WPF-层级控件-TreeView

<?xml version="1.0" encoding="utf-8" ?>
<Data xmlns="">
  <Grade Name="一年级">
    <Class Name="1班">
      <Group Name="A组"/>
      <Group Name="B组"/>
      <Group Name="C组"/>
      <Group Name="D组"/>
    </Class>
  </Grade>
  <Grade Name="二年级">
    <Class Name="1班">
      <Group Name="A组"/>
      <Group Name="B组"/>
      <Group Name="C组"/>
      <Group Name="D组"/>
    </Class>
  </Grade>
  <Grade Name="三年级">
    <Class Name="1班">
      <Group Name="A组"/>
      <Group Name="B组"/>
      <Group Name="C组"/>
      <Group Name="D组"/>
    </Class>
  </Grade>
</Data>

<Window x:Class="TreeView.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <!--数据源-->
        <XmlDataProvider x:Key="ds" Source="Data.xml" XPath="Data/Grade"/>
        <!--年级模板-->
        <HierarchicalDataTemplate DataType="Grade" ItemsSource="{Binding XPath=Class}">
            <TextBlock Text="{Binding XPath=@Name}"></TextBlock>
        </HierarchicalDataTemplate>
        <!--班级模板-->
        <HierarchicalDataTemplate DataType="Class" ItemsSource="{Binding XPath=Group}">
            <RadioButton Content="{Binding XPath=@Name}" GroupName="gn"></RadioButton>
        </HierarchicalDataTemplate>
        <!--小组模板-->
        <HierarchicalDataTemplate DataType="Group" ItemsSource="{Binding XPath=Student}">
            <CheckBox Content="{Binding XPath=@Name}"></CheckBox>
        </HierarchicalDataTemplate>
    </Window.Resources>
    <Grid>
        <TreeView ItemsSource="{Binding Source={StaticResource ds}}"></TreeView>
    </Grid>
</Window>