附加属性不更新我的视图模型

Question

我使用this SO post（Ray Burn的答案）中提供的技术将我的RichTextBox绑定到我的视图模型（VM）的基础string - 类型属性。我面临的问题是，即使在焦点更改后，当用户在RichTextBox中输入内容时，基础字段似乎也不会更新。以下是相关的XAML（SyntaxHighlighterTextBox是一个继承自RichTextBox的自定义控件）：

<local:SyntaxHighlighterTextBox 
      local:RichTextBoxHelper.DocumentXaml="{Binding Query, UpdateSourceTrigger=PropertyChanged}" />

以下是基础VM的Query属性：

Public Property Query() As String
  Get
    Return mQuery
  End Get
  Set(ByVal value As String)
    mQuery = value
    RaiseEvent PropertyChanged(Me, New System.ComponentModel.PropertyChangedEventArgs("Query"))
  End Set
End Property

修改

这是我的处理程序代码：

Public Shared Sub MyHandler(obj As DependencyObject, e As DependencyPropertyChangedEventArgs)
    Dim rtb = DirectCast(obj, SyntaxHighlighterTextBox)

    Dim xaml = GetDocumentXaml(rtb)
    Dim doc = New FlowDocument()
    range = New TextRange(doc.ContentStart, doc.ContentEnd)

    range.Load(New MemoryStream(Encoding.UTF8.GetBytes(xaml)), DataFormats.Text)

    rtb.Document = doc

    AddHandler range.Changed, Sub(obj2, e2)
                                If rtb.Document Is doc Then
                                  Dim buffer As New MemoryStream()
                                  range.Save(buffer, DataFormats.Text)
                                  SetDocumentXaml(rtb, Encoding.UTF8.GetString(buffer.ToArray()))
                                End If
                              End Sub
End Sub