Notes

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using System;
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using System.Collections.Generic;
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using System.Globalization;
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using System.IO;
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using System.Linq;
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using System.Text;
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using System.Threading;
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using System.Threading.Tasks;
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using System.Windows;
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using System.Windows.Controls;
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using System.Windows.Data;
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using System.Windows.Documents;
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using System.Windows.Markup;
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namespace ViewModel.Services
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{
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    public class RichTextBoxHelper : DependencyObject
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    {
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        private static HashSet<Thread> _recursionProtection = new HashSet<Thread>();
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        public static string GetDocumentXaml(DependencyObject obj)
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        {
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            return (string)obj.GetValue(DocumentXamlProperty);
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        }
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        public static void SetDocumentXaml(DependencyObject obj, string value)
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        {
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            _recursionProtection.Add(Thread.CurrentThread);
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            obj.SetValue(DocumentXamlProperty, value);
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            _recursionProtection.Remove(Thread.CurrentThread);
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        }
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        public static readonly DependencyProperty DocumentXamlProperty = DependencyProperty.RegisterAttached(
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            "DocumentXaml",
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            typeof(string),
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            typeof(RichTextBoxHelper),
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            new FrameworkPropertyMetadata(
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                "",
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                FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
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                (obj, e) =>
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                {
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                    if (_recursionProtection.Contains(Thread.CurrentThread))
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                        return;
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                    var richTextBox = (RichTextBox)obj;
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                    // Parse the XAML to a document (or use XamlReader.Parse())
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                    try
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                    {
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                        var stream = new MemoryStream(Encoding.UTF8.GetBytes(GetDocumentXaml(richTextBox)));
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                        var doc = (FlowDocument)XamlReader.Load(stream);
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                        // Set the document
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                        richTextBox.Document = doc;
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                    }
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                    catch (Exception)
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                    {
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                        richTextBox.Document = new FlowDocument();
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                    }
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                    // When the document changes update the source
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                    richTextBox.TextChanged += (obj2, e2) =>
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                    {
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                        RichTextBox richTextBox2 = obj2 as RichTextBox;
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                        if (richTextBox2 != null)
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                        {
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                            SetDocumentXaml(richTextBox, XamlWriter.Save(richTextBox2.Document));
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                        }
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                    };
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                }
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            )
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        );
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    }
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}
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