Does Treeview use command bindings for expand/collapse?

Question

The WPF Treeview responds to + and - keystrokes to expand and collapse nodes in the tree. Great!

Is there an existing command I can bind my toolbar buttons or menu items to to perform the same actions in the treeview? I don't see anything related to expand/collapse in the stock command constants.

Vernacularize

Answer 1

The TreeView handles the expansion of a TreeViewItem with the mouse by binding ToggleButton.IsChecked to TreeViewItem.IsExpanded in the ControlTemplate and handles expansion with the keyboard in an override TreeViewItem.OnKeyDown. So, no, it doesn't use commands in its implementation.

But you can add your own commands without much effort. In this example, I've added a behavior to a TreeView so that it responds to the standard Open and Close application commands:

<DockPanel>
    <Menu DockPanel.Dock="Top">
        <MenuItem Header="Open" CommandTarget="{Binding ElementName=treeView1}" Command="Open"/>
        <MenuItem Header="Close" CommandTarget="{Binding ElementName=treeView1}" Command="Close"/>
    </Menu>
    <TreeView>
        <i:Interaction.Behaviors>
            <local:TreeViewCommandsBehavior/>
        </i:Interaction.Behaviors>
        <TreeViewItem Header="Root">
            <TreeViewItem Header="Item1">
                <TreeViewItem Header="Subitem1"/>
                <TreeViewItem Header="Subitem2"/>
            </TreeViewItem>
            <TreeViewItem Header="Item2">
                <TreeViewItem Header="Subitem3"/>
                <TreeViewItem Header="Subitem4"/>
            </TreeViewItem>
        </TreeViewItem>
    </TreeView>
</DockPanel>

and here is the behavior that makes that work:

public class TreeViewCommandsBehavior : Behavior<TreeView>
{
    private TreeViewItem selectedTreeViewItem;

    protected override void OnAttached()
    {
        AssociatedObject.AddHandler(TreeViewItem.SelectedEvent, new RoutedEventHandler(TreeViewItem_Selected));
        AssociatedObject.CommandBindings.Add(new CommandBinding(ApplicationCommands.Open, CommandExecuted));
        AssociatedObject.CommandBindings.Add(new CommandBinding(ApplicationCommands.Close, CommandExecuted));
    }

    private void TreeViewItem_Selected(object sender, RoutedEventArgs e)
    {
        selectedTreeViewItem = e.OriginalSource as TreeViewItem;
    }

    private void CommandExecuted(object sender, ExecutedRoutedEventArgs e)
    {
        bool expand = e.Command == ApplicationCommands.Open;
        if (selectedTreeViewItem != null)
            selectedTreeViewItem.IsExpanded = expand;
    }
}

If you are not familiar with behaviors, first add this namespace:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

and add the corresponding reference to your project.

Alvardo

Note that the System.Windows.Interactivity assembly is not part of the .NET 4.0 runtime distribution; it's available in the Microsoft Expression Blend SDK redistributables. – Vernacularize

Somehow, that fact had eluded me, probably because the name of the assembly. Although it creates a dependency on an external assembly, using behaviors is a big win because it makes the feature reusable without involving any code-behind. Thanks for pointing that out. – Alvardo