Model–view–controller

Model–view–controller (MVC) is a software architecture,[1] currently considered an architectural pattern used in software engineering. The pattern isolates "domain logic" (the application logic for the user) from the user interface (input and presentation), permitting independent development, testing and maintenance of each (separation of concerns).

Model View Controller (MVC) pattern creates applications that separate the different aspects of the application (input logic, business logic, and UI logic), while providing a loose coupling between these elements.[2]

Contents

History

MVC was first described in 1979[3] by Trygve Reenskaug, then working on Smalltalk at Xerox PARC. The original implementation is described in depth in the influential paper "Applications Programming in Smalltalk-80: How to use Model–View–Controller".[4]

Overview

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

  1. The user interacts with the user interface in some way (for example, by pressing a mouse button).
  2. The controller handles the input event from the user interface, often via a registered handler or callback, and converts the event into an appropriate user action, understandable for the model.
  3. The controller notifies the model of the user action, possibly resulting in a change in the model's state. (For example, the controller updates the user's shopping cart.)[5]
  4. A view queries the model in order to generate an appropriate user interface (for example the view lists the shopping cart's contents). The view gets its own data from the model. In some implementations, the controller may issue a general instruction to the view to render itself. In others, the view is automatically notified by the model of changes in state (Observer) that require a screen update.
  5. The user interface waits for further user interactions, which restarts the control flow cycle.

Some implementations such as the W3C XForms also use the concept of a dependency graph to automate the updating of views when data in the model changes.

The goal of MVC is, by decoupling models and views, to reduce the complexity in architectural design and to increase flexibility and maintainability of code. MVC has also been used to simplify the design of Autonomic and Self-Managed systems[6]

Concepts

The model manages the behavior and data of the application domain, responds to requests for information about its state (usually from the view), and responds to instructions to change state (usually from the controller). In event-driven systems, the model notifies observers (usually views) when the information changes so that they can react.

The 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. A view port typically has a one to one correspondence with a display surface and knows how to render to it.

The controller receives user input and initiates a response by making calls on model objects. A controller accepts input from the user and instructs the model and a view port to perform actions based on that input.

An MVC application may be a collection of model/view/controller triads, each responsible for a different UI element. The Swing GUI system, for example, models almost all interface components as individual MVC systems.

MVC is often seen in web applications where the view is the HTML or XHTML generated by the app. The controller receives GET or POST input and decides what to do with it, handing over to domain objects (i.e. the model) that contain the business rules and know how to carry out specific tasks such as processing a new subscription, and which hand control to (X)HTML-generating components such as templating engines, XML pipelines, Ajax callbacks, etc.

The model is not necessarily merely a database; the 'model' in MVC is both the data and the business/domain logic needed to manipulate the data in the application. 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. Models are not data access objects; however, in very simple apps that have little domain logic there is no real distinction to be made. Active Record is an accepted design pattern that merges domain logic and data access code — a model which knows how to persist itself.

Architecture vs. frameworks

Although MVC is typically associated with frameworks, it is essentially an architecture.[7] This means that it can be implemented even without an object-oriented language or a specific class hierarchy. For example, using as little as jQuery's trigger() and bind(), it is possible to build robust MVC applications in a browser using JavaScript. The key is simply to divide up the responsibilities of the MVC components into clearly defined sections of code. As stated in the overview, the code that embodies the model takes care of state, business logic, persistence, and notifications. The persistence can be implemented via cookies or AJAX. The notifications can be taken care of by the jQuery.trigger(). The code that embodies the view takes care of querying the model and rendering the view. The view code can be implemented in a variety of ways, including inserting HTML DOM nodes or changing CSS styles. The code that embodies the controller takes care of initialization of the model and wiring up the events between the view's HTML DOM elements and controller and between the model and the view code, using jQuery.bind().

Example

Here is a simple application of the pattern, implementing Java Servlets and Java Server Pages from Java EE:

Model
The model is a collection of Java classes that form a software application intended to store, and optionally separate, data. A single front end class that can communicate with any user interface (for example: a console, a graphical user interface, or a web application).
View
The view is represented by a Java Server Page, with data being transported to the page in the HttpServletRequest or HttpSession.
Controller
The Controller servlet communicates with the front end of the model and loads the HttpServletRequest or HttpSession with appropriate data, before forwarding the HttpServletRequest and Response to the JSP using a RequestDispatcher.

The Servlet is a Java class, and it communicates and interacts with the model, but does not need to generate HTML or XHTML output; the JSPs do not have to communicate with the model because the Servlet provides them with the information—they can concentrate on creating output.

Implementations of MVC as GUI frameworks

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

Implementations of MVC as web-based frameworks

Main article: Comparison of web application frameworks

In the design of web applications, MVC is implemented by web template systems as a "view for web" component.

MVC is typically implemented as a "Model 2" architecture in Sun parlance. Model 2 focuses on efficiently handling and dispatching full page form posts and reconstructing the full page via a front controller. Complex web applications continue to be more difficult to design than traditional applications because of this "full page" effect. More recently "view for web" Ajax driven frameworks that focus on firing focused UI events at specific UI Components on the page are emerging. This is causing MVC to be revisited for web application development using traditional desktop programming techniques.

ABAP Objects

ActionScript

C++

CFML - Adobe ColdFusion, Railo, and Open BlueDragon

MS-DOS

Flex

Groovy

Java

MVC web application frameworks:

Java Stand-alone Application Toolkit:

JavaScript

MVC web application frameworks:

Informix 4GL

.NET

Perl

PHP

Python

Ruby

Smalltalk

XML

See also

References

  1. ^ Reenskaug, Trygve. "MVC XEROX PARC 1978-79". http://heim.ifi.uio.no/~trygver/themes/mvc/mvc-index.html. Retrieved 2008-06-09. 
  2. ^ Model View Controller aspnet4.com
  3. ^ Trygve M. H. Reenskaug/MVC—XEROX PARC 1978-79
  4. ^ How to use Model–View–Controller (MVC)
  5. ^ Complex controllers are often structured using the command pattern to encapsulate actions and simplify extension.
  6. ^ E. Curry and P. Grace, “Flexible Self-Management Using the Model-View-Controller Pattern,” IEEE Software, vol. 25, no. 3, pp. 84-90, May. 2008.
  7. ^ Sagar Vasekar; Michael Rimov; Larry Hamel; et. al. (2004-12). "Expresso Developer's Guide". http://www.jcorporate.com/expresso/doc/edg/edg_WhatIsMVC.html: Jcorporate. "Flexibility in large component based systems raise questions on how to organize a project for easy development and maintenance while protecting your data and reputation, especially from new developers and unwitting users. The answer is in using the Model, View, Control (MVC) architecture." 
  8. ^ http://dod.codeplex.com/
  9. ^ "Product Review: WaveMaker’s point-and-click Java". Infoworld. April 24, 2008. http://www.infoworld.com/article/08/04/17/16TC-wavemaker-studio_1.html. Retrieved 2008-04-25. 
  10. ^ Django appears to be a MVC framework, but you call the controller the “view”, and the view the “template”. How come you don’t use the standard names?, FAQ: General, Django

External links