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 = )
         {
             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="" Width="">
     <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="">
                     <Setter Property="IsExpanded" Value="True"/>
                 </DataTrigger>
             </Style.Triggers>
         </Style>
     </Window.Resources>
     <Grid x:Name="mTopLevelGrid">
         <TreeView x:Name="mTreeView" Grid.Column=""
                   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="">
            <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。