Handling the window closing event with WPF / MVVM Light Toolkit

Question

I'd like to handle the Closing event (when a user clicks the upper right 'X' button) of my window in order to eventually display a confirm message or/and cancel the closing.

I know how to do this in the code-behind: subscribe to the Closing event of the window then use the CancelEventArgs.Cancel property.

But I'm using MVVM so I'm not sure it's the good approach.

I think the good approach would be to bind the Closing event to a Command in my ViewModel.

I tried that:

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Closing">
        <cmd:EventToCommand Command="{Binding CloseCommand}" />
    </i:EventTrigger>
</i:Interaction.Triggers>

With an associated RelayCommand in my ViewModel but it doesn't work (the command's code is not executed).

Answer 1

I would simply associate the handler in the View constructor:

MyWindow() 
{
    // Set up ViewModel, assign to DataContext etc.
    Closing += viewModel.OnWindowClosing;
}

Then add the handler to the ViewModel:

using System.ComponentModel;

public void OnWindowClosing(object sender, CancelEventArgs e) 
{
   // Handle closing logic, set e.Cancel as needed
}

In this case, you gain exactly nothing except complexity by using a more elaborate pattern with more indirection (5 extra lines of XAML plus Command pattern).

The "zero code-behind" mantra is not the goal in itself, the point is to decouple ViewModel from the View. Even when the event is bound in code-behind of the View, the ViewModel does not depend on the View and the closing logic can be unit-tested.