wpf Treeview怎么在空白处绑定右键菜单

[hanyujack]

<Style TargetType="{x:Type base:BopTreeViewBase}" x:Key="{ComponentResourceKey {x:Type local:CConditionSetOutlineListBox}, myStyle}">
<Setter Property="ItemContainerStyle">
<Setter.Value>
<!--
This Style binds a TreeViewItem to a TreeViewItemViewModel.
-->
<Style TargetType="{x:Type TreeViewItem}">
<Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
<Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
<Setter Property="Foreground" Value="Black" />
<Setter Property="ContextMenu" Value="{StaticResource {ComponentResourceKey {x:Type base:BopTreeViewBase}, bopMenuPopupOutlineConditionSetList }}" /> //这里已经绑定了Tree节点中的菜单
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Foreground" Value="White" />

</Trigger>
</Style.Triggers>
</Style>
</Setter.Value>

</Setter>
</Style>

请问如何在treeview在空白空点添加一个右键菜单.