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Model-view-controller

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 and the business rules used to manipulate the data; the view corresponds to elements of the user interface such as text, checkbox items, and so forth; and the controller manages details involving the communication to the model of user actions such as keystrokes and mouse movements.

History

The pattern was first described in 1979 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.

After that numerous derivatives of the MVC pattern appeared. Probably one of the most known of them is the Model View Presenter pattern, which appeared in the early 90s and was designed to be an evolution of MVC. However Model-View-Controller still remains very widely used.

Pattern description

Model-view-controller is both an architectural pattern and a design pattern, depending on where it is used.

As an architectural pattern

It is common to split an application into separate layers that run on different computers: presentation (UI), domain logic, and data access. In MVC the presentation layer is further separated into view and controller.

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 in XML nodes, and the business rules that transform that content based on user actions.

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

  1. The user interacts with the user interface in some way (e.g. presses a button).
  2. A controller handles the input event from the user interface, often via a registered handler or callback.
  3. The controller notifies the model of the user action, possibly resulting in a change in the model's state. (e.g. controller updates user's Shopping cart).
  4. 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.
  5. The user interface waits for further user interactions, which begins the cycle anew.

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.

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

As a design pattern

MVC encompasses more of the architecture of an application than is typical for a design pattern.Model

The domain-specific representation of the information on which 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.

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 model-- Like other frameworks, the design of the real model is usually left to the developer.
Control level model-- Swing also supports models on the level of controls (elements of the graphical user interface). 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 (the Event dispatching thread). It catches and propagates the events to the view and model.

Combined frameworks

Java: Java Enterprise Edition (Java EE)

Simple Version using just Servlets and JSPs from J2EE:Model

The model is a collection of java classes that together do something useful - they make up a software application which might involve moving data around. There is a single front end class that can communicate with any UI - eg a console, a thick GUI or a web application.View
The view is represented by 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 simple version uses the best parts of Servlet and JSP technology: the servlet is essentially a Java class and communicates and interacts with the model but does not have to do extensive generation of xhtml output; the JSPs do not have to communicate with the model as they are provided with the information they need by the servlet - they can concentrate on creating xhtml output.

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 (JSF).

XForms

XForms is an XML format for the specification of a data processing model for XML data and user interface(s) for the XML data, such as web forms.Model XForms stores the Model as XML in the browser.View The Views are XForms controls for screen elements and can be placed directly in a web page.

The model and views are bound together using reference or binding statements. These binding statements are used by the XForms dependancy graph to ensure that the correct views are updated when data in the model changes.Controller All mouse events are processed by XForms controls and XML events are dispatched.

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

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

MVC is typically implemented as a "Model 2" architecture in Sun parlance. Model2 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 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.

.NET

ColdFusion

  • Mach-II A framework that focuses on trying to ease software development and maintenance
  • Model-Glue Through a simple implementation of Implicit Invocation and Model-View-Controller, they allow applications to be well organized without sacrificing flexibility.
  • FuseBox Fusebox does not force the Model-View-Controller (MVC) pattern or Object-Oriented Programming (OOP) on the developer. However, either or both of these development approaches can be used with Fusebox.

Java

MVC web application frameworks:

JavaScript

MVC web application frameworks:

  • SproutCore
  • Wavemaker WYSIWYG development platform for Ajax web applications
  • DojoMVC JavaScript MVC based upon Dojo core. Includes jQuery integration.

ABAP Objects

Informix 4GL

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

Perl

PHP

Python

  • Django A complete Python web application framework. Django prefers to call its MVC implementation MTV, for Model-Template-View.
  • Pylons - Python Web Framework
  • TurboGears for Python
  • web2py Web Framework
  • Zope Content Management Framework

Ruby

Smalltalk

XML

See also

References

External links

General information regarding MVC

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