Software development

// Observer pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Observer.Structural
{

// MainApp test application

class MainApp
{
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.Read();
}
}

// "Subject"

abstract class Subject
{
private ArrayList observers = new ArrayList();

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();
}
}
}

// "ConcreteSubject"

class ConcreteSubject : Subject
{
private string subjectState;

// Property
public string SubjectState
{
get{ return subjectState; }
set{ subjectState = value; }
}
}

// "Observer"

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

// "ConcreteObserver"

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);
}

// Property
public ConcreteSubject Subject
{
get { return subject; }
set { subject = value; }
}
}
}

Output expected:

Observer X's new state is ABC
Observer Y's new state is ABC
Observer Z's new state is ABC