Inner GroupBox does not swallow the Interactivity event - wpf

Question

I have a GroupBox within a parent GroupBox. Both of them have their own
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown">
<i:InvokeCommandAction Command="{Binding ...}" />
</i:EventTrigger>
</i:Interaction.Triggers>
When I press the inner GroupBox it fires its own Command and then the parent Command is also triggered.
How do I prevent that? How do I make the inner GroupBox swallow the event?

Answer 1

You could use another implementation of TriggerAction that supports passing the event args as a command parameter to the command, for example the EventToCommand class in the MvvmLight library:
<GroupBox Header="Outer" xmlns:mvvm="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Platform">
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown">
<i:InvokeCommandAction Command="{Binding OuterCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock>...</TextBlock>
<GroupBox Header="Inner" Grid.Row="1">
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown">
<mvvm:EventToCommand Command="{Binding InnerCommand}" PassEventArgsToCommand="True" />
</i:EventTrigger>
</i:Interaction.Triggers>
<TextBlock>inner...</TextBlock>
</GroupBox>
</Grid>
</GroupBox>
public class ViewModel
{
public ViewModel()
{
OuterCommand = new RelayCommand(arg => true, (arg)=> { MessageBox.Show("outer"); });
InnerCommand = new RelayCommand(arg => true,
(arg) =>
{
MessageBox.Show("inner");
MouseButtonEventArgs mbea = arg as MouseButtonEventArgs;
if (mbea != null)
mbea.Handled = true;
});
}
public RelayCommand OuterCommand { get; }
public RelayCommand InnerCommand { get; }
}
The ugly thing with this solution is that the view model has a dependency upon the view related MouseButtonEventArgs type though. If you don't like this you can implement your own behaviour as suggested by #adabyron here:
MVVM Passing EventArgs As Command Parameter
Then you could set the Handled property of the MouseButtonEventArgs directly in the behaviour instead of passing it along to the view model.