I am sure most of you would have seen tons of definitions, tutorials and examples of MVVM pattern and its context with respect to Windows Presentation Foundation (WPF) and Silverlight (a stripped down WPF for the internet browser environment). However, many people tell me that those examples don’t really help them understand the pattern fully well to use it in their applications or at least think about the pattern’s applicability in a given scenario. This post is an attempt to address this gap and demystify MVVM enough to start implementing it via code.

Like its ancestors – MVC, MVP/variations, MVVM pattern serves to a specific purpose: separating presentation (ASP.NET, Windows Forms, WPF and Silverlight) from data (business data such as order, customer, contact, etc.) and the logic of displaying it (responding to control events, displaying data using various controls, etc.). By this separation, another benefit that MVVM brings to the table is the testability of the presentation logic independent of the UI. This especially makes sense where UI designers and developers have to work on the same artifact to do their respective jobs and follow their own workflow. Imagine a UI designer working on an .aspx page and a developer on the page’s code-behind. The designer may do multiple iterations trying various design combinations, themes, reviewing it by the business, etc. while the developer wouldn’t really care about it. On the flip side, a developer would want to do unit testing, code analysis, bug fixing, etc. that the designer doesn’t care about. These two roles can work independently without challenges if there is a mechanism that helps seamlessly integrate their work at end with ease. MVVM is that mechanism, which addresses this exact problem with less or no effort!

Model: Data we intend to display in the UI via various controls. It could be any data source such as a WCF services, OData data provider, SQL Server and XML File.

View: The actual UI – web forms, Windows forms, WPF/Silverlight (consumer of data, the Model)

ViewModel (VM): The confusing piece in the pattern by its name for many people! VM acts as the data source for any component (consumer) that would be interested in the data exposed by the VM; yet it does not know anything about its consumer. The consumer could be a View (which is the case most of the time) or just anything that knows how to make the best out of VM’s data. As a matter of fact, the VM doesn’t care how the consumer would display the data (if it is a UI), use it or whatsoever. It is this fact that makes a VM testable without an UI via mocking. After testing, you just replace the mock with the actual UI and everything just works as intended.

Generally ViewModels are designed based on its consumer(s) data requirements (Model for the UI and hence the name ViewModel). For example, a VM for a dialog showing a list of products will have public properties and methods for product list (an ICollection may be?), sorting and filtering all for that dialog’s consumption.

Remember that there can be more than one consumer pulling data from a VM. You might have come across cases where a single VM provides data to more than one ViewModels. Who by the way provides data to a VM to serve to its consumer(s)? Model.

So, how MVVM fits into the WPF/Silverlight development? Stay tuned!