How to open a new window using MVVM Light Toolkit

Question

Hi guys,

I am using MVVM light toolkit in my wpf application. I would like to know what is the best approach for opening a new window from an existing window. I have got this MainViewModel, which is respnsible for MainWindow of my application. Now in the MainView, on a button click, I would like to open a second window on top of it. I have got RelayCommmand binded to the button command. In the relay command method, I can create a new window object and simply call show, something like this

var view2 = new view2()
view2.Show()

but I don't think the viewmodel should be responsible for creating the new view2 object. I have read this post http://stackoverflow.com/questions/2761162/wpf-mvvm-get-parent-from-view-model where Bugnion has suggested to pass message to the view1 from the viewmodel1 and then view1 should create the new view2. But I am not sure what does he actually mean by passing the message to the view1? how should the view1 handle the message? In its code behind or what?

Regards, Nabeel

Answer 1

Passing a message from ViewModel1 to View1 means to use the messaging capabilities in the MVVM Light Toolkit.

For example, your ViewModel1 could have a command called ShowView2Command, then it would send a message to display the view.

public class ViewModel1 : ViewModelBase
{
    public RelayCommand ShowView2Command { private set; get; }

    public ViewModel1() : base()
    {
        ShowView2Command = new RelayCommand(ShowView2CommandExecute);
    }

    public void ShowView2CommandExecute()
    {
        Messenger.Default.Send(new NotificationMessage("ShowView2"));
    }
}

View1 would register to receive messages in its code behind and display View2 when it receives the correct message.

public partial class View1 : UserControl
{
    public View1()
    {
        InitializeComponent();
        Messenger.Default.Register<NotificationMessage>(this, NotificationMessageReceived);
    }

    private void NotificationMessageReceived(NotificationMessage msg)
    {
        if (msg.Notification == "ShowView2")
        {
            var view2 = new view2();
            view2.Show();
        }
    }
}

Answer 2

Unless I am missing the point here - if I were to use the code behind, then why not directly implement button_click event and open the second view?

What Bugnion seems to be suggesting is view1 -> button click -> relay command -> viewmodel1 -> message -> view1 -> view1.cs -> open view 2.

You are going to sacrifice testability anyhow by writing code-behind, so why take such a long route?