MvvmLight Good Practice : Show a Form by a ViewModel

Question

I'm a fresher with WPF, MVVM. Now, I have been investigating MVVM Light. I got some confused. I appreciate any help from all of you. Thanks in advance for taking a look at this post.

Can we absolutely remove code behind of View (such as: Invoking InitializeComponent() somewhere inside .xaml --> Does not need view.xaml.cs anymore in some simple case).
I have many views insight my project, how many locator is necessary? I intend to make separate locator for each view. And I wonder that if I register all locators in app.xml, are all of views initialized and registered right after user run the application? If Yes, Is it not good for performance?
Main question: I have a scenario: I have many forms: such as :
1. MainForm: which is the 1st form invoked by application.
2. ImportForm: which is invoked when User click Import (from MainForm)

Assumption that: I did finish all stuff related to binding (such as button Import --> RelayCommand(OnImport)) What is the best practice for me to implement this scenario? 1. Just implement to init and show ImportForm like below:

public void OnImport()
{
   ImportForm importForm = new ImportForm();
   importForm.ShowDialog();
}

It's simple, but I wonder if this way follow the MVVM's paradigm?

So, I did some researching and do another way like:

public void OnImport() { //// Just send message Messenger.Default.Send( new NotificationMessage(this, "OnImport")); }

In Code Behind: MainForm.xaml.cs

public MainForm()
    {
        InitializeComponent();
        Messenger.Default.Register<NotificationMessage>(this, (nm) =>
        {
           if (nm.Sender == this.DataContext)
           {    
                if (nm.Notification == "OnImport")
                {  
                    ImportForm importForm = new ImportForm();
                    importForm.ShowDialog();
                }
           }
        });
    }

By this way, I must write some code inside code behind --> Is it a problem?

Both above solutions can finish mentioned scenario correctly, but I confused which one is better or is there any actually right solution out there?

Thanks again for your patience with the long question.

Ekk Ekk · Accepted Answer · 2012-12-06T07:41:34

No, you cannot. InitializeComponent() paints UI on the screen. The purpose of MVVM is to separate logic that does not related to View and store it in a ViewModel. It does not tend or aim to remove code-behind.
It depends on you. You can create one Locator for all ViewModels or one Locator per one ViewModel. Anyway, I found that Locator does not scale well and hard to manage in a larger project. It creates dependency between View, Locators and ViewModels. I personal prefer to use DI framework instead of Locator even if it is a small project.
You can do both, depends on your requirement. (a) If clicking the button on the main form does nothing more than show a dialog then I would use Click event because it is View related. It has nothing to do with any logic, so keep it in the code behind is the best solution for me. (b) By the way, if clicking the button does something, for example, connect to a database then show a dialog if a condition is true. In this case, I would use Messenger to keep View and ViewModel separate from each other.

MvvmLight Good Practice : Show a Form by a ViewModel

1 Answers