What is MVVM
Model–View–ViewModel (MVVM) is a software architectural pattern.
MVVM facilitates a separation of development of the graphical user interface – be it via a markup language or GUI code – from development of the business logic or back-end logic (the data model). The view model of MVVM is a value converter, meaning the view model is responsible for exposing (converting) 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.
The MVVM pattern lends itself naturally to XAML application platforms such as Silverlight. This is because the MVVM pattern leverages some of the specific capabilities of Silverlight, such as data binding, commands, and behaviors. MVVM is similar to many other patterns that separate the responsibility for the appearance and layout of the UI from the responsibility for the presentation logic; for example, if you’re familiar with the Model-View-Controller (MVC) pattern, you’ll find that MVVM has many similar concepts.
Motivation for the MVVM Pattern
Development technologies like Windows Forms, WPF, Silverlight, and Windows Phone provide a default experience that leads a developer down the path of dragging controls from a toolbox to a design surface, then writing code in the form’s code-behind file. As such applications grow in size and scope and are modified, complex maintenance issues begin to arise. These issues include the tight coupling between the UI controls and the business logic, which increases the cost of making UI modifications, and the difficulty of unit testing such code.
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