Get Selected TreeViewItem Using MVVM

Based on Martin's answer I made a simple application showing how to apply the proposed solution.

The sample code uses the Cinch V2 framework to support MVVM but it can be easily changed to use the framework of you preference.

For those interested, here is the code on GitHub

Hope it helps.

I would probably use the SelectedItemChanged event to set a respective property on your VM.

To do what you want you can modify the ItemContainerStyle of the TreeView:

<TreeView>
  <TreeView.ItemContainerStyle>
    <Style TargetType="TreeViewItem">
      <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
    </Style>
  </TreeView.ItemContainerStyle>
</TreeView>

Your view-model (the view-model for each item in the tree) then has to expose a boolean IsSelected property.

If you want to be able to control if a particular TreeViewItem is expanded you can use a setter for that property too:

<Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}"/>

Your view-model then has to expose a boolean IsExpanded property.

Note that these properties work both ways so if the user selects a node in the tree the IsSelected property of the view-model will be set to true. On the other hand if you set IsSelected to true on a view-model the node in the tree for that view-model will be selected. And likewise with expanded.

If you don't have a view-model for each item in the tree, well, then you should get one. Not having a view-model means that you are using your model objects as view-models, but for this to work these objects require an IsSelected property.

To expose an SelectedItem property on your parent view-model (the one you bind to the TreeView and that has a collection of child view-models) you can implement it like this:

public ChildViewModel SelectedItem {
  get { return Items.FirstOrDefault(i => i.IsSelected); }
}

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If you don't want to track selection on each individual item on the tree you can still use the SelectedItem property on the TreeView. However, to be able to do it "MVVM style" you need to use a Blend behavior (available as various NuGet packages - search for "blend interactivity").

Here I have added an EventTrigger that will invoke a command each time the selected item changes in the tree:

<TreeView x:Name="treeView">
  <i:Interaction.Triggers>
    <i:EventTrigger EventName="SelectedItemChanged">
      <i:InvokeCommandAction
        Command="{Binding SetSelectedItemCommand}"
        CommandParameter="{Binding SelectedItem, ElementName=treeView}"/>
    </i:EventTrigger>
  </i:Interaction.Triggers>
</TreeView>

You will have to add a property SetSelectedItemCommand on the DataContext of the TreeView returning an ICommand. When the selected item of the tree view changes the Execute method on the command is called with the selected item as the parameter. The easiest way to create a command is probably to use a DelegateCommand (google it to get an implementation as it is not part of WPF).

A perhaps better alternative that allows two-way binding without the clunky command is to use BindableSelectedItemBehavior provided by Steve Greatrex here on Stack Overflow.