DocumentTextBindingBehavior.cs

using System;
using Avalonia;
using Avalonia.Xaml.Interactivity;
using AvaloniaEdit;
using AvaloniaEdit.Utils;

namespace SharpFM.Behaviors;

public class DocumentTextBindingBehavior : Behavior<TextEditor>
{
    private TextEditor _textEditor = null!;

    public static readonly StyledProperty<string> TextProperty =
        AvaloniaProperty.Register<DocumentTextBindingBehavior, string>(nameof(Text));

    public string Text
    {
        get => GetValue(TextProperty);
        set => SetValue(TextProperty, value);
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        if (AssociatedObject is TextEditor textEditor)
        {
            _textEditor = textEditor;
            _textEditor.TextChanged += TextChanged;
            this.GetObservable(TextProperty).Subscribe(TextPropertyChanged);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (_textEditor != null)
        {
            _textEditor.TextChanged -= TextChanged;
        }
    }

    private void TextChanged(object? sender, EventArgs eventArgs)
    {
        if (_textEditor != null && _textEditor.Document != null)
        {
            Text = _textEditor.Document.Text;
        }
    }

    private void TextPropertyChanged(string text)
    {
        if (_textEditor != null && _textEditor.Document != null && text != null)
        {
            var caretOffset = _textEditor.CaretOffset;
            _textEditor.Document.Text = text;
            if (caretOffset <= text.Length)
            {
                _textEditor.CaretOffset = caretOffset;
            }
        }
    }
}