The MVC Pattern and Beyond in VB.NET

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3 The MVC Pattern and Beyond
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Important Although a significant design difference exists between MVP and the original MVC,
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MVP and Model2 have a lot in common. The biggest difference remains the interaction between view and controller it s strictly based on a contract in MVP, and it s kind of free form in Model2. This said, you can find particular implementations of the patterns that blur this difference significantly. In ASP.NET MVC, for instance, you don t have an interface for the view, but using the Model property of the ViewData dictionary, you can define an equally strongly typed model for each view. Another difference between MVP and Model2 is the driver of the action. In MVP, the action is triggered by the view; in Model2, the entry point is the controller. Precisely for this reason, in Model2 (unlike MVP) the runtime environment is responsible for instantiating the controller. All in all, if you employ Model2 as your definition of MVC, you can hardly see the difference between it and MVP. But the difference does exist; and it s not even small. To grasp it, though, you must read the full story.
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Presentation Model Pattern (Also Known as MVVM)
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Developed by Martin Fowler, the Presentation Model (PM) pattern is fully described here: http://martinfowler.com/eaaDev/PresentationModel.html. How does PM differ from MVP Ultimately, it s not an entirely different type of animal. It s correct to consider it yet another variation of MVP that is particularly suited to supporting a rich and complex user interface. On the Windows platforms, PM works well with user interfaces built with Windows Presentation Foundation and Silverlight. Microsoft recommends it here: http://msdn.microsoft.com/en-us/library/cc707885.aspx. However, Microsoft also developed a WPF-specific version of PM that goes under the name of Model-View-ViewModel (MVVM). As I see things, PM and MVVM are not different things MVVM is just a WPF-specific implementation of PM. In this book, I ll consider PM and MVVM to be the same thing. PM, like MVP, is based on three actors: the view, the model, and the presenter.
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PM Actors at a Glance
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In MVP, the view exposes a contract to the presenter and the presenter talks back to the view through that interface. Binding of the data occurs through the implementation of the interface in the view class the Page class in ASP.NET, the Form class in Windows Forms, and the Window class in WPF. The code that does the binding belongs to the view and can be as simple as a property assignment or as sophisticated as data-binding. In PM, the view doesn t expose any interface, but a data model for the view is incorporated in the presenter. The view elements are directly bound to properties on the model. In summary, in PM the view is passive and doesn t implement any interface. The interface is transformed into a model class and incorporated in the presenter. See Figure 3-9.
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Part I The Programming Paradigm View View Forwards user actions Data-binding
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Model Presenter
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Presenter Redirect to a new PM/MVVM triad Yes
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Model No
Return values Invoke method Middle Tier
New view
FIGuRE 3-9 The triad in the Presentation Model pattern
Let s examine the role played by the actors in a bit more detail.
Implementation of the Model Actor
In PM, the model plays the same role it played in the original formulation of MVC: a container for any data being worked on in the view. Unlike MVC, though, there s no bidirectional link between the view and model in the form of an Observer relationship. The view is bound to the model and uses any stored information to generate the response. The actual form of the binding is an implementation detail, but it s always something close to data-binding. The innovative point of PM is that the presenter doesn t operate on the view. The presenter, instead, exposes an object model tailor-made for the view and takes care of populating it with fresh data. The view, in turn, gains access to the presenter s object model in some way. In the .NET space, data-binding is a common way in which this is achieved.
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