Model-View-ViewModel Design Pattern - An implementation of RelayCommand for SilverLight

A common pattern for developping applications with SilverLight and WPF is the Model-View-ViewModel (MVVM) Design Pattern. The MVVP pattern is an evolution of the MVC and MVP well-known pattern, specially adapted to the advanced data binding features of WPF and SilverLight. Please refer to the following excellent MSDN article from Josh Smith for a complete description and implementation of the MVVM Pattern for WPF:

WPF Apps With The Model-View-ViewModel Design Pattern

However, as stated in this article title, we're talking about an MVVM pattern for WPF. For those who have tried to develop portable applications working on both WPF and SilverLight, you know this frustration of discovering small differences between WPF and SilverLight and missing classes in  SilverLight as compared to WPF. 

And in our specific case, Josh defines a RelayCommand class, used to simplified the binding with user interface elements commands with underlying actions in the ViewModel class.

Here is the definition of the RelayCommand class:

public class RelayCommand : ICommand
{
 #region Fields

 readonly Action

This class makes use of the CommandManager object and the RequerySuggested event. When a user actions the UI in WPF, this triggers the CommandManager to requery all the commands. In Silverlight the CommandManager class does not exist: you have to do the work yourself!

So I created the following RelayCommand implementation, working for SilverLight:

 public class RelayCommand : ICommand
 {
 #region Fields

 readonly Action

And here is the definition of the ICommandManager interface:

 public interface ICommandManager
 {
 event EventHandler RequerySuggested;
 }

Now when creating a new command, you should pass a reference to an object implementing the ICommandManager interface. This object will usually be the ViewModel class itself. It is then the responsability of the ViewModel class to trigger the RequerySuggested event when anything in the state of the ViewModel changed that requires the command to be rebinded. That's it, this completes the SilverLight implementation of the MVVM pattern.