<TreeView x:Class="EArchiveMaster.View.MyTreeView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
            <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
            <EventSetter Event="LostFocus" Handler="EventSetter_OnHandler" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

public partial class MyTreeView
{
    public event Action SomeItemLostFocus;

    public MyTreeView()
    {
        InitializeComponent();
    }

    private void EventSetter_OnHandler(object sender, RoutedEventArgs e)
    {
        e.Handled = true;
        if (SomeItemLostFocus != null)
            SomeItemLostFocus();
    }
}

public static List<TreeViewItem> FindTreeViewItems(this Visual @this)
        {
            if (@this == null)
                return null;

            var result = new List<TreeViewItem>();

            var frameworkElement = @this as FrameworkElement;
            if (frameworkElement != null)
            {
                frameworkElement.ApplyTemplate();
            }

            Visual child = null;
            for (int i = 0, count = VisualTreeHelper.GetChildrenCount(@this); i < count; i++)
            {
                child = VisualTreeHelper.GetChild(@this, i) as Visual;

                var treeViewItem = child as TreeViewItem;
                if (treeViewItem != null)
                {
                    result.Add(treeViewItem);
                    if (!treeViewItem.IsExpanded)
                    {
                        treeViewItem.IsExpanded = true;
                        treeViewItem.UpdateLayout();
                    }
                }
                foreach (var childTreeViewItem in FindTreeViewItems(child))
                {
                    result.Add(childTreeViewItem);
                }
            }
            return result;
        }