WPF - update a dependency property from a DataTrigger - wpf

Question

I have a listView and want when an item is selected to update a public value from the DataContext.
I know I can do this through the SelectedItem from listview, I want to do this (if possible) with a data trigger.
Code:
<Window.DataContext>
<local:MainWindowViewModel></local:MainWindowViewModel>
</Window.DataContext>
<Grid>
<ListView ItemsSource="{Binding listsToDisplay}">
<ListView.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding listName}"></TextBlock>
<DataTemplate.Triggers>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ListViewItem}}, Path=IsSelected}" Value="True">
<Setter Property="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:MainWindowViewModel}}, Path=listWidth}" Value="1"></Setter>
</DataTrigger>
</DataTemplate.Triggers>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</Grid>

Answer 1

I want to do this (if possible) with a data trigger.
This is not possible. You cannot set a value of a view model property in a setter of a style.
A style setter can only set the value of a property that belongs to the element to which the style is applied.

Answer 2

You could bind the selected item of the listView to a property on the viewmodel then do the update in the viewmodel when that value changes through the property.
<ListView ItemsSource="{Binding listsToDisplay}" SelectedItem="{Binding SelectedItem}">
<ListView.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding listName}"></TextBlock>
</ListView.ItemTemplate>
</ListView>
This also has the advantage of allowing you to get or set the selected item in the viewModel.