AttachedStateEffect.cs

using Xamarin.Forms.CustomAttributes;
using Xamarin.Forms.Internals;

namespace Xamarin.Forms.Controls.Effects
{
	[Preserve(AllMembers = true)]
	public class AttachedStateEffect : RoutingEffect
	{
		public enum AttachedState
		{
			Unknown,
			Attached,
			Detached
		}

		public AttachedState State { get; set; } = AttachedState.Unknown;

		public const string EffectName = "AttachedStateEffect";
		public AttachedStateEffect() : base($"{Issues.Effects.ResolutionGroupName}.{EffectName}")
		{
		}
	}
}




using System;
using System.Linq;
using Xamarin.Forms;
using Xamarin.Forms.ControlGallery.iOS;
using Xamarin.Forms.Controls.Effects;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportEffect(typeof(AttachedStateEffectRenderer), AttachedStateEffect.EffectName)]
namespace Xamarin.Forms.ControlGallery.iOS
{
	public class AttachedStateEffectRenderer : PlatformEffect
	{
		AttachedStateEffect Effect;
		protected override void OnAttached()
		{
			Effect = (AttachedStateEffect)Element.Effects.First(e => e is AttachedStateEffect);

			if(Effect.State != AttachedStateEffect.AttachedState.Unknown)
			{
				throw new InvalidOperationException($"Invalid State: {Effect.State} expected {AttachedStateEffect.AttachedState.Unknown}");
			}

			Effect.State = AttachedStateEffect.AttachedState.Attached;
			
		}

		protected override void OnDetached()
		{
			if (Effect.State != AttachedStateEffect.AttachedState.Attached)
			{
				throw new InvalidOperationException($"Invalid State: {Effect.State} expected {AttachedStateEffect.AttachedState.Attached}");
			}

			Effect.State = AttachedStateEffect.AttachedState.Detached;
		}
	}
}