Model-View-Controller - Definition and Overview

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Model-View-Controller (MVC) is a software architecture that separates an application's data model, user interface, and control logic into three distinct components so that modifications to the view component can be made with minimal impact to the data model component. This is useful since models typically enjoy a fair degree of stability (owing to the stability of the domain being modeled), whereas user interface code usually undergoes frequent and sometimes dramatic change (owing to usability problems, the need to support growing classes of users, or simply the need to keep the application looking "fresh"). Separating the view from the model makes the model more robust, because the developer is less likely to "break" the model while reworking the view.

The pattern was first described in 1979 (http://heim.ifi.uio.no/~trygver/themes/mvc/mvc-index.html) by Trygve Reenskaug, then working on Smalltalk at Xerox research labs.

Though MVC comes in different flavors, control flow generally works as follows:

1. user interacts with the user interface in some way (e.g., user presses a button)
2. controller receives notification of the user action from the user interface objects
3. controller accesses the model, possibly updating it in a way appropriate to the user's action (e.g., controller updates user's shopping cart)
4. controller delegates to the view for presentation
5. view uses the model to generate an appropriate user interface (e.g., view produces a screen listing the shopping cart contents). Note: In some implementations, the view does not have direct access to an external model -- instead, the controller pushes the data from the external model into the view.
6. user interface waits for further user interactions, which begin the cycle anew.

In the design of web applications, MVC is also known as a "Model 2" architecture. 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.

Implementations

• JavaServer Faces and Jakarta Struts are currently the most popular web oriented MVC implementations. Struts works at a page level; JSF works at a component level.
• Smalltalk as described in Applications Programming in Smalltalk-80(TM):How to use Model-View-Controller (http://st-www.cs.uiuc.edu/users/smarch/st-docs/mvc.html)
• The OPENSTEP and NeXTSTEP development environments encourage the use of MVC. Cocoa, based on these technologies, also uses MVC.
• The WebObjects development/deployment environment is strongly based on MVC.
• MVC-Frameworks written in PHP (Wiki) (http://wact.sourceforge.net/index.php/MvcFrameworksWrittenInPhp)
• Fusebox
• Maypole
• Ruby on Rails

See also

• Design patterns
• Model view controller triad

External links

• Description in the Portland Pattern Repository (http://c2.com/cgi/wiki?ModelViewController)
• Core J2EE Patterns - Front Controller (http://java.sun.com/blueprints/corej2eepatterns/Patterns/FrontController.html)
• Fusebox (http://www.fusebox.org/)
• Maypole (http://maypole.perl.org/)
• Ruby on Rails (http://www.rubyonrails.org/show/HomePage)
• Model View Controller in Web Applications (http://wact.sourceforge.net/index.php/ModelViewController)

• .mvc is also the file extension for MivaScript files.
• MVC is Move Character in the IBM System/360 instruction set