mvvm light Messenger.Default.Register in View codebehind?

BookShelf解决方案John Papa在Mix11上展示的东西听起来有点奇怪…它使用MVVM模式和MVVM Light工具包…一切都很好。我唯一不能理解的是:在视图的代码后面，它注册了一对消息，下面是代码:

public partial class BookView : Page
{
    public BookView()
    {
        InitializeComponent();
        //btnEdit.SetBinding(Button.IsEnabledProperty, new Binding("User.IsAuthenticated") { Source = Application.Current.Resources["WebContext"] });
        Title = ApplicationStrings.HomePageTitle;
        RegisterMessages();
    }
    private void RegisterMessages()
    {
        Messenger.Default.Register<LaunchEditBookMessage>(this, OnLaunchEditBook);
        Messenger.Default.Register<SavedBookDialogMessage>(this, OnSaveBookDialogMessageReceived);
    }
    private void OnLaunchEditBook(LaunchEditBookMessage msg)
    {
        var editBook = new EditBookWindow();
        editBook.Show();
    }
    private void OnSaveBookDialogMessageReceived(SavedBookDialogMessage msg)
    {
        MessageBox.Show(msg.Content, msg.Caption, msg.Button);
    }
//...

这是一个业务应用程序，如果您从该页面切换到另一个页面，然后回到那里，页面再次实例化并继续为那些消息注册，导致那些多次着火…

怎么来它订阅那些消息在代码后，而不是在ViewModels?这是否与UI线程有关?这是一个正确的实现吗?

如果用户导航到另一个页面，您将如何注销这些消息?

EDIT: refactoring CODE

XAML

<sdk:Page Loaded="Page_Loaded" Unloaded="Page_Unloaded">

代码后面

private void Page_Loaded(object sender, RoutedEventArgs e)
    {
        RegisterMessages();
    }
    private void Page_Unloaded(object sender, RoutedEventArgs e)
    {
        Messenger.Default.Unregister(this);
    }