how can I set the xaml content in viewmodel - silverlight

Question

I've had a hyperlink button where i set in the button click in code behind the content to a new view if the login success.
private void OkButtonClick(object sender, RoutedEventArgs e)
{
LoginOperation loginOp = FLS.Utilities.RIAWebContext.Current.Authentication.Login(
new LoginParameters(usernameTextBox.Text, passwordTextBox.Text));
loginOp.Completed += (s2, e2) =>
{
if (loginOp.HasError)
{
errorTextBlock.Text = loginOp.Error.Message;
loginOp.MarkErrorAsHandled();
return;
}
else if (!loginOp.LoginSuccess)
{
errorTextBlock.Text = "Login failed.";
return;
}
else
{
errorTextBlock.Text = string.Empty;
Content = new WelcomeView();
}
};
}
I've now moved the code behind for MVVM in a view model and use a delegateCommand on the hyperlink button.
<UserControl ... >
<Grid ... >
...
<HyperlinkButton Content="Login" Height="23" HorizontalAlignment="Left" Margin="313,265,0,0" Name="loginButton" Command="{Binding Path=LoginCommand}" VerticalAlignment="Top" Width="75"/>
...
</Grid>
</UserControl>
But I don't know, how I make the Content = new WelcomeView(); from the code behind in the viewmodel?

Answer 1

A good design pattern would be to have two different Data Template, one to present the data before the login, and the second Data Template to be used after the login.
There are several ways to achieve that. The one that I typically use simply put the ViewModel (directly of using binding) the only children of Window.
In your ViewModel implement a content selector class. This is a class derived from DataTemplateSelector and uses the FindResource API to get the appropriate data template.
<Window ...>
<Window.Resources>
<DataTemplate x:key="beforeLogin">
...
</DataTemplate>
<DataTemplate x:Key="afterLogin">
...
</DataTemplate>
</Window.Resources>
<Window.ContentTemplateSelector>
<code:MyTemplateSelector />
</Window.ContentTemplateSelector>
<-- Here is the content of Window. It's the view model (data). The View will be
bind by the TemplateSelector
<code:YourViewModel />
</Window>
Check out this page: http://msdn.microsoft.com/en-us/library/system.windows.controls.contentcontrol.contenttemplateselector.aspx for a related example.
There are other design pattern. Another common idiom is simply firing a "UiRequest" event, which will be picked up by the code-behind of the view. Remember that MVVM dictates the ViewModel is "view agnostic", but it really doesn't mean "no code behind". This means the VM cannot reference anything in the view. Communication this way happens view events (e.g. data binding is just a wrapper around property changed events). So have an event UiRequest in your View Model, and design a protocol. In the constructor of the View - register a handler. In the handler, change the content (people use this idiom mainly to start a popup window, but it can be used anywhere).