Migrate from Basic to MVVM and MEF Composable Patterns for a Silverlight Application - Part 3

using System;
using System.Linq;
using System.Windows.Controls;
using System.ComponentModel;
using System.ComponentModel.Composition;
using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using GalaSoft.MvvmLight.Messaging;
using ProductApp.Common;

namespace ProductApp.Main.ViewModels
{
    [Export(typeof(IModule)), ExportMetadata(MetadataKeys.Name, ModuleID.MainPageViewModel)]
    [PartCreationPolicy(CreationPolicy.NonShared)]
    public class MainPageViewModel : ViewModelBase, IModule
    {
        private ModuleCatalogService _catalogService = ModuleCatalogService.Instance;
        private string _currentViewText = string.Empty;

        public MainPageViewModel()
        {            
        }

        // Defined with string type to pass the command parameter
        private RelayCommand<string> _loadModuleCommand;
        //
        public RelayCommand<string> LoadModuleCommand
        {
            get
            {
                if (_loadModuleCommand == null)
                {
                    // Parameter 1: define a delegate method to be executed
                    // Parameter 2: the delegate function CanExedute
                    // with one-in(object) and one-out(bool) parameters
                    // to make the command enabled or disabled
                    _loadModuleCommand = new RelayCommand<string>(
                                  OnLoadModuleCommand,
                                  moduleId => moduleId != null);
                }
                return _loadModuleCommand;
            }
        }
        //
        private void OnLoadModuleCommand(String moduleId)
        {
            string xapUri;
            try
            {
                if (_currentViewText != moduleId)
                {
                    // For loading Xap or View
                    switch (moduleId)
                    {
                        // Load ProductApp.Xap on-demand
                        case ModuleID.ProductListView:
                            xapUri = "/ClientBin/ProductApp.Views.xap";
                            _catalogService.AddXap(xapUri, arg => ProductApp_OnXapDownloadCompleted(arg));
                            break;
                        
                        default:
                            throw new NotImplementedException();
                    }
                }
            }
            catch (Exception ex)
            {
                // Pass error object to View code-behind
                Messenger.Default.Send(ex, MessageToken.RaiseErrorMessage);
            }
        }

        private void ProductApp_OnXapDownloadCompleted(AsyncCompletedEventArgs e)
        {
            // Send message back to View code-behind to load ProductList View
            Messenger.Default.Send(ModuleID.ProductListView, MessageToken.LoadScreenMessage);

            // Cache for check repeat commands later
            _currentViewText = ModuleID.ProductListView;
        }
    }
}