Where to put events when using MVVM?

Question

Should I put all events in views code behind or there is a more proper way, like place commands in ViewModel? For example, I want to open Tab on double click on the datagrid row, where should I handle this event?

Answer 1

Kyle is correct in that your handlers should appear in the view model. If a command property doesn't exist then you can use an interaction trigger instead:

<DataGrid>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="MouseDoubleClick">
            <i:InvokeCommandAction Command="{Binding Mode=OneWay, Path=OpenClientCommand}" CommandParameter="{Binding ElementName=searchResults, Path=SelectedItems}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>

    ... other stuff goes here ...

</DataGrid>

Or you can use MVVM Lite's EventToCommand, which also allows you to pass in the message parameters:

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

Which is used in in this case to cancel the window close event in response to the "Are you sure you want to quit?" dialog:

public ICommand ClosingCommand { get { return new RelayCommand<CancelEventArgs>(OnClosing); } }

private void OnClosing(CancelEventArgs args)
{
    if (UserCancelsClose())
        args.Cancel = true;
}

Relevant namespaces are as follows:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:cmd ="http://www.galasoft.ch/mvvmlight"