c# - The best approach to create new window in WPF using MVVM

Question

In the neighbour post: How should the ViewModel close the form? I've posted my vision how to close windows with MVVM usage. And now I have a question: how to open them.

I have a main window (main view). If user clicks on the "Show" button then "Demo" window (modal dialog) should be displayed. What is a preferable way to create and open windows using MVVM pattern? I see two general approaches:

The 1st one (probably the simplest). Event handler "ShowButton_Click" should be implemented in the code behind of the main window in way like this:

        private void ModifyButton_Click(object sender, RoutedEventArgs e)
        {
            ShowWindow wnd = new ShowWindow(anyKindOfData);
            bool? res = wnd.ShowDialog();
            if (res != null && res.Value)
            {
                //  ... store changes if neecssary
            }
        }

1. If we "Show" button state should be changed (enabled/disabled) we will need to add logic that will manage button state;
2. The source code is very similar to "old-style" WinForms and MFC sources - I not sure if this is good or bad, please advise.
3. Something else that I've missed?

Another approach:

In the MainWindowViewModel we will implement "ShowCommand" property that will return ICommand interface of the command. Comman in turn:

• will raise "ShowDialogEvent";
• will manage button state.

This approach will be more suitable for the MVVM but will require additional coding: ViewModel class can't "show dialog" so MainWindowViewModel will only raise "ShowDialogEvent", the MainWindowView we will need to add event handler in its MainWindow_Loaded method, something like this:

((MainWindowViewModel)DataContext).ShowDialogEvent += ShowDialog;

(ShowDialog - similar to the 'ModifyButton_Click' method.)

So my questions are: 1. Do you see any other approach? 2. Do you think one of the listed is good or bad? (why?)

Any other thoughts are welcome.

Thanks.

Answer 1

Some MVVM frameworks (e.g. MVVM Light) make use of the Mediator pattern. So to open a new Window (or create any View) some View-specific code will subscribe to messages from the mediator and the ViewModel will send those messages.

Like this:

Subsription

Messenger.Default.Register<DialogMessage>(this, ProcessDialogMessage);
...
private void ProcessDialogMessage(DialogMessage message)
{
     // Instantiate new view depending on the message details
}

In ViewModel

Messenger.Default.Send(new DialogMessage(...));

I prefer to do the subscription in a singleton class, which "lives" as long as the UI part of the application does. To sum up: ViewModel passes messages like "I need to create a view" and the UI listens to those messages and acts on them.

There's no "ideal" approach though, for sure.