Two-Way Binding on TreeView.SelectedItem

In this post I’m going to describe an attached-behavior for a TreeView control that allows you to achieve two-way binding on the SelectedItem property. You can grab the source code here.

[UPDATE 6 Dec 2010]: I fixed a bug that was causing it to fail to update the TreeView when the selected node was set (through binding) before the view had loaded.

Background

If you have ever tried using the TreeView control with it’s ItemsSource bound to some kind of data context then you probably know how frustrating it can be to work with the SelectedItem property. This is especially true if you are trying to follow an MVVM pattern.

Typically when you use an ItemsControl you want to create a two-way binding on two of its properties: ItemsSource and SelectedItem. But TreeView, unlike other ItemsControl subclasses, has a read-only SelectedItem property which means no two-way binding like this:

<sdk:TreeView 
    ItemsSource="{Binding TreeNodes}" 
    SelectedItem="{Binding SelectedNode, Mode=TwoWay}"/>

There are a number of workarounds for this challenge (and it is a challenge – just try a Google search for Silverlight TreeView SelectedItem) but most of the solutions I’ve read either seem too complex to use, require you to compromise the MVVM pattern, or are unfriendly with Blend. Wouldn’t it be nice if you could just attach a behavior to the TreeView to make it work?

BindableTreeViewSelectedItemBehavior

Yeah – it’s a long name for a behavior, but at least you know what it does just by reading its name. Using it in Blend is pretty easy:

Just drop it on the TreeView control and set its Binding property.The XAML binding will look like this:

<sdk:TreeView ItemsSource="{Binding TreeNodes}" ...>
    <i:Interaction.Behaviors>
        <Behaviors:BindableTreeViewSelectedItemBehavior 
            Binding="{Binding SelectedNode}"/>
    </i:Interaction.Behaviors>
</sdk:TreeView>

You don’t need to make it a two-way binding, the behavior looks after that for you. Here is a sample application that has two TreeView controls bound to the same data context. If you change the selected item on one TreeView it will update the selected item on the other TreeView:

Under The Hood

This attached behavior works by acting as a go-between using the TreeView.SelectedItemChanged event, the TreeView.SelectedItem property, and a private two-way binding on the data context. Here is a diagram that shows how the behavior wires itself up:

Click on the diagram to see a larger version. The diagram can be broken down as follows:

• The behavior examines it’s Binding property and uses that information to create a private two-way binding between the property on the data context of the tree view (in our example it’s against the SelectedNode property) and the behavior’s private SelectedItemProperty DependencyProperty.
  
• When the value for the DataContext’s SelectedNode property changes, the change event for the SelectedItemProperty DependencyProperty (AssociatedObjectSelectedItemChanged) fires and we set the new value on the TreeView using the TreeView.SelectedItem property. This allows the tree view to have its selected item set from the data context.
  
• The behavior also attaches an event handler for the TreeView.SelectedItemChanged event (also called SelectedItemChanged) which fires whenever the user changes the selected item.
  
• When the SelectedItemChanged event handler is called, the behavior updates the value of its SelectedItemProperty DependencyProperty. This allows the data context to have its SelectedNode value updated when the user changes the selected item in the TreeView.

Summary

In this post I described an attached behavior that lets you achieve two-way binding on the TreeView.SelectedItem property. The behavior is especially useful if you are using the MVVM pattern to keep your views and view models separate, and want to avoid code in the View’s code-behind file. The source can be downloaded here.