AvaloniaEdit Binding

AvaloniaEdit 默认不支持Binding操作，因此需要自定义Behavior，将字符串与其`.Document.Text`绑定。

首先创建自定义Behavior：

```c#
using System;
using Avalonia;
using Avalonia.Xaml.Interactivity;
using AvaloniaEdit;

namespace MatoEditor.Utils;

public sealed class AvalonEditBehavior : Behavior<TextEditor>
{
    private TextEditor _textEditor = null;

public static readonly StyledProperty<string> TextProperty =
        AvaloniaProperty.Register<AvalonEditBehavior, 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 > _textEditor.Document.TextLength)
            {
                _textEditor.CaretOffset = _textEditor.Document.TextLength;
            }
            else
            {
                _textEditor.CaretOffset = caretOffset;
            }
        }
    }
}
```

注意官方Wiki给出的代码有部分是：

```c#
var caretOffset = _textEditor.CaretOffset;
_textEditor.Document.Text = text;
_textEditor.CaretOffset = caretOffset;
```

这个地方肯会导致索引超出文本长度报错，因此要加上一个判断。

然后在axaml文件中使用时加入Behaviors:
```xml
<avaloniaEdit:TextEditor
                Name="TextEditor"
                HorizontalScrollBarVisibility="Auto"
                VerticalScrollBarVisibility="Auto"
                WordWrap="True">
                <Interaction.Behaviors>
                    <utils:AvalonEditBehavior Text="{Binding ContentString, Mode=TwoWay}"/>
                </Interaction.Behaviors>
            </avaloniaEdit:TextEditor>
```

AvaloniaEdit Binding

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