designPatternGuru/DesignPatternGuru/Observer/Observers/Subject.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Observer.Observers
{
    internal class Subject : ISubject
    {
        public int State { get; set; } = -0;

        private List<IObserver> _observers = new List<IObserver>();

        public void Attach(IObserver observer)
        {
            _observers.Add(observer);
            Console.WriteLine("Subject: Attached an observers.");
        }

        public void Detach(IObserver observer)
        {
            _observers.Remove(observer);
            Console.WriteLine("Subject: Detached an observers.");
        }

        public void Notify()
        {
            Console.WriteLine("Subject: Notifying observers...");
            foreach (IObserver observer in _observers)
            {
                observer.Update(this);
            }
        }

        public void SomeBusinessLogic()
        {
            Console.WriteLine();
            Console.WriteLine("Subject: I'm doing something important.");
            this.State = new Random().Next(0, 10);

            Thread.Sleep(15);

            Console.WriteLine($"Subject: My state has just changed to: {this.State}");
            Notify();
        }
    }
}