Model–view–viewmodel is a software architectural pattern that facilitates the separation of the development of the graphical user interface – be it via a markup language or GUI code – from the development of the business logic or back-end logic so that the view is not dependent on any specific model platform. The view model of MVVM is a value converter, meaning the view model is responsible for exposing the data objects from the model in such a way that objects are easily managed and presented. In this respect, the view model is more model than view, and handles most if not all of the view's display logic. The view model may implement a mediator pattern, organizing access to the back-end logic around the set of use cases supported by the view. MVVM is a variation of Martin Fowler's Presentation Model design pattern. It was invented by Microsoft architects Ken Cooper and Ted Peters specifically to simplify event-driven programming of user interfaces. The pattern was incorporated into Windows Presentation Foundation and Silverlight. John Gossman, one of Microsoft's WPF and Silverlight architects, in 2005. Model–view–viewmodel is also referred to as model–view–binder, especially in implementations not involving the.NET platform. ZK and KnockoutJS use model–view–binder.
Components of MVVM pattern
;Model ;View ;View model ;Binder
Rationale
MVVM was designed to make use of data bindingfunctions in WPF to better facilitate the separation of view layer development from the rest of the pattern, by removing virtually all GUI code from the view layer. Instead of requiring user experience developers to write GUI code, they can use the framework markup language and create data bindings to the view model, which is written and maintained by application developers. The 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 entirecode 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 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. 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.
Criticism
A criticism of the pattern comes from MVVM creator John Gossman himself, who points out that the overhead in implementing MVVM is "overkill" for simple UI operations. He says 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.
