WPF DataGridComboBoxColumn 一失去焦点就丢失text
在我的DataGrid中定义两个级联的DataGridComboBoxColumn，A1，A2，A1变的时候也把A2的ItemsSource改变。现在改变起来没问题，但是A2一旦失去焦点，那么本应显示的Text也不显示，十分奇怪。背后的绑定显示数据更改已经生效了，但是就是不显示在DataGrid上：
<DataGrid x:Name="Dg_Main" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"
CanUserAddRows="True" CanUserDeleteRows="True" SelectionMode="Single" SelectionUnit="FullRow"
AlternatingRowBackground="Wheat" RowHeaderWidth="10" AlternationCount="2"
AutoGenerateColumns="False" ItemsSource="{Binding Path=Displays}" PreparingCellForEdit="Dg_Main_PreparingCellForEdit">
<DataGrid.Columns>
<DataGridComboBoxColumn Header="A1" MinWidth="80" ItemsSource="{Binding Source={StaticResource A1Source}}" SelectedValueBinding="{Binding Path=a1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="PrimitiveValue" DisplayMemberPath="Content"/>
<DataGridComboBoxColumn Header="A2" MinWidth="80" SelectedValueBinding="{Binding Path=a2, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="PrimitiveValue" DisplayMemberPath="Content" EditingElementStyle="{StaticResource EditCombobox}">
<DataGridComboBoxColumn.ElementStyle>
<Style TargetType="ComboBox">
<!--<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="{Binding Path=Content}"></TextBlock>
</DataTemplate>
</Setter.Value>
</Setter>-->
<!--<Setter Property="DisplayMemberPath" Value="{Binding Path=Content}"></Setter>-->
<Setter Property="IsSynchronizedWithCurrentItem" Value="False" />
</Style>
</DataGridComboBoxColumn.ElementStyle>
</DataGridComboBoxColumn>
</DataGrid.Columns>
</DataGrid>
