当绑定源更改时,不会通知FlowDocumentReader文档,为什么?

所以我在.xaml文件中有这个xaml

<StackPanel>
        <Button Width="200" Height="30" Content="Change Words" 
                Click="Button_Click"/>
        <FlowDocumentReader 
            ViewingMode="Scroll" Zoom="90"
            Focusable="True"
            Background="White"
            IsFindEnabled="True"
            IsPageViewEnabled="True"
            IsScrollViewEnabled="True"
            x:Name="FDR"
            Document="{Binding Path=WordDocument}"
            Width="400" Height="400">            
        </FlowDocumentReader>
    </StackPanel>

在后面的代码中, 载重,

public partial class Window1 : Window
    {
        MyDoc _myDoc = null;
        FlowDocument _theFlowDocument;

        public Window1()
        {
            InitializeComponent();
            _myDoc  = new MyDoc().Create(); // Create returns MyDoc, that has a WordDocument property with some FlowDocument contents
            this.DataContext = _myDoc ;
        }
 private void Button_Click(object sender, RoutedEventArgs e)
        {
            _myDoc.WordDocument = _myDoc.CreateFlowDocument("Now it's changed");
        }
 }

单击按钮,我将更改Word文档的内容。CreateFlowDocument创建一个段落和一个传递字符串的运行。

单击按钮时,虽然我已将其绑定到WordDocument属性,但FlowDocumentReader不会显示更改的内容。

我做错什么了?