareBonesMVC A one-file, no-configuration, MVC framework for PHP5.
CakePHP webapplication framework modeled after the concepts of Ruby on Rails.
CodeIgniter A PHP MVC framework.
DragonPHP MVC2 Framework for PHP 5.
Fusebox Framework for building ColdFusion and PHP web applications.
FUSE A powerful but easy-to-use PHP 5 Framework for MVC development
KohanaPHP A powerful, lightweight, easily extendable PHP 5 MVC Framework.
LightVC Lightweight PHP 5 Strict MVC Framework with decoupling of Model and other non-View-Controller essentials to promote code reuse.
Odin Assemble Small footprint PHP based MVC Framework.
phpXCore A MVC design pattern based PHP content management framework compatible with PHP4 and PHP5.
PRADO A PHP 5 MVC framework.
SilverStripe contains a fully fledged PHP 5.2 ORM/MVC Framework focused on building websites.
Solar PHP 5 framework Solar is a PHP 5 framework for rapid application development. It is fully name-spaced and uses enterprise application design patterns, with built-in support for localization and configuration at all levels.
Switch board with Routing PHP 5 MVC Framework with Routing.
Symfony Framework PHP 5 MVC Framework.
TinyMVC Framework Simple and lightweight PHP5 MVC (Model-View-Controller) framework.
TYPO3 extension library lib/div PHP 4/5 MVC framework for TYPO3 extension development
Qcodo is an open-source PHP 5 web application framework
Zend Framework A PHP 5-based MVC framework.
ZNF PHP5 MVC framework for enterprise web applications
Zoop Framework A Mature PHP 4/5 MVC framework.

Python

Django A complete Python web application framework. Django prefers to call its MVC implementation MTV, for Model-Template-View [1].
Gluon A full-stack python web application framework
Pylons - Python Web Framework
TurboGears for Python
Spring Python