Questions about ObservableValidator in MVVM Toolkit Sample App

In the MVVM Toolkit Sample App there is a sample of the ObservableValidator. I have a couple questions (thank you in advance for reading through all this)
 
```
public ValidationFormWidgetViewModel(IDialogService dialogService)
{
     DialogService = dialogService;
}
```
 
I assume the IDialogService is being passed in via DI - is that correct? Is this the preferred way to invoke a dialog from the VM?
 
There is this declaration:
 
`public event EventHandler? FormSubmissionCompleted;`
 
Later, if there are no errors it is invoked with:
 
`FormSubmissionCompleted?.Invoke(this, EventArgs.Empty);`
 
Can you explain that construct? It does not show what gets called when you invoke that. The XAML on the other hand has 
 
```

       
           
               
                   
                   
               
           
       
```
 
So I'm guessing that when FormSubmissionCompleted is invoked it tiggers the display of the SuccessInfoBar??
 
Is EventTriggerBehavior documented anywhere? I can't find it.
 
Also what is with this construct: 
 
`TargetObject="{x:Bind SuccessInfoBar}"`
 
Is that the (new?) (preferred?) way of handling a binding?

Unfortunately the namespaces are not shown as far as I can tell. Is there a complete sample that has the complete contents of all the files for ObservableValidator?

Thanks!