Binding to a RichTextbox

[Scottintexas]

There are a few examples of binding to a RichTextbox on the internet. I searched all over and it comes down to two or three examples. I chose one of the examples to work with as there were a number of people that claimed success and it did not require installing new tools (which, for security reasons, I cannot do). The idea is to attach a bindable dependency property to a rich text box and bind the property with the text to it. So that's what I did, but when I run the program the dependency property never gets hit. When the program starts a break point in the dependency property demonstrates that the dependency property is getting registered. But when you set a string to the property in the view model, nothing happens. The dependency property never gets called.

Here is the Dependency Property

namespace VBADeconstructor.View_Model
{
    public class RichTextBoxHelper : DependencyObject
    {


        private static HashSet<Thread> _recursionProtection=new HashSet<Thread>();


        public static string GetDocumentXaml(DependencyObject obj)
        {
            return (string)obj.GetValue(DocumentXamlProperty);
        }




        public static void SetDocumentXaml(DependencyObject obj, string value)
        {
            _recursionProtection.Add(Thread.CurrentThread);
            obj.SetValue(DocumentXamlProperty, value);
            _recursionProtection.Remove(Thread.CurrentThread);
        }


        public static readonly DependencyProperty DocumentXamlProperty = DependencyProperty.RegisterAttached
            (
            "DocumentXaml", 
            typeof(string), 
            typeof(RichTextBoxHelper), 
            new FrameworkPropertyMetadata
            (
                "",FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.AffectsRender,
                (obj,e )=>
                {
                    if(_recursionProtection.Contains(Thread.CurrentThread)) return;
                    var richTextBox=(RichTextBox)obj;


                    try
                    {
                        string docXml = GetDocumentXaml(richTextBox);
                        var stream = new MemoryStream(Encoding.UTF8.GetBytes(docXml));
                        FlowDocument doc;
                        if (!string.IsNullOrEmpty(docXml))
                        {
                            doc= (FlowDocument)XamlReader.Load(stream);
                        }
                        else
                        {
                            doc = new FlowDocument();
                        }


                        richTextBox.Document = doc;
                    }
                    catch (Exception)
                    {
                        richTextBox.Document = new FlowDocument();
                    }


                    richTextBox.TextChanged += (obj2, e2) =>
                    {
                        RichTextBox richbox = obj2 as RichTextBox;
                        if(richbox != null)
                        {
                            SetDocumentXaml(richTextBox,XamlWriter.Save(richbox.Document));
                        }
                    };
                }
                )
            );


    }
}

In the MainWindowViewModel I have this property.

        public string Procedure
        {
            get { return _procedure; }
            set
            {
                _procedure = value;
                RaisePropertyChanged();
            }
        }

And in the View I have this

        <RichTextBox Grid.Row="3"
                     Grid.Column="0"
                     Grid.ColumnSpan="2"
                     vm:RichTextBoxHelper.DocumentXaml="{Binding vm:MainWindowViewModel.Procedure, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

I don't know what is missing. I cannot ask the people who originally posted their solutions. So, hopefully, one of the geniuses here can tell how to make it work.

Thank you