MSDN Magazine - February 2009 - (Page 74)

The single most important aspect signing an application, you can often of WPF that makes MVVM a great decide whether something should be pattern to use is the data binding inin the view or the ViewModel by imagfrastructure. By binding properties of ining that you want to write a unit test a view to a ViewModel, you get loose to consume the ViewModel. If you can coupling between the two and entirely write unit tests for the ViewModel withremove the need for writing code in a out creating any UI objects, you can also ViewModel that directly updates a view. completely skin the ViewModel because The data binding system also supports it has no dependencies on specific viinput validation, which provides a stansual elements. dardized way of transmitting validation Lastly, for developers who work with Figure 1 Workspaces errors to a view. visual designers, using MVVM makes it Two other features of WPF that make much easier to create a smooth designer/ this pattern so usable are data templates developer workflow. Since a view is just and the resource system. Data teman arbitrary consumer of a ViewModel, plates apply Views to ViewModel obit is easy to just rip one view out and drop jects shown in the user interface. You in a new view to render a ViewModel. can declare templates in XAML and let This simple step allows for rapid protothe resource system automatically lotyping and evaluation of user interfaces cate and apply those templates for you made by the designers. at run time. You can learn more about The development team can focus on binding and data templates in my July creating robust ViewModel classes, and 2008 article, “Data and WPF: Customthe design team can focus on making Figure 2 New Customer Data Entry Form ize Data Display with Data Binding and user-friendly Views. Connecting the WPF” (msdn.microsoft.com/magazine/cc700358). output of both teams can involve little more than ensuring that If it were not for the support for commands in WPF, the MVVM the correct bindings exist in a view’s XAML file. pattern would be much less powerful. In this article, I will show you how a ViewModel can expose commands to a View, thus al- The Demo Application lowing the view to consume its functionality. If you aren’t familAt this point, I have reviewed MVVM’s history and theory of iar with commanding, I recommend that you read Brian Noyes’s operation. I also examined why it is so popular amongst WPF decomprehensive article, “WPF: Understanding Routed Events and velopers. Now it is time to roll up your sleeves and see the pattern Commands in WPF,” from the September 2008 issue (msdn.micro- in action. The demo application that accompanies this article uses soft.com/magazine/cc785480). MVVM in a variety of ways. It provides a fertile source of examIn addition to the WPF (and Silverlight 2) features that make ples to help put the concepts into a meaningful context. I created MVVM a natural way to structure an application, the pattern is also the demo application in Visual Studio 2008 SP1, against the Microsoft .NET Framework 3.5 SP1. The unit tests run in the Visual Studio unit testing system. The application can contain any number of “workspaces,” each of which the user can open by clicking on a command link in the navigation area on the left. All workspaces live in a TabControl on the main content area. The user can close a workspace by clicking the Close button on that workspace’s tab item. The application has two available workspaces: “All Customers” and “New Customer.” After running the application and opening some workspaces, the popular because ViewModel classes are easy to unit test. When an UI looks something like Figure 1. application’s interaction logic lives in a set of ViewModel classes, Only one instance of the “All Customers” workspace can be open you can easily write code that tests it. In a sense, Views and unit at a time, but any number of “New Customer” workspaces can be tests are just two different types of ViewModel consumers. Having open at once. When the user decides to create a new customer, she a suite of tests for an application’s ViewModels provides free and must fill in the data entry form in Figure 2. fast regression testing, which helps reduce the cost of maintaining After filling in the data entry form with valid values and clickan application over time. ing the Save button, the new customer’s name appears in the tab In addition to promoting the creation of automated regression item and that customer is added to the list of all customers. The aptests, the testability of ViewModel classes can assist in properly plication does not have support for deleting or editing an existing designing user interfaces that are easy to skin. When you are de- customer, but that functionality, and many other features similar Just like any other design pattern, MVVM is a set of guidelines, not rules. 74 msdn magazine MVVM Pattern http://msdn.microsoft.com/magazine/cc700358 http://msdn.microsoft.com/magazine/cc785480 http://msdn.microsoft.com/magazine/cc785480

Table of Contents Feed for the Digital Edition of MSDN Magazine - February 2009

MSDN Magazine - February 2009 Contents Toolbox CLR Inside Out Data Points Cutting Edge Patterns In Practice Best Practices .Net Interop "Oslo" Basics Patterns Silverlight Under The Table Foundations Windows With C++ .NET Matters Going Places { End Bracket }

MSDN Magazine - February 2009

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