Model View ViewModel

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Model View ViewModel (MVVM) is an architectural pattern for software development.

MVVM is a variation of Martin Fowler's Presentation Model design pattern.[1][2] Like Fowler's Presentation Model, MVVM abstracts a view's state and behavior.[1] However, whereas the Presentation Model abstracts a view (i.e., creates a view model) in a manner that does not depend on a specific user-interface platform, MVVM was developed at Microsoft specifically to simplify event-driven programming of user interfaces—by exploiting features of Windows Presentation Foundation (WPF) (Microsoft's .NET graphics system) and Silverlight (WPF's Internet application derivative).[1]

John Gossman, one of Microsoft's WPF and Silverlight architects, announced MVVM on his blog in 2005.

MVVM and Presentation Model both derive from the model–view–controller pattern (MVC). MVVM facilitates a separation of the development of the graphical user interface (either as markup language or GUI code) from the development of the business logic or back-end logic (the data model). The view model of MVVM is a value converter;[3] this means that the view model is responsible for exposing the data objects from the model in such a way that the objects are easily managed and consumed. In this respect, the view model is more model than view, and handles most if not all of the view’s display logic.[3] The view model may also implement a mediator pattern, organising access to the backend logic around the set of use cases supported by the view.

Model View ViewModel is also called model-view-binder, especially in implementations that don't involve the .NET platform. ZK (a web application framework written in Java) and KnockoutJS (a JavaScript library) use model-view-binder.[1][4][5]

Components of the MVVM pattern[edit]

Model
Model refers either to a domain model, which represents the real state content (an object-oriented approach), or to the data access layer that represents that content (a data-centric approach).[citation needed]
View
As in the MVC and MVP patterns, the view is the user interface (UI).[further explanation needed]
View model
The view model is an abstraction of the view that exposes public properties and commands. Instead of the controller of the MVC pattern, or the presenter of the MVP pattern, MVVM has a binder. In the view model, this binder mediates communication between the view and the data binder.[clarification needed] The view model has been described as a state of the data in the model.[6]
Binder
Declarative data- and command-binding are implicit in the MVVM pattern. In the Microsoft solution stack, the binder is a markup language called XAML.[7] The binder frees the developer from being obliged to write boiler-plate logic to synchronise the view model and view. When implemented outside of the Microsoft stack the presence of a declarative databinding technology is a key enabler of the pattern.[4][8][clarification needed]

Rationale[edit]

MVVM was designed to make use of data binding functions in WPF to better facilitate the separation of view layer development from the rest of the pattern, by removing virtually all GUI code (“code-behind”) from the view layer.[9] Instead of requiring user experience (UX) developers to write GUI code, they can use the framework markup language (e.g., XAML) and create data bindings to the view model, which is written and maintained by application developers. This separation of roles allows interactive designers to focus on UX needs rather than programming of business logic. The layers of an application can thus be developed in multiple work streams for higher productivity. Even when a single developer works on the entire code base a proper separation of the view from the model is more productive as the user interface typically changes frequently and late in the development cycle based on end-user feedback.

The MVVM pattern attempts to gain both the advantages of separation of functional development provided by MVC, while leveraging the advantages of data bindings and the framework by binding data as close to the pure application model as possible.[9][10][11][clarification needed] It uses the binder, view model, and any business layers' data-checking features to validate incoming data. The result is that the model and framework drive as much of the operations as possible, eliminating or minimizing application logic which directly manipulates the view (e.g., code-behind).

Criticism[edit]

A criticism of the pattern comes from MVVM creator John Gossman himself,[12] who points out that the overhead in implementing MVVM is "overkill" for simple UI operations. He also states that for larger applications, generalizing the ViewModel becomes more difficult. Moreover, he illustrates that data binding in very large applications can result in considerable memory consumption.

References[edit]

External links[edit]