Tuesday, March 02, 2010

The Need for ViewModels

A Recap:

  1. A GameView –> ChessboardView –> ItemsTemplate –> Items (BoardSquareView in DataTemplate)
  2. GameView uses Game object (Model) as a resource or creates and assigns the Game object to GameView’s DataContext property
  3. Game exposes Chessboard as property
  4. Chessboard exposes ObservableCollection of BoardSquare objects

The code we’ve seen thus far there’s no real difference in functionality if we do not have ViewModels.  In fact, we could use the code-behinds to infuse Views with ViewModel behaviour.  Of course, at design time, there’s still a clean separation:

  1. XAML is in a separate file
  2. Code-behind is a separate file too
    • In classic implementation, you typically would have tons of code assigning values to the controls and objects here.  Let’s just forget for now what pundits would say about such implementation!
  3. Models

Unfortunately, at compile time and hence at runtime, the view requires the code-behind.  Since code-behind is so intricately entwined with the view, it’s important to shun it for holding the UI state. 

Why is UI state such a big deal?  Now, let’s say we want to hold state of the controls.  For example, we have a ‘Turn’ property in the Chessboard model indicating whose turn it is.  We could use it to control which ones to enable and disable.  Assume, we’ll use triggers ingeniously to control and make the view hold this state. The more complex the interactions, the more complex the view would become.  Does that belong there?

Another question is do we care?  Why is this important that we use this another level of indirection? 

It’s elementary, my dear Watson!

It’s all about replacing the View at will.   When XAML allows you to craft your individual controls on screen to such granularity, why turn your back for replacing your application’s entire UI?  Assume, you want to deliver your applications in multiple flavors.  One that takes full advantage of WPF and yet another that runs inside the browser. 

Silverlight 3.0, for example, doesn’t support many of the features that WPF provides.  Triggers are one.  We’ll see what it takes to do this when we release Shatranj for Silverlight 4.0.

Lo and behold!  Enter ViewModels!


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