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6/02/2009

Model View Presenter

Model View Presenter is a software pattern considered a derivative of the Model-view-controller.

Pattern description

Model-View-Presenter is a user interface design pattern engineered to facilitate automated unit testing and improve the separation of concerns in presentation logic.

The Model is an interface defining the data to be displayed or otherwise acted upon in the user interface.

The View is an interface that displays data (the Model) and routes user commands to the Presenter to act upon that data.

The Presenter acts upon the Model and the View. It retrieves data from repositories, persists it, manipulates it, and determines how it will be displayed in the View.

Normally, the View implementation instantiates the concrete Presenter object, providing a reference to itself. The following C# code demonstrates a simple View constructor, where ConcreteDomainPresenter implements the IDomainPresenter interface:

public class DomainView: IDomainView
{
private IDomainPresenter mDomainPresenter;

public DomainView()
{
this.mDomainPresenter = new ConcreteDomainPresenter(this);
}
}

The degree of logic permitted in the View varies among different implementations.

At one extreme, the View is entirely passive, forwarding all interaction operations to the Presenter. In this formulation, when a user triggers an event method of the View, it does nothing but invoke a method of the Presenter which has no parameters and no return value. The Presenter then retrieves data from the View through methods defined by the View interface. Finally, the Presenter then operates on the Model and updates the View with the results of the operation.

Other versions of Model-View-Presenter allow some latitude with respect to which class handles a particular interaction, event, or command. This is often more suitable for web-based architectures, where the View, which executes on a client's browser, may be the best place to handle a particular interaction or command.

From a layering point of view, the Presenter class might be considered as belonging to the application layer in a multilayered architectured object-oriented system with common layers but it can also be seen as a Presenter layer of its own between the Application layer and the User Interface layer.

Pattern history

The Model View Presenter ("MVP") software pattern originated in the early 1990s at Taligent, a joint venture of Apple, IBM, and HP, and was the underlying programming model for application development in Taligent's C++-based CommonPoint environment. The pattern was later migrated by Taligent to Java and popularized in a paper by Taligent CTO Mike Potel[1]. After the demise of Taligent in 1997, the MVP pattern was adapted by Andy Bower and Blair McGlashan of Dolphin Smalltalk to form the basis for their Smalltalk user interface framework[2]. In 2006, Microsoft began incorporating MVP into their documentation and examples for user interface programming in the .NET framework[3]. The evolution and multiple variants of the MVP pattern, including the relationship of MVP to other design patterns such as MVC, were analyzed in detail in articles by Martin Fowler[4] and Derek Greer[5].

It is often claimed that this pattern has "Retired" (11 Jul 06) since Martin Fowler said so[6]. He split the pattern into Supervising Controller [7] and Passive View [8]. Nevertheless, the pattern name still lives on the internet, and a Google search for the pattern still generates 34000+ hits, including a link to an article that has been published in MSDN Magazine [9].

Pattern implementation in .NET

In a .NET environment the same Presenter class can be used for ASP.NET application and a Windows Forms application. The presenter gets and sets information from/to the View through an interface that in .NET can be implemented by both Windows Forms class and an ASPX page (the code-behind class can implement the View interface).

Instead of manually implementing the pattern, one of the Model-View-Presenter frameworks may be used. Below are listed some of such frameworks under the .NET platform.

Model-View-Presenter frameworks under .NET

Pattern implementation in Java

In a Java (AWT/Swing/SWT) application, the MVP pattern can be used by letting the UI class implement a view interface.

The same approach can be used for Java thin (web based) applications since modern Java Component Based Web Frameworks allow development of client side logic using the same component approach as thick clients. To implement MVP in Google Web Toolkit one just needs to let any Component implement the view interface. The same approach is possible using the Echo2 web framework since the web framework uses Swing classes in its core.


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