Illustrated GOF Design Patterns in C# Part V: Behavioral II

//	example of implementing the Observer pattern

using System;
using	System.Collections;
using	Concrete;

namespace	Concrete
{
	public delegate void	PriceChanged(BaseProduct p);

	public class	BaseProduct
	{
		private static int	m_nIDPool = 1;
		private int	m_nID;
		private string	m_strName;
		private double	m_yPrice;

		public event PriceChanged	PriceChangedObservers;

		public BaseProduct(string name, double price)
		{
			m_nID = m_nIDPool++;
			m_strName = name;
			m_yPrice = price;
		}

		public int	id	{	get	{	return m_nID;	}	}

		public string	name
		{
			get	{	return m_strName;	}
			set	{	m_strName = value;	}
		}

		public double	price
		{
			get	{	return m_yPrice;	}
			set
			{
				m_yPrice = value;

				//	notify listeners
				PriceChangedObservers(this);
			}
		}
	}

	public class	BaseObserver
	{
		virtual public void OnPriceChanged(BaseProduct p)
		{
			Console.WriteLine("{0} detected price change on {1}, ${2}",
				this.GetType().ToString(), p.name, p.price);
		}

		public void	ObserveProduct(BaseProduct p)
		{
			p.PriceChangedObservers += new PriceChanged(this.OnPriceChanged);
		}
	}

	public class	POObserver : BaseObserver
	{
		override public void	OnPriceChanged(BaseProduct p)
		{
			base.OnPriceChanged(p);
			Console.WriteLine("{0} updating PO pricing", this.GetType().ToString());
		}
	}

	public class	UIObserver : BaseObserver
	{
		override public void	OnPriceChanged(BaseProduct p)
		{
			base.OnPriceChanged(p);
			Console.WriteLine("{0} updating UI view", this.GetType().ToString());
		}
	}
}

namespace	Client
{
	public class	TheClient
	{
		public static void	Main(string[] args)
		{
			BaseProduct	p1 = new BaseProduct("Product A", 14.99);
			BaseProduct	p2 = new BaseProduct("Product B", 9.99);
			UIObserver	ui = new UIObserver();
			POObserver	po = new POObserver();

			ui.ObserveProduct(p1);
			ui.ObserveProduct(p2);
			po.ObserveProduct(p1);
			po.ObserveProduct(p2);

			p1.price = 24.99;
			p2.price = 11.99;
		}
	}
}