Model–view–controller

Model–view–controller (MVC) is a software design pattern^[1] commonly used for developing user interfaces that divides the related program logic into three interconnected elements. These elements are the internal representations of information (the model), the interface (the view) that presents information to and accepts it from the user, and the controller software linking the two.^[2]^[3]

Traditionally used for desktop graphical user interfaces (GUIs), this pattern became popular for designing web applications.^[4] Popular programming languages have MVC frameworks that facilitate the implementation of the pattern.

History[edit]

One of the seminal insights in the early development of graphical user interfaces, MVC became one of the first approaches to describe and implement software constructs in terms of their responsibilities.^[5]

Trygve Reenskaug created MVC while working on Smalltalk-79 as a visiting scientist at the Xerox Palo Alto Research Center (PARC) in the late 1970s.^[6]^[7]^[8]^: 330 He wanted a pattern that could be used to structure any program where users interact with a large, convoluted data set. His design initially had four parts: Model, view, thing, and editor. After discussing it with the other Smalltalk developers, he and the rest of the group settled on model, view, and controller instead.^[6]

In their final design, a model represents some part of the program purely and intuitively. A view is a visual representation of a model, retrieving data from the model to display to the user and passing requests back and forth between the user and the model. A controller is an organizational part of the user interface that lays out and coordinates multiple Views on the screen, and which receives user input and sends the appropriate messages to its underlying Views. This design also includes an Editor as a specialized kind of controller used to modify a particular view, and which is created through that view.^[6]

Smalltalk-80 supports a version of MVC that evolved from this one.^[6] It provides abstract view and controller classes as well as various concrete subclasses of each that represent different generic widgets. In this scheme, a View represents some way of displaying information to the user, and a controller represents some way for the user to interact with a view. A view is also coupled to a model object, but the structure of that object is left up to the application programmer. The Smalltalk-80 environment also includes an "MVC Inspector", a development tool for viewing the structure of a given model, view, and controller side-by-side.^[9]

In 1988, an article in The Journal of Object Technology (JOT) by two ex-PARC employees presented MVC as a general "programming paradigm and methodology" for Smalltalk-80 developers. However, their scheme differed from both Reenskaug et al.'s and that presented by the Smalltalk-80 reference books. They defined a view as covering any graphical concern, with a controller being a more abstract, generally invisible object that receives user input and interacts with one or many views and only one model.^[10]

The MVC pattern subsequently evolved,^[11] giving rise to variants such as hierarchical model–view–controller (HMVC), model–view–adapter (MVA), model–view–presenter (MVP), model–view–viewmodel (MVVM), and others that adapted MVC to different contexts.

The use of the MVC pattern in web applications grew after the introduction of NeXT's WebObjects in 1996, which was originally written in Objective-C (that borrowed heavily from Smalltalk) and helped enforce MVC principles. Later, the MVC pattern became popular with Java developers when WebObjects was ported to Java. Later frameworks for Java, such as Spring (released in October 2002), continued the strong bond between Java and MVC.

In 2003, Martin Fowler published Patterns of Enterprise Application Architecture, which presented MVC as a pattern where an "input controller" receives a request, sends the appropriate messages to a model object, takes a response from the model object, and passes the response to the appropriate view for display.^[8]^: 56 This is close to the approach taken by the Ruby on Rails web application framework (August 2004), which has the client send requests to the server via an in-browser view, these requests are handled by a controller on the server, and the controller communicates with the appropriate model objects.^[12] The Django framework (July 2005, for Python) put forward a similar "model template view" (MTV) take on the pattern, in which a view retrieves data from models and passes it to templates for display.^[13] Both Rails and Django debuted with a strong emphasis on rapid deployment, which increased MVC's popularity outside the traditional enterprise environment in which it has long been popular.

The model is responsible for managing the data of the application. It receives user input from the controller.

The view renders presentation of the model in a particular format.

The controller responds to the user input and performs interactions on the data model objects. The controller receives the input, optionally validates it and then passes the input to the model.

In addition to dividing the application into these components, the model–view–controller design defines the interactions between them.^[37]

As with other software patterns, MVC expresses the "core of the solution" to a problem while allowing it to be adapted for each system.^[38] Particular MVC designs can vary significantly from the traditional description here.^[39]

independence of presentation and data, e.g. multiple views on one model simultaneously,

composable presentation widgets, e.g. one view used as a subview of another,

switchable input modes, by swapping one controller out for another during runtime, and

independence of input and output processing, via the separate responsibilities of controllers and views.

[41]

As Alan Kay wrote in 2003, the original motivation behind the MVC was to allow creation of a graphical interface for any object.^[40] That was outlined in detail in Richard Pawson's book Naked Objects.^[40]

Trygve Reenskaug, originator of MVC at PARC, has written that "MVC was conceived as a general solution to the problem of users controlling a large and complex data set."^[6]

In their 1991 guide Inside Smalltalk, Carleton University computer science professors Wilf LaLonde and John Pugh described the advantages of Smalltalk-80-style MVC as:

Use in web applications[edit]

Although originally developed for desktop computing, MVC has been widely adopted as a design for World Wide Web applications in major programming languages. Several web frameworks have been created that enforce the pattern. These software frameworks vary in their interpretations, mainly in the way that the MVC responsibilities are divided between the client and server.^[42] Early MVC frameworks took a thin client approach that placed almost the entire model, view and controller logic on the server. In this approach, the client sends hyperlink requests or form submissions to the controller and then receives a complete and updated web page (or other document) from the view; the model exists entirely on the server.^[42] Later frameworks have allowed the MVC components to execute partly on the client, using Ajax to synchronize data.