Two Way Binding to AvalonEdit Document Text using MVVM

Create a Behavior class that will attach the TextChanged event and will hook up the dependency property that is bound to the ViewModel.

AvalonTextBehavior.cs

public sealed class AvalonEditBehaviour : Behavior<TextEditor> 
{
    public static readonly DependencyProperty GiveMeTheTextProperty =
        DependencyProperty.Register("GiveMeTheText", typeof(string), typeof(AvalonEditBehaviour), 
        new FrameworkPropertyMetadata(default(string), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, PropertyChangedCallback));

    public string GiveMeTheText
    {
        get { return (string)GetValue(GiveMeTheTextProperty); }
        set { SetValue(GiveMeTheTextProperty, value); }
    }

    protected override void OnAttached()
    {
        base.OnAttached();
        if (AssociatedObject != null)
            AssociatedObject.TextChanged += AssociatedObjectOnTextChanged;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        if (AssociatedObject != null)
            AssociatedObject.TextChanged -= AssociatedObjectOnTextChanged;
    }

    private void AssociatedObjectOnTextChanged(object sender, EventArgs eventArgs)
    {
        var textEditor = sender as TextEditor;
        if (textEditor != null)
        {
            if (textEditor.Document != null)
                GiveMeTheText = textEditor.Document.Text;
        }
    }

    private static void PropertyChangedCallback(
        DependencyObject dependencyObject,
        DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
    {
        var behavior = dependencyObject as AvalonEditBehaviour;
        if (behavior.AssociatedObject!= null)
        {
            var editor = behavior.AssociatedObject as TextEditor;
            if (editor.Document != null)
            {
                var caretOffset = editor.CaretOffset;
                editor.Document.Text = dependencyPropertyChangedEventArgs.NewValue.ToString();
                editor.CaretOffset = caretOffset;
            }
        }
    }
}

View.xaml

 <avalonedit:TextEditor
        WordWrap="True"
        ShowLineNumbers="True"
        LineNumbersForeground="Magenta"
        x:Name="textEditor"
        FontFamily="Consolas"
        SyntaxHighlighting="XML"
        FontSize="10pt">
        <i:Interaction.Behaviors>
            <controls:AvalonEditBehaviour GiveMeTheText="{Binding Test, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        </i:Interaction.Behaviors>
    </avalonedit:TextEditor>

i must be defined as

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

ViewModel.cs

    private string _test;
    public string Test
    {
        get { return _test; }
        set { _test = value; }
    }

That should give you the Text and push it back to the ViewModel.