WPF TreeView IsExpanded 绑定不上的问题

最近项目上需要通过MVVM来控制TreeView，其中需要需要控制通过搜索来定位某个节点，正常逻辑下，首先通过需要在树上面找到该节点，然后选中该节点，并将该节点的父节点展开，这个时候需要通过MVVM来控制，需要绑定起来，只是一直没有binding上，代码如下：

MVVM示例代码：

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DragDrop
{
    class DataItem : NotifyPropertyBase, ICloneable
    {
        public DataItem(string header, int deepth = 1)
        {
            Header = header;
            Deepth = deepth;
        }

        public object Clone()
        {
            DataItem dataItem = new DataItem(Header, Deepth);
            dataItem.IsExpanded = IsExpanded;
            dataItem.IsSelected = IsSelected;
            dataItem.Deepth = Deepth;
            foreach (DataItem item in Items)
                dataItem.Items.Add((DataItem)item.Clone());
            return dataItem;
        }

        private string header;
        public string Header
        {
            get { return header; }
            set
            {
                SetProperty(ref header, value);
            }
        }

        private bool isSelected;
        public bool IsSelected
        {
            get { return isSelected; }
            set { SetProperty(ref isSelected, value); }
        }

        private bool isExpanded;
        public bool IsExpanded
        {
            get { return isExpanded; }
            set
            {
                SetProperty(ref isExpanded, value);
                Console.WriteLine("{0}--{1}", Header, IsExpanded);
            }
        }

        private int deepth;
        public int Deepth
        {
            get { return deepth; }
            set
            {
                if (deepth != value)
                {
                    deepth = value;
                    SetProperty(ref deepth, value);
                }
            }
        }

        private ObservableCollection<DataItem> mItems = null;
        public ObservableCollection<DataItem> Items
        {
            get
            {
                if (mItems == null)
                    mItems = new ObservableCollection<DataItem>();
                return mItems;
            }
        }       
        
    }

    class Data
    {
        private static Data mInstance = new Data();

        public static Data Instance
        {
            get { return mInstance; }
        }

        private ObservableCollection<DataItem> GenerateTreeViewItems()
        {
            ObservableCollection<DataItem> items = new ObservableCollection<DataItem>();

            DataItem item1 = new DataItem("TreeViewItem1");
            item1.Items.Add(new DataItem("SubItem1", item1.Deepth));
            item1.Items.Add(new DataItem("SubItem2", item1.Deepth));
            item1.Items.Add(new DataItem("SubItem3", item1.Deepth));
            item1.Items.Add(new DataItem("SubItem4", item1.Deepth));
            items.Add(item1);

            DataItem item2 = new DataItem("TreeViewItem2");
            item2.Items.Add(new DataItem("SubItem1", item2.Deepth));
            item2.Items.Add(new DataItem("SubItem2", item2.Deepth));
            items.Add(item2);

            DataItem item3 = new DataItem("TreeViewItem3");
            item3.Items.Add(new DataItem("SubItem1", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem2", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem3", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem4", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem5", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem6", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem7", item3.Deepth));
            item3.Items.Add(new DataItem("SubItem8", item3.Deepth));
            items.Add(item3);

            return items;
        }

        private ObservableCollection<DataItem> GenerateListItems()
        {
            ObservableCollection<DataItem> items = new ObservableCollection<DataItem>();
            items.Add(new DataItem("ListBoxItem1"));
            items.Add(new DataItem("ListBoxItem2"));
            items.Add(new DataItem("ListBoxItem3"));
            items.Add(new DataItem("ListBoxItem4"));
            items.Add(new DataItem("ListBoxItem5"));
            return items;
        }

        public ObservableCollection<DataItem> TreeViewItems
        {
            get
            {
                if (mTreeViewItems == null)
                    mTreeViewItems = GenerateTreeViewItems();
                return mTreeViewItems;
            }
        }

        public ObservableCollection<DataItem> ListBoxItems
        {
            get
            {
                if (mListBoxItems == null)
                    mListBoxItems = GenerateListItems();
                return mListBoxItems;
            }
        }

        private ObservableCollection<DataItem> mTreeViewItems = null;
        private ObservableCollection<DataItem> mListBoxItems = null;
    }
}

界面代码：

<Window x:Class="DragDrop.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DragDrop"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <HierarchicalDataTemplate x:Key="treeViewTemplate" DataType="{x:Type local:DataItem}" ItemsSource="{Binding Items}">
            <TextBlock Text="{Binding Header}"/>
        </HierarchicalDataTemplate>

        <Style x:Key="TreeViewStyle" TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
            <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Deepth}" Value="1">
                    <Setter Property="IsExpanded" Value="True"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <Grid x:Name="mTopLevelGrid">
        <TreeView x:Name="mTreeView" Grid.Column="0"
                  ItemsSource="{Binding Source={x:Static local:Data.Instance}, Path=TreeViewItems}"
                  ItemTemplate="{StaticResource treeViewTemplate}"
                  ItemContainerStyle="{StaticResource TreeViewStyle}"/>
    </Grid>
</Window>

数据的绑定没有问题，界面没有问题，奇怪的是IsSelected可以正常绑定，但是IsExpanded就是不行，后来发现这两个属性唯一的区别就是在TreeView的式样中

<Style.Triggers>
     <DataTrigger Binding="{Binding Deepth}" Value="1">
           <Setter Property="IsExpanded" Value="True"/>
      </DataTrigger>
</Style.Triggers>

后面把这个DataTrigger这段代码注释掉，编译一下，重新运行，IsExpanded就可以正常绑定。后面自己思考了一下，MVVM模式实现将ViewModel与界面上面的显示绑定起来，实际上也就是使用了观察者模式，而Trigger的原理应该和数据binding是一样的，而且这个Trigger写在后面，可能直接把前面的绑定取代了，但是试了一下，将DataTrigger放到前面，也还是绑定不上，这种可能性排除；个人猜测可能是Trigger的优先级比较高，从而造成IsExpanded一直Binding不上去。