Observer.cs

using System;
using System.Collections.Generic;

namespace Prototype.Scripts.Contexts.Common
{
    public interface IObserver<T, Tenum> where Tenum : struct, IConvertible
    {
        void OnContextChanged(T context, Tenum property);
        void OnAttached(T context);
        void OnDetached();
    }
    public interface ISubject<T, Tenum> where Tenum : struct, IConvertible
    {
        void Attach(IObserver<T, Tenum> observer);
        void Detach(IObserver<T, Tenum> observer);
        void Notify(Tenum property);
    }

    public class ContextSubject<T, Tenum> : ISubject<T, Tenum> where Tenum : struct, IConvertible
    {
        private readonly List<IObserver<T, Tenum>> _observers = new List<IObserver<T, Tenum>>();
        private readonly T _context;

        public ContextSubject(T context)
        {
            _context = context;
        }

        public void Attach(IObserver<T, Tenum> observer)
        {
            _observers.Remove(observer);

            _observers.Add(observer);

            observer.OnAttached(_context);
        }
        public void Detach(IObserver<T, Tenum> observer)
        {
            _observers.Remove(observer);

            observer.OnDetached();
        }

        public void Notify(Tenum property)
        {
            for (int i = _observers.Count-1; i >= 0; i--)
            {
                _observers[i].OnContextChanged(_context, property);
            }
        }

        public void Clear()
        {
            _observers.Clear();
        }
    }
}