How do I switch views from within a view?

Question

I'm making a WPF application making use of the MVVM light framework.

What I'm trying to do is have a login form in a view, when the user presses a button within that view it launches a LoginCommand for the attached ViewModel. From there I either want to launch a new window which holds the rest of the application, or simply switch views from the same window.

Currently I have it so that there is a view called MainView which has a content control inside bound to View1. However to switch to View2 I need to put the button for this on MainView, and not within View1 where it belongs.

Any advice?

Answer 1

Usually I do this one of two ways:

If the login window is a one-time thing required before starting the application, I will put it in the OnStartup() method of the Application object

protected override void OnStartup(StartupEventArgs e)
{
    base.OnStartup(e);

    // Login
    var login = new LoginDialog();
    var loginVm = new LoginViewModel();

    login.DataContext = loginVm;
    login.ShowDialog();

    if (!login.DialogResult.GetValueOrDefault())
    {
        // Error is handled in login class, not here
        Environment.Exit(0);
    }

    // If login is successful, show main application
    var app = new ShellView();
    var appModel = new ShellViewModel();

    app.DataContext = viewModel;
    app.Show();
}

The other way I usually do this is through a ShellViewModel or ApplicationViewModel which handles all my window management. This method uses DataTemplates to define each screen, and uses a ContentControl as a placeholder for the current screen in the ShellView or ApplicationView.

I usually combine this with an Event System of some kind, like Microsoft Prism's EventAggregator, so it can listen for messages of a specific type, such as OpenWindow or CloseWindow messages. If you're interested, I have a blog post about Communication between ViewModels that should give you a better idea of what an event system looks like.

For example, my ShellViewModel might start by displaying a LoginViewModel (a DataTemplate is used to tell WPF to draw the LoginViewModel with the LoginView), and it would subscribe to receive messages of type SuccessfulLogin. Once the LoginViewModel broadcasts a SuccessfulLogin message, the ShellViewModel will close the LoginViewModel and replace it with the ApplicationViewModel. You can see an example of this in my article on Navigation with MVVM