Question

I’m trying to make Avalon MVVM compatible in my WPF application. From googling, I found out that AvalonEdit is not MVVM friendly and I need to export the state of AvalonEdit by making a class derived from TextEditor then adding the necessary dependency properties. I’m afraid that I’m quite lost in Herr Grunwald’s answer here:

If you really need to export the state of the editor using MVVM, then I suggest you create a class deriving from TextEditor which adds the necessary dependency properties and synchronizes them with the actual properties in AvalonEdit.

Does anyone have an example or have good suggestions on how to achieve this?

Answer 1

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));
        }
    }
}