Applying the Model-View-ViewModel Pattern to a Windows Phone App

A dominant pattern that has emerged in XAML UI based technologies, in particular WPF and Silverlight, is the Model-View-ViewModel (MVVM) pattern. MVVM is an architectural pattern largely based on the Model-View-Controller (MVC) pattern, which, like the MVC pattern, serves to isolate the domain logic from the user interface logic. In addition, MVVM leverages the strong data binding capabilities of XAML based technologies, which allows loose coupling between the view and the viewmodel so that the viewmodel does not need to directly manipulate the view. This eliminates the need for almost all code-beside, which has a number of benefits, including freeing interactive designers from writing view specific code.

The following are the principal elements of the MVVM pattern:

Model—The model is responsible for managing and delivering data.

View—The view is responsible for displaying data. The view is ordinarily a UI element, and, in the case of XAML-based Windows Phone apps, it is a UserControl such as a PhoneApplicationPage.

ViewModel—A bridge or intermediary between the model and the view, which commonly retrieves model objects and exposes them to the view. Often the viewmodel is designed to respond to commands that are bound to UI elements in the view. The viewmodel can be thought of as the model of the view.

With the release of the Windows Phone 7.1 SDK came Silverlight 4 and support for ICommands. The use of commands is discussed in the section “Using Commands” later in the chapter.

There are numerous benefits to using MVVM in your apps. MVVM can improve an app’s testability because it is easier to test code from a unit test that does not rely on surfacing UI objects. Testing apps is discussed further in Chapter 24, “Unit Testing Apps.”

Placing application interaction logic in a viewmodel also makes it easier to redesign your app while reducing the need to refactor interaction logic. Occasionally you may like to reuse some of your UI logic in different apps or, to a lesser extent, you might want to target different UI technologies, such as WPF, Silverlight, or Windows Store XAML applications. Decoupling interaction logic from any particular UI technology makes it easier to target multiple platforms.