Binding SelectedItem in a HierarchicalDataTemplate-applied WPF TreeView

Question

I have a data-bound TreeView and I want to bind SelectedItem. This attached behavior works perfectly without HierarchicalDataTemplate but with it the attached behavior only works one way (UI to data) not the other because now e.NewValue is MyViewModel not TreeViewItem.

This is a code snippet from the attached behavior:

private static void OnSelectedItemChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    var item = e.NewValue as TreeViewItem;
    if (item != null)
    {
        item.SetValue(TreeViewItem.IsSelectedProperty, true);
    }
}

This is my TreeView definition:

<Window xmlns:interactivity="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity">
    <TreeView ItemsSource="{Binding MyItems}" VirtualizingStackPanel.IsVirtualizing="True">
        <interactivity:Interaction.Behaviors>
            <behaviors:TreeViewSelectedItemBindingBehavior SelectedItem="{Binding SelectedItem, Mode=TwoWay}" />
        </interactivity:Interaction.Behaviors>
        <TreeView.Resources>
            <HierarchicalDataTemplate DataType="{x:Type local:MyViewModel}" ItemsSource="{Binding Children}">
                <TextBlock Text="{Binding Name}"/>
            </HierarchicalDataTemplate>
        </TreeView.Resources>
    </TreeView>
</Window>

If I can get a reference to the TreeView in the attached behavior method OnSelectedItemChanged, maybe I can use the answers in this question to get the TreeViewItem but I don't know how to get there. Does anyone know how and is it the right way to go?

Answer 1

Here is an improved version of the above mentioned attached behavior. It fully supports twoway binding and also works with HeriarchicalDataTemplate and TreeViews where its items are virtualized. Please note though that to find the 'TreeViewItem' that needs to be selected, it will realize (i.e. create) the virtualized TreeViewItems until it finds the right one. This could potentially be a performance problem with big virtualized trees.

/// <summary>
///     Behavior that makes the <see cref="System.Windows.Controls.TreeView.SelectedItem" /> bindable.
/// </summary>
public class BindableSelectedItemBehavior : Behavior<TreeView>
{
    /// <summary>
    ///     Identifies the <see cref="SelectedItem" /> dependency property.
    /// </summary>
    public static readonly DependencyProperty SelectedItemProperty =
        DependencyProperty.Register(
            "SelectedItem",
            typeof(object),
            typeof(BindableSelectedItemBehavior),
            new UIPropertyMetadata(null, OnSelectedItemChanged));

    /// <summary>
    ///     Gets or sets the selected item of the <see cref="TreeView" /> that this behavior is attached
    ///     to.
    /// </summary>
    public object SelectedItem
    {
        get
        {
            return this.GetValue(SelectedItemProperty);
        }

        set
        {
            this.SetValue(SelectedItemProperty, value);
        }
    }

    /// <summary>
    ///     Called after the behavior is attached to an AssociatedObject.
    /// </summary>
    /// <remarks>
    ///     Override this to hook up functionality to the AssociatedObject.
    /// </remarks>
    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.SelectedItemChanged += this.OnTreeViewSelectedItemChanged;
    }

    /// <summary>
    ///     Called when the behavior is being detached from its AssociatedObject, but before it has
    ///     actually occurred.
    /// </summary>
    /// <remarks>
    ///     Override this to unhook functionality from the AssociatedObject.
    /// </remarks>
    protected override void OnDetaching()
    {
        base.OnDetaching();
        if (this.AssociatedObject != null)
        {
            this.AssociatedObject.SelectedItemChanged -= this.OnTreeViewSelectedItemChanged;
        }
    }

    private static Action<int> GetBringIndexIntoView(Panel itemsHostPanel)
    {
        var virtualizingPanel = itemsHostPanel as VirtualizingStackPanel;
        if (virtualizingPanel == null)
        {
            return null;
        }

        var method = virtualizingPanel.GetType().GetMethod(
            "BringIndexIntoView",
            BindingFlags.Instance | BindingFlags.NonPublic,
            Type.DefaultBinder,
            new[] { typeof(int) },
            null);
        if (method == null)
        {
            return null;
        }

        return i => method.Invoke(virtualizingPanel, new object[] { i });
    }

    /// <summary>
    /// Recursively search for an item in this subtree.
    /// </summary>
    /// <param name="container">
    /// The parent ItemsControl. This can be a TreeView or a TreeViewItem.
    /// </param>
    /// <param name="item">
    /// The item to search for.
    /// </param>
    /// <returns>
    /// The TreeViewItem that contains the specified item.
    /// </returns>
    private static TreeViewItem GetTreeViewItem(ItemsControl container, object item)
    {
        if (container != null)
        {
            if (container.DataContext == item)
            {
                return container as TreeViewItem;
            }

