MyAvaloniaDemo/Attached/TextEditorAssist.cs

using Avalonia;
using Avalonia.Data;
using AvaloniaEdit;
using AvaloniaEdit.Document;

namespace MyAvaloniaApp.Attached;

/// <summary>
/// 为 <see cref="TextEditor"/> 提供可绑定的 <see cref="TextDocument"/> 附加属性，
/// 便于在 MVVM 模式下直接绑定到 ViewModel。
/// </summary>
public static class TextEditorAssist
{
    public static readonly AttachedProperty<TextDocument?> DocumentProperty =
        AvaloniaProperty.RegisterAttached<TextEditor, TextDocument?>(
            "Document",
            typeof(TextEditorAssist),
            defaultBindingMode: BindingMode.TwoWay);

    private static readonly AttachedProperty<bool> IsInternalUpdateProperty =
        AvaloniaProperty.RegisterAttached<TextEditor, bool>(
            "IsInternalUpdate",
            typeof(TextEditorAssist));

    static TextEditorAssist()
    {
        DocumentProperty.Changed.AddClassHandler<TextEditor>(OnDocumentPropertyChanged);
        TextEditor.DocumentProperty.Changed.AddClassHandler<TextEditor>(OnEditorDocumentChanged);
    }

    public static void SetDocument(TextEditor editor, TextDocument? value) =>
        editor.SetValue(DocumentProperty, value);

    public static TextDocument? GetDocument(TextEditor editor) =>
        editor.GetValue(DocumentProperty);

    private static void OnDocumentPropertyChanged(TextEditor editor, AvaloniaPropertyChangedEventArgs change)
    {
        if (editor.GetValue(IsInternalUpdateProperty))
            return;

        var document = change.NewValue as TextDocument;
        if (!ReferenceEquals(editor.Document, document))
        {
            editor.SetValue(IsInternalUpdateProperty, true);
            editor.Document = document;
            editor.SetValue(IsInternalUpdateProperty, false);
        }
    }

    private static void OnEditorDocumentChanged(TextEditor editor, AvaloniaPropertyChangedEventArgs change)
    {
        if (editor.GetValue(IsInternalUpdateProperty))
            return;

        var document = change.NewValue as TextDocument;
        if (!ReferenceEquals(editor.GetValue(DocumentProperty), document))
        {
            editor.SetValue(IsInternalUpdateProperty, true);
            editor.SetValue(DocumentProperty, document);
            editor.SetValue(IsInternalUpdateProperty, false);
        }
    }
}