Very late to the party with my answer but for those wanting a pure MVVM solution this can be done with an Event Trigger (to update the binding when the user selects a new item) and a Data Trigger (to update the selected item when the value of the binding changes).
For this to work the main ViewModel needs the items, a property for the currently selected item and a command property that will be called when the currently selected item changes:
public class MainViewModel : ViewModelBase
{
// the currently selected node, can be changed programmatically
private Node _CurrentNode;
public Node CurrentNode
{
get { return this._CurrentNode; }
set { this._CurrentNode = value; RaisePropertyChanged(() => this.CurrentNode); }
}
// called when the user selects a new node in the tree view
public ICommand SelectedNodeChangedCommand { get { return new RelayCommand<Node>(OnSelectedNodeChanged); } }
private void OnSelectedNodeChanged(Node node)
{
this.CurrentNode = node;
}
// list of items to display in the tree view
private ObservableCollection<Node> _Items;
public ObservableCollection<Node> Items
{
get { return this._Items; }
set { this._Items = value; RaisePropertyChanged(() => this.Items); }
}
}
The TreeView needs an event trigger to call SelectedNodeChangedCommand when the selection changes, and a DataTrigger in the TreeViewItem style so that the control items get selected when the value of CurrentNode is changed programatically in the code:
<TreeView x:Name="treeView" ItemsSource="{Binding Items}"
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:cmd ="http://www.galasoft.ch/mvvmlight">
<TreeView.Resources>
<conv:EqualityConverter x:Key="EqualityConverter" />
<Style TargetType="TreeViewItem">
<Setter Property="IsExpanded" Value="True" />
<Setter Property="IsSelected" Value="False" />
<Style.Triggers>
<!-- DataTrigger updates TreeViewItem selection when vm code changes CurrentNode -->
<DataTrigger Value="True">
<DataTrigger.Binding>
<MultiBinding Converter="{StaticResource EqualityConverter}">
<Binding RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type TreeView}}" Path="DataContext.CurrentNode" />
<Binding />
</MultiBinding>
</DataTrigger.Binding>
<Setter Property="IsSelected" Value="True" />
</DataTrigger>
</Style.Triggers>
</Style>
<!-- *** HierarchicalDataTemplates go here *** -->
</TreeView.Resources>
<!-- EventTrigger invokes SelectedNodeChangedCommand when selection is changed by user interaction -->
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectedItemChanged">
<cmd:EventToCommand Command="{Binding SelectedNodeChangedCommand}" CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=TreeView}, Path=SelectedItem}" />
</i:EventTrigger>
</i:Interaction.Triggers>
</TreeView>
The DataTrigger works by detecting when the value of CurrentNode matches the Node for the current list item. Unfortunately DataTriggers can't bind their Value, so it has to test with an EqualityConverter instead which just does a simple comparison:
public class EqualityConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
return values[0] == values[1];
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
