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

using System;
using System.Windows;
using System.Windows.Controls;
using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Messaging;
using ProductApp.Common;

namespace ProductApp.Main.Views
{
    public partial class MainPage : UserControl
    {
        private ModuleCatalogService _catalogService = ModuleCatalogService.Instance;

        public MainPage()
        {
            InitializeComponent();

            // Register MVVMLight messages
            Messenger.Default.Register(this, MessageToken.LoadScreenMessage, new Action<string>(OnLoadScreenMessage));
            Messenger.Default.Register(this, MessageToken.RaiseErrorMessage, new Action<Exception>(OnRaiseErrorMessage));
            Messenger.Default.Register(this, MessageToken.UseDialogMessage, new Action<DialogMessage>(OnUseDialogMessage));

            if (!ViewModelBase.IsInDesignModeStatic)
            {
                // Import the ViewModel module into the View DataContext so that 
                // the members of the ViewModel can be exposed
                DataContext = _catalogService.GetModule(ModuleID.MainPageViewModel);
            }
        }

        // Method to be executed when receiving the message
        private void OnLoadScreenMessage(string moduleId) 
        {           
            object newScreen;
            try
            {
                // Import selected View module and then
                // set the commandArg for UI changes
                switch (moduleId)
                {
                    case ModuleID.ProductListView:
                        newScreen = _catalogService.GetModule(ModuleID.ProductListView);
                        break;
                    case ModuleID.AnotherScreenView:
                        newScreen = _catalogService.GetModule(ModuleID.AnotherScreenView);
                        break;
                    case ModuleID.AnotherXapView:
                        newScreen = _catalogService.GetModule(ModuleID.AnotherXapView);
                        break;
                    default:
                        throw new NotImplementedException();
                }

                // Set the existing View module as object of ICleanup type                 
                var viewToCleanUp = MainContent.Content as ICleanup;
                if (viewToCleanUp != null)
                {
                    // Start clean-up by calling Cleanup() in the existing View
                    viewToCleanUp.Cleanup();

                    // Remove the existing View from Category Service - the last step
                    _catalogService.ReleaseModule((IModule)MainContent.Content);
                }

                // Set the new screen
                MainContent.Content = newScreen;

                // Set link button properties
                SetLinkButtonState(moduleId);
            }
            catch (Exception ex)
            {
                OnRaiseErrorMessage(ex);
            }
        }

        // UI
        private void SetLinkButtonState(string buttonArg)
        {
            foreach (UIElement child in LinksStackPanel.Children)
            {
                HyperlinkButton hb = child as HyperlinkButton;
                if (hb != null && hb.Command != null)
                {
                    if (hb.CommandParameter.ToString().Equals(buttonArg))
                    {
                        VisualStateManager.GoToState(hb, "ActiveLink", true);
                    }
                    else
                    {
                        VisualStateManager.GoToState(hb, "InactiveLink", true);
                    }
                }
            }
        }

        private void OnRaiseErrorMessage(Exception ex)
        {
            // Error message display
            ChildWindow errorWin = new ErrorWindow(ex.Message, ex.StackTrace);
            errorWin.Show();
        }

        private void OnUseDialogMessage(DialogMessage dialogMessage)
        {
            // MVVMLight DialogMessage callback processes
            if (dialogMessage != null)
            {
                MessageBoxResult result = MessageBox.Show(dialogMessage.Content,
                    dialogMessage.Caption, dialogMessage.Button);
                dialogMessage.ProcessCallback(result);
            }
        }
    }
}