如何强制FlowDocument在DataContext集上刷新其内容

我正在尝试使用带有绑定的FlowDocument来拥有单独的模板，该模板能够用数据模型中的实际数据填充。然后我将其转换为图像并打印或保存在硬盘上。为了将FlowDocument的Run与数据模型绑定，我使用了本文中的代码：https://msdn.microsoft.com/en-us/magazine/dd569761.aspx

FlowDocument模板如下：

<FlowDocument xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
          xmlns:p="clr-namespace:Labels;assembly=DataModel.Impl"
PageWidth="200" MinPageWidth="200" PageHeight="200" MinPageHeight="200">
<Section>
  <Paragraph>
    <p:BindableRun BoundText="{Binding Path=Text}"/>
  </Paragraph>
</Section>
</FlowDocument>

BindableRun的代码：

public class BindableRun : Run
  {
    public static readonly DependencyProperty BoundTextProperty = DependencyProperty.Register("BoundText", typeof(string), typeof(BindableRun),
      new FrameworkPropertyMetadata(string.Empty, FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure, OnBoundTextChanged, CoerceText));
    public BindableRun()
    {
      FlowDocumentHelpers.FixupDataContext(this);
    }
    private static void OnBoundTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
      ((Run)d).Text = (string)e.NewValue;
    }
    private static object CoerceText(DependencyObject d, object value)
    {
      return value;
    }
    public String BoundText
    {
      get { return (string)GetValue(BoundTextProperty); }
      set { SetValue(BoundTextProperty, value); }
    }
  }

然后我加载模板并在其中设置DataContext：

private class DataClass
{
  public string Text { get; set; }
}
private static FlowDocument LoadFlowDocument(string path)
{
  using (var xamlFile = new FileStream(path, FileMode.Open, FileAccess.Read))
  {
    return XamlReader.Load(xamlFile) as FlowDocument;
  }
}
    private static void FlowDoc2Image(FlowDocument document, DataClass dataContext, Stream imageStream)
{
  var flowDocumentScrollViewer = new FlowDocumentScrollViewer
  {
    VerticalScrollBarVisibility = ScrollBarVisibility.Hidden,
    HorizontalScrollBarVisibility = ScrollBarVisibility.Hidden,
    DataContext = dataContext
  };
  flowDocumentScrollViewer.Document = document;
  flowDocumentScrollViewer.Measure(new Size(999999999, 999999999));
  //1st pass
  flowDocumentScrollViewer.Arrange(new Rect(0, 0, flowDocumentScrollViewer.ActualWidth, flowDocumentScrollViewer.ActualHeight));
  //2nd pass. It's not code duplication! Do not remove!
  flowDocumentScrollViewer.Arrange(new Rect(0, 0, flowDocumentScrollViewer.ActualWidth, flowDocumentScrollViewer.ActualHeight));
  var bitmapRenderer =
    new RenderTargetBitmap((int)flowDocumentScrollViewer.ActualWidth, (int)flowDocumentScrollViewer.ActualHeight, 96, 96, PixelFormats.Pbgra32);
  bitmapRenderer.Render(flowDocumentScrollViewer);
  var pngEncoder = new PngBitmapEncoder { Interlace = PngInterlaceOption.On };
  pngEncoder.Frames.Add(BitmapFrame.Create(bitmapRenderer));
  pngEncoder.Save(imageStream);
}
public void Test()
{
  var doc = LoadFlowDocument("C:''Experiments''DocWithBinding.xaml");
  var context = new DataClass {Text = "SomeText"};
  doc.DataContext = context;
  using (var imageStream = new FileStream("C:''Experiments''image.png", FileMode.OpenOrCreate, FileAccess.Write))
  {
    FlowDoc2Image(doc, context, imageStream);
  }
}

但什么也没发生。我试图在BindableRun中设置断点来更改它的值。我从来没有到过那里。更改DataContext不会影响文档。