using System;

using System.Collections.Generic;

interface ISubject {

    void AddObserver(Observer observer);

    void RemoveObserver(Observer observer);

    void NotifyEvent(string evt);

}

​

public class Subject:ISubject {

    private List<Observer> observers = new List<Observer>();

    public void AddObserver(Observer observer) {

        observers.Add(observer);

        Console.WriteLine("{0} subscribes", observer.ObserverName);

    }

    public void RemoveObserver(Observer observer) {

        observers.Remove(observer);

        Console.WriteLine("{0} UN-subscribes", observer.ObserverName);

    }

    public void NotifyEvent(string evt) {

        observers.ForEach(x => x.onNotifyEvent(evt));

    }

}

​

interface IObserver {

    void onNotifyEvent(string evt);

}

public class Observer:IObserver {

    public string ObserverName { get; private set; }

    public Observer(string name) {

        this.ObserverName = name;

    }

    public virtual void onNotifyEvent(string evt) {

        Console.WriteLine("{0} : {1}", this.ObserverName, evt);

    }

}

​

public class Physics:Subject {

    public void updateEntity(object entity) {

        // ... entity.update();

    }

    public void update() {

        if (true) { //player is dead

            NotifyEvent("death");

        }

    }

}

​

public class AchievementsManager:Observer {

    public AchievementsManager(string name):base(name) {}

    private bool deathUnlocked = false;

    override public void onNotifyEvent(string evt) {

        switch (evt) {

            case "death":

                if (!deathUnlocked) {

                    unlock(evt);

                    deathUnlocked = true;

                }

                break;

        }

    }

    private void unlock(string evt) {

        Console.WriteLine("{0} UNLOCKING {1}", this.ObserverName, evt);

    }

}

​

// ----------------------------- MAIN -----------------------

public class Program {

    static public void Main(string[] args) {

        Physics subject = new Physics();

        // Observer1 takes a subscription to the store

        Observer observer1 = new AchievementsManager("AchievementsManager");

        subject.AddObserver(observer1);

        // Observer2 takes a subscription to the store

        Observer observer2 = new Observer("AudioManager");

        subject.AddObserver(observer2);

        subject.update();

        // Observer1 unsubscribes and Observer3 subscribes to notifications.

        subject.RemoveObserver(observer2);

        subject.AddObserver(new Observer("ScoreManager"));

        subject.update();

    }

}