            // Expand the current container
            if (container is TreeViewItem && !((TreeViewItem)container).IsExpanded)
            {
                container.SetValue(TreeViewItem.IsExpandedProperty, true);
            }

            // Try to generate the ItemsPresenter and the ItemsPanel.
            // by calling ApplyTemplate.  Note that in the 
            // virtualizing case even if the item is marked 
            // expanded we still need to do this step in order to 
            // regenerate the visuals because they may have been virtualized away.
            container.ApplyTemplate();
            var itemsPresenter =
                (ItemsPresenter)container.Template.FindName("ItemsHost", container);
            if (itemsPresenter != null)
            {
                itemsPresenter.ApplyTemplate();
            }
            else
            {
                // The Tree template has not named the ItemsPresenter, 
                // so walk the descendents and find the child.
                itemsPresenter = container.GetVisualDescendant<ItemsPresenter>();
                if (itemsPresenter == null)
                {
                    container.UpdateLayout();
                    itemsPresenter = container.GetVisualDescendant<ItemsPresenter>();
                }
            }

            var itemsHostPanel = (Panel)VisualTreeHelper.GetChild(itemsPresenter, 0);

            // Ensure that the generator for this panel has been created.
#pragma warning disable 168
            var children = itemsHostPanel.Children;
#pragma warning restore 168

            var bringIndexIntoView = GetBringIndexIntoView(itemsHostPanel);
            for (int i = 0, count = container.Items.Count; i < count; i++)
            {
                TreeViewItem subContainer;
                if (bringIndexIntoView != null)
                {
                    // Bring the item into view so 
                    // that the container will be generated.
                    bringIndexIntoView(i);
                    subContainer =
                        (TreeViewItem)container.ItemContainerGenerator.
                                                ContainerFromIndex(i);
                }
                else
                {
                    subContainer =
                        (TreeViewItem)container.ItemContainerGenerator.
                                                ContainerFromIndex(i);

                    // Bring the item into view to maintain the 
                    // same behavior as with a virtualizing panel.
                    subContainer.BringIntoView();
                }

                if (subContainer == null)
                {
                    continue;
                }

                // Search the next level for the object.
                var resultContainer = GetTreeViewItem(subContainer, item);
                if (resultContainer != null)
                {
                    return resultContainer;
                }

                // The object is not under this TreeViewItem
                // so collapse it.
                subContainer.IsExpanded = false;
            }
        }

        return null;
    }

    private static void OnSelectedItemChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var item = e.NewValue as TreeViewItem;
        if (item != null)
        {
            item.SetValue(TreeViewItem.IsSelectedProperty, true);
            return;
        }

        var behavior = (BindableSelectedItemBehavior)sender;
        var treeView = behavior.AssociatedObject;
        if (treeView == null)
        {
            // at designtime the AssociatedObject sometimes seems to be null
            return;
        }

        item = GetTreeViewItem(treeView, e.NewValue);
        if (item != null)
        {
            item.IsSelected = true;
        }
    }

    private void OnTreeViewSelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        this.SelectedItem = e.NewValue;
    }
}

And for the sake of completeness hier is the implementation of GetVisualDescentants:

/// <summary>
///     Extension methods for the <see cref="DependencyObject" /> type.
/// </summary>
public static class DependencyObjectExtensions
{
    /// <summary>
    ///     Gets the first child of the specified visual that is of tyoe <typeparamref name="T" />
    ///     in the visual tree recursively.
    /// </summary>
    /// <param name="visual">The visual to get the visual children for.</param>
    /// <returns>
    ///     The first child of the specified visual that is of tyoe <typeparamref name="T" /> of the
    ///     specified visual in the visual tree recursively or <c>null</c> if none was found.
    /// </returns>
    public static T GetVisualDescendant<T>(this DependencyObject visual) where T : DependencyObject
    {
        return (T)visual.GetVisualDescendants().FirstOrDefault(d => d is T);
    }

    /// <summary>
    ///     Gets all children of the specified visual in the visual tree recursively.
    /// </summary>
    /// <param name="visual">The visual to get the visual children for.</param>
    /// <returns>All children of the specified visual in the visual tree recursively.</returns>
    public static IEnumerable<DependencyObject> GetVisualDescendants(this DependencyObject visual)
    {
        if (visual == null)
        {
            yield break;
        }

        for (var i = 0; i < VisualTreeHelper.GetChildrenCount(visual); i++)
        {
            var child = VisualTreeHelper.GetChild(visual, i);
            yield return child;
            foreach (var subChild in GetVisualDescendants(child))
            {
                yield return subChild;
            }
        }
    }
}