WPF how to binding an attached event to viewmodel? - wpf

Question

<TreeView x:Name="TestTree"
ItemsSource="{Binding Children}"
ItemTemplateSelector="{StaticResource TemplateSelector}" >
<i:Interaction.Triggers>
<i:EventTrigger EventName="TreeViewItem.Seleted">
<i:InvokeCommandAction
Command="{Binding SelectedCommand}"
CommandParameter="{Binding RelativeSource={RelativeSource Self}}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</TreeView>
As above codes mentioned, I wanna get the selected treeviewitem data from the view, while the binding event TreeViewItem.Seleted which is an attached event(member event is okay) cannot be received in the viewmodel. How to binding an attached event to viewmodel?

Answer 1

then, you won't get TreeViewItem event in the TreeView ? you need a custom item template. I do it like this on a datagrid with MVVMLight (but no item)
</DataGrid.Columns>
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseDoubleClick" >
<command:EventToCommand Command="{Binding Path=OpenEquipementCommand}" PassEventArgsToCommand="True"/>
</i:EventTrigger>
<i:EventTrigger EventName="SelectionChanged">
<command:EventToCommand Command="{Binding Path=SelectionChangedCommand}" PassEventArgsToCommand="True" />
</i:EventTrigger>
</i:Interaction.Triggers>
</DataGrid>
so i think you must do
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectedItemChanged" >
<command:EventToCommand Command="{Binding Path=SelectionChangedCommand}" PassEventArgsToCommand="True" />
</i:EventTrigger>
</i:Interaction.Triggers>

Answer 2

Why don't just use plain old eventhandler?
<TreeView x:Name="TreeView1" SelectedItemChanged="TreeView_SelectedItemChanged" />
public partial class MainWindow : Window
{
MainWindowViewModel ViewModel => (MainWindowViewModel) DataContext;
private void TreeView1_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
var element = (FrameworkElement)sender;
var item = element.DataContext;
ViewModel.SelectionChangedCommand.Invoke(item);
//alternativelly:
ViewModel.SelectedItem = TreeView1.SelectedItem;
}
}
Just because WPF support binding, it does not mean you have to use it everywhere even if it's very complicated. The code I've written is not violation of MVVM.
If you have a good reason to avoid code behind, you may implement this eventhandler in a custom behavior implemented as attached property, so it would look like this:
<TreeView local:TreeViewBehavior.SelectionChangedCommand="{Binding SelectionChangedCommand}" />