Shaker/Avalonia/Avalonia.Xaml.Interactions.Custom/BoundsObserverBehavior.cs

using System.Reactive;
using System.Reactive.Disposables;
using Avalonia.Controls;
using Avalonia.Data;

namespace Avalonia.Xaml.Interactions.Custom;

/// <summary>
/// Observes the bounds of an associated <see cref="Control"/> and updates its Width and Height properties.
/// </summary>
public class BoundsObserverBehavior : DisposingBehavior<Control>
{
    /// <summary>
    /// Defines the <see cref="Bounds"/> property.
    /// </summary>
    public static readonly StyledProperty<Rect> BoundsProperty =
        AvaloniaProperty.Register<BoundsObserverBehavior, Rect>(nameof(Bounds), defaultBindingMode: BindingMode.OneWay);

    /// <summary>
    /// Defines the <see cref="Width"/> property.
    /// </summary>
    public static readonly StyledProperty<double> WidthProperty =
        AvaloniaProperty.Register<BoundsObserverBehavior, double>(nameof(Width),
            defaultBindingMode: BindingMode.TwoWay);

    /// <summary>
    /// Defines the <see cref="Height"/> property.
    /// </summary>
    public static readonly StyledProperty<double> HeightProperty =
        AvaloniaProperty.Register<BoundsObserverBehavior, double>(nameof(Height),
            defaultBindingMode: BindingMode.TwoWay);

    /// <summary>
    /// Gets or sets the bounds of the associated control. This is a styled Avalonia property.
    /// </summary>
    public Rect Bounds
    {
        get => GetValue(BoundsProperty);
        set => SetValue(BoundsProperty, value);
    }

    /// <summary>
    /// Gets or sets the width of the associated control. This is a two-way bound Avalonia property.
    /// </summary>
    public double Width
    {
        get => GetValue(WidthProperty);
        set => SetValue(WidthProperty, value);
    }

    /// <summary>
    /// Gets or sets the height of the associated control. This is a two-way bound Avalonia property.
    /// </summary>
    public double Height
    {
        get => GetValue(HeightProperty);
        set => SetValue(HeightProperty, value);
    }

    /// <summary>
    /// Attaches the behavior to the associated control and starts observing its bounds to update the Width and Height properties accordingly.
    /// </summary>
    /// <param name="disposables">A composite disposable used to manage the lifecycle of subscriptions and other disposables.</param>
    protected override void OnAttached(CompositeDisposable disposables)
    {
        if (AssociatedObject is not null)
        {
            disposables.Add(this.GetObservable(BoundsProperty)
                .Subscribe(new AnonymousObserver<Rect>(bounds =>
                {
                    Width = bounds.Width;
                    Height = bounds.Height;
                })));
        }
    }
}