Making AvalonEdit MVVM compatible

Herr Grunwald is talking about wrapping the TextEditor properties with dependency properties, so that you can bind to them. The basic idea is like this (using the CaretOffset property for example):

Modified TextEditor class

public class MvvmTextEditor : TextEditor, INotifyPropertyChanged
{
    public static DependencyProperty CaretOffsetProperty = 
        DependencyProperty.Register("CaretOffset", typeof(int), typeof(MvvmTextEditor),
        // binding changed callback: set value of underlying property
        new PropertyMetadata((obj, args) =>
        {
            MvvmTextEditor target = (MvvmTextEditor)obj;
            target.CaretOffset = (int)args.NewValue;
        })
    );

    public new string Text
    {
        get { return base.Text; }
        set { base.Text = value; }
    }

    public new int CaretOffset
    {
        get { return base.CaretOffset; }
        set { base.CaretOffset = value; }
    }

    public int Length { get { return base.Text.Length; } }

    protected override void OnTextChanged(EventArgs e)
    {
        RaisePropertyChanged("Length");
        base.OnTextChanged(e);
    }

    public event PropertyChangedEventHandler PropertyChanged;
    public void RaisePropertyChanged(string info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

Now that the CaretOffset has been wrapped in a DependencyProperty, you can bind it to a property, say Offset in your View Model. For illustration, bind a Slider control's value to the same View Model property Offset, and see that when you move the Slider, the Avalon editor's cursor position gets updated:

Test XAML

<Window x:Class="AvalonDemo.TestWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:avalonEdit="http://icsharpcode.net/sharpdevelop/avalonedit"
    xmlns:avalonExt="clr-namespace:WpfTest.AvalonExt"
    DataContext="{Binding RelativeSource={RelativeSource Self},Path=ViewModel}">
  <StackPanel>
    <avalonExt:MvvmTextEditor Text="Hello World" CaretOffset="{Binding Offset}" x:Name="editor" />
    <Slider Minimum="0" Maximum="{Binding ElementName=editor,Path=Length,Mode=OneWay}" 
        Value="{Binding Offset}" />
    <TextBlock Text="{Binding Path=Offset,StringFormat='Caret Position is {0}'}" />
    <TextBlock Text="{Binding Path=Length,ElementName=editor,StringFormat='Length is {0}'}" />
  </StackPanel>
</Window>

Test Code-behind

namespace AvalonDemo
{
    public partial class TestWindow : Window
    {
        public AvalonTestModel ViewModel { get; set; }

        public TestWindow()
        {
            ViewModel = new AvalonTestModel();
            InitializeComponent();
        }
    }
}

Test View Model

public class AvalonTestModel : INotifyPropertyChanged
{
    private int _offset;

    public int Offset 
    { 
        get { return _offset; } 
        set 
        { 
            _offset = value; 
            RaisePropertyChanged("Offset"); 
        } 
    }

    public event PropertyChangedEventHandler PropertyChanged;
    public void RaisePropertyChanged(string info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

Not sure if this fits your needs, but I found a way to access all the "important" components of the TextEditor on a ViewModel while having it displayed on a View, still exploring the possibilities though.

What I did was instead of instantiating the TextEditor on the View and then binding the many properties that I will need, I created a Content Control and bound its content to a TextEditor instance that I create in the ViewModel.

View:

<ContentControl Content="{Binding AvalonEditor}" />

ViewModel:

using ICSharpCode.AvalonEdit;
using ICSharpCode.AvalonEdit.Document;
using ICSharpCode.AvalonEdit.Highlighting;
// ...
private TextEditor m_AvalonEditor = new TextEditor();
public TextEditor AvalonEditor => m_AvalonEditor;

Test code in the ViewModel (works!)

// tests with the main component
m_AvalonEditor.SyntaxHighlighting = HighlightingManager.Instance.GetDefinition("XML");
m_AvalonEditor.ShowLineNumbers = true;
m_AvalonEditor.Load(@"C:\testfile.xml");

// test with Options
m_AvalonEditor.Options.HighlightCurrentLine = true;

// test with Text Area
m_AvalonEditor.TextArea.Opacity = 0.5;

// test with Document
m_AvalonEditor.Document.Text += "bla";

At the moment I am still deciding exactly what I need my application to configure/do with the textEditor but from these tests it seems I can change any property from it while keeping a MVVM approach.

You can use the Document property from the editor and bind it to a property of your ViewModel.

Here is the code for the view :

<Window x:Class="AvalonEditIntegration.UI.View"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:AvalonEdit="clr-namespace:ICSharpCode.AvalonEdit;assembly=ICSharpCode.AvalonEdit"
        Title="Window1"
        WindowStartupLocation="CenterScreen"
        Width="500"
        Height="500">
    <DockPanel>
        <Button Content="Show code"
                Command="{Binding ShowCode}"
                Height="50"
                DockPanel.Dock="Bottom" />
        <AvalonEdit:TextEditor ShowLineNumbers="True"
                               Document="{Binding Path=Document}"
                               FontFamily="Consolas"
                               FontSize="10pt" />
    </DockPanel>
</Window>

And the code for the ViewModel :

namespace AvalonEditIntegration.UI
{
    using System.Windows;
    using System.Windows.Input;
    using ICSharpCode.AvalonEdit.Document;

    public class ViewModel
    {
        public ViewModel()
        {
            ShowCode = new DelegatingCommand(Show);
            Document = new TextDocument();
        }

        public ICommand ShowCode { get; private set; }
        public TextDocument Document { get; set; }

        private void Show()
        {
            MessageBox.Show(Document.Text);
        }
    }
}

source : blog nawrem.reverse