The Observer/Publish-Subscribe Pattern in C#

The Observer Pattern is most commonly known also as the Publish/Subscribe