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

namespace Observer
{
    /// <summary>

    /// MainApp startup class for Structural 

    /// Observer Design Pattern.

    /// </summary>
    //Define a one-to-many dependency between objects so that when one object changes state, 
    //all its dependents are notified and updated automatically.

    class MainApp
    {
        /// <summary>

        /// Entry point into console application.

        /// </summary>

        static void Main()
        {
            // Configure Observer pattern

            ConcreteSubject s = new ConcreteSubject();

            s.Attach(new ConcreteObserver(s, "X"));
            s.Attach(new ConcreteObserver(s, "Y"));
            s.Attach(new ConcreteObserver(s, "Z"));

            // Change subject and notify observers

            s.SubjectState = "ABC";
            s.Notify();

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>

    /// The 'Subject' abstract class

    /// </summary>

    abstract class Subject
    {
        private List<Observer> _observers = new List<Observer>();

        public void Attach(Observer observer)
        {
            _observers.Add(observer);
        }

        public void Detach(Observer observer)
        {
            _observers.Remove(observer);
        }

        public void Notify()
        {
            foreach (Observer o in _observers)
            {
                o.Update();
            }
        }
    }

    /// <summary>

    /// The 'ConcreteSubject' class

    /// </summary>

    class ConcreteSubject : Subject
    {
        private string _subjectState;

        // Gets or sets subject state

        public string SubjectState
        {
            get { return _subjectState; }
            set { _subjectState = value; }
        }
    }

    /// <summary>

    /// The 'Observer' abstract class

    /// </summary>

    abstract class Observer
    {
        public abstract void Update();
    }

    /// <summary>

    /// The 'ConcreteObserver' class

    /// </summary>

    class ConcreteObserver : Observer
    {
        private string _name;
        private string _observerState;
        private ConcreteSubject _subject;

        // Constructor

        public ConcreteObserver(
          ConcreteSubject subject, string name)
        {
            this._subject = subject;
            this._name = name;
        }

        public override void Update()
        {
            _observerState = _subject.SubjectState;
            Console.WriteLine("Observer {0}'s new state is {1}",
              _name, _observerState);
        }

        // Gets or sets subject

        public ConcreteSubject Subject
        {
            get { return _subject; }
            set { _subject = value; }
        }
    }
}