wpf中datagird绑定CellEditEnding并设置UpdateSourceTrigger不生效

Question

XAML

<DataGrid ItemsSource="{Binding Products}" CanUserAddRows="False" AutoGenerateColumns="False" SelectedItem="{Binding SelectedProduct}">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="CellEditEnding">
            <cmd:EventToCommand Command="{Binding ProdcutCellEditCmd,UpdateSourceTrigger=PropertyChanged}" PassEventArgsToCommand="True"></cmd:EventToCommand>
        </i:EventTrigger>
    </i:Interaction.Triggers>
    <DataGrid.Columns>
        <DataGridTextColumn Binding="{Binding id}" Header="ID" IsReadOnly="True"></DataGridTextColumn>
        <DataGridTextColumn Binding="{Binding ParentLCSKU}" Header="LCSKU(Parent)" IsReadOnly="True"></DataGridTextColumn>
        <DataGridTextColumn Binding="{Binding ChildLCSKU}" Header="LCSKU(Child)" IsReadOnly="True"></DataGridTextColumn>
        <DataGridComboBoxColumn ItemsSource="{Binding Source={StaticResource ProductColors}}" SelectedValueBinding="{Binding Color}" Header="颜色" IsReadOnly="True"></DataGridComboBoxColumn>
        <DataGridComboBoxColumn ItemsSource="{Binding Source={StaticResource ProductSizes}}" SelectedValueBinding="{Binding Size}" Header="尺寸" IsReadOnly="True"></DataGridComboBoxColumn>
        <DataGridComboBoxColumn ItemsSource="{Binding Source={StaticResource ProductCategories}}" SelectedValueBinding="{Binding Category}" Header="类别" IsReadOnly="True"></DataGridComboBoxColumn>
        <DataGridTextColumn Binding="{Binding Cost}" Header="成本"></DataGridTextColumn>
        <DataGridCheckBoxColumn Binding="{Binding IsOEM,UpdateSourceTrigger=PropertyChanged}" Header="OEM"></DataGridCheckBoxColumn>
    </DataGrid.Columns>
</DataGrid>

vm

public ProductVM()
{
    ProdcutCellEditCmd = new RelayCommand<DataGridCellEditEndingEventArgs>(prodcutDataGridCellEditEnding);

}
public RelayCommand<DataGridCellEditEndingEventArgs> ProdcutCellEditCmd { get; set; }
private void prodcutDataGridCellEditEnding(DataGridCellEditEndingEventArgs e)
{
    if (e.EditAction == DataGridEditAction.Commit)
    {
        var prodcut = e.Row.DataContext as BaseProduct;
        SelectedProduct = prodcut;
        productSave();
    }
}

在xaml里面设置了UpdateSourceTrigger=PropertyChanged但是实际运行的时候还是需要失去焦点时才能触发那个事件。
使用的是mvvmlight。