implementing-wpf-adorners

# WPF Adorner Patterns

Adorner is a mechanism for overlaying decorative visual elements on top of UIElements.

## 1. Adorner Concept

### 1.1 Characteristics

- **AdornerLayer**: Separate rendering layer that holds Adorners
- **Z-Order**: Always renders above the adorned element
- **Layout Independent**: No effect on target element's layout
- **Event Support**: Can receive mouse/keyboard events

### 1.2 Usage Scenarios

| Scenario | Description |
|----------|-------------|
| **Validation Display** | Input field error display |
| **Drag Handles** | Element move/resize handles |
| **Watermark** | Hint text for empty TextBox |
| **Selection Display** | Highlight selected elements |
| **Tooltip/Badge** | Additional info display on elements |
| **Drag and Drop** | Preview during drag |

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## 2. Basic Adorner Implementation

### 2.1 Simple Adorner

```csharp
namespace MyApp.Adorners;

using System.Windows;
using System.Windows.Documents;
using System.Windows.Media;

/// <summary>
/// Adorner that draws border around element
/// </summary>
public sealed class BorderAdorner : Adorner
{
    private readonly Pen _borderPen;

    public BorderAdorner(UIElement adornedElement) : base(adornedElement)
    {
        _borderPen = new Pen(Brushes.Red, 2)
        {
            DashStyle = DashStyles.Dash
        };
        _borderPen.Freeze();

        // Disable mouse events (decoration only)
        IsHitTestVisible = false;
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        var rect = new Rect(AdornedElement.RenderSize);

        // Draw border
        drawingContext.DrawRectangle(null, _borderPen, rect);
    }
}
```

### 2.2 Applying Adorner

```csharp
// Get AdornerLayer
var adornerLayer = AdornerLayer.GetAdornerLayer(targetElement);

if (adornerLayer is not null)
{
    // Add Adorner
    var adorner = new BorderAdorner(targetElement);
    adornerLayer.Add(adorner);
}
```

### 2.3 Removing Adorner

```csharp
// Remove all Adorners from s