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 line indicate an indirect association (e.g., observer pattern).
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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 line indicate an indirect association (e.g., observer pattern).

Model-View-Controller (MVC) is a design pattern used in software engineering. In complex computer applications that present lots of data to the user, one often wishes to separate data (Model) and user interface (View) concerns, so that changes to the user interface do not impact the data handling, and that the data can be reorganized without changing the user interface. The Model-View-Controller design pattern solves this problem by decoupling data access and business logic from data presentation and user interaction, by introducing an intermediate component: the Controller.

Contents

[edit] Pattern Description

It is common to split an application into separate layers: presentation (UI), domain, 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. Hence the term architectural pattern may be useful [1], or perhaps an aggregate design pattern.

Model
The domain-specific representation of the information on which the application operates. The model is another name for the domain layer. Domain logic adds meaning to raw data (e.g., calculating if 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. MVC is often seen in web applications, where the view is the HTML page and the code which gathers dynamic data for the page.
Controller
Processes and responds to events, typically user actions, and may invoke changes on the model.

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., user presses a button)
  2. A controller handles the input event from the user interface, often via a registered handler or callback.
  3. 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).[2]
  4. A view uses the model to generate an appropriate user interface (e.g., 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. (However, the observer pattern can be used to allow the model to indirectly notify interested parties – potentially including views – of a change.)
  5. The user interface waits for further user interactions, which begins the cycle anew.

[edit] Implementations

The pattern was first described in 1979 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.

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

  • The NeXTSTEP and OPENSTEP development environments encourage the use of MVC. Cocoa and GNUstep, based on these technologies, also use MVC.
  • The Core Data framework is the most powerful MVC implementation for the Apple Macintosh to date. It is a modern addition to Cocoa frameworks and is fully supported by the latest Xcode IDE.
  • Microsoft Foundation Classes (MFC) (called Document/View architecture here)
  • The Java Swing
  • The Qt Toolkit since Qt4 Release.
  • XForms has a clear separation of model (stored inside the HTML head section) from the presentation (stored in the HTML body section). XForms uses simple bind commands to link the presentation to the model.
  • Web template systems (see list of implementation on article) are widely used on web and other contexts. It is a kind of "view subsystem" in a MVC.

[edit] MVC in various languages

[edit] .NET

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[edit] ASP.NET

In ASP.NET, the patterns for the view and controller are well defined. The model is left to the developer to design.

Model
ASP.NET does not strictly require a model. The developer has the option to create a model class, but may choose to forget it and have the event handlers in the controller perform any calculations and data persistence. That said, using a model to encapsulate business rules and database access is both possible and preferable.
View
The ASPX and ASCX files handle the responsibilities of the view. With this design, the view object actually inherits from the controller object. This is different from the Smalltalk implementation, in which separate classes have pointers to one another.
Controller
The duties of the controller are split between two places. The generation and passing of events is part of the framework and more specifically the Page and Control classes. The handling of events is usually done in the code-behind class. However, moving code specific to the transition between views in a separate Controller is a good practice. In turn, it becomes possible to centralizing the registration of Observers in the isolated Controller.

[edit] Windows Forms

In WinForms, a .NET framework, the patterns for the view and controller are well defined. The model is left to the developer to design.

Model
Just like ASP.Net, WinForms does not strictly require a model. The developer has the option to create a model class, but may choose to forget it and have the event handlers in the controller perform any calculations and data persistence. Again, using a model to encapsulate business rules and database access is both possible and preferable.
View
A class inheriting from either Form or Control handles the responsibilities of the view. In the case of WinForms, the View and Controller are compiled into the same class. This differs from ASP.Net, which uses inheritance, and Smalltalk, which have separate classes with pointers to one other
Controller
The duties of the controller are split between three places. The generation and passing of events starts at the OS level. Inside the .Net framework, the Form and Control classes route the event to the proper event handler. The handling of events is usually done in the code-behind class.

[edit] Java

[edit] Java Platform, Enterprise Edition (Java EE)

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

Model
The model is represented by entity beans.
View
The view in a Java EE application may be represented by a Java Server Page. Alternately, 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.

[edit] Java MVC Frameworks

  • Struts is one of the original web-application frameworks which makes extensive use of MVC logic.
  • The Spring Framework is a newer (as of late 2006) J2EE application framework for both native Java applications as well as Java-served web-applications. Spring utilizes a multi-tier approach to java applications. One of the layers enables struts-like MVC implementations.
  • Java Server Faces (JSF) framework is one of the most advanced, Java standardized web-application framework.

[edit] Java Swing

Java Swing is different from the other frameworks, in that it actually 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.

[edit] Ruby

The Ruby on Rails framework is a popular Model-view-controller architecture for Ruby.

[edit] Python

Python has many MVC frameworks. The two most popular frameworks are Django and TurboGears.

[edit] Perl

The most popular MVC for Perl is Catalyst. Catalyst borrows from other frameworks such as Ruby on Rails and Apache Struts. It makes extensive use of the CPAN archive to provide the various components as follows.

Model

Catalyst is also very flexible on the Model, examples being DBIx::Class, Class::DBI, Class::DBI::SQLite

View

CPAN modules such as Template Toolkit, Mason, HTML::Template and Petal can all be used as the View.

Controller

Catalyst uses advanced URL to action dispatching to map the URL to the correct controller.

[edit] PHP

There are many different MVC frameworks for PHP, some of which seek to imitate framework features from Java, .NET, and elsewhere. Often the Smarty template language is used to separate presentation from program logic. Zend, the company that develops PHP, recently began developing a MVC framework for PHP.

[edit] ColdFusion

ColdFusion also does not have a built in MVC structure, but has many third-party frameworks such as Model-Glue, Fusebox and Mach-II which stress the MVC ideal

[edit] Architectures for Web-based Interfaces

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.

[edit] See also

[edit] External links

General information regarding MVC

Specific aspects of MVC or alternatives to MVC

MVC frameworks in PHP

[edit] References

  1. ^ Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Michael Stal (1996). Pattern-Oriented Software Architecture. John Wiley and Sons. ISBN 0-471-95869-7.
  2. ^ Complex controllers are often structured using the command pattern to encapsulate actions and simplify extension.
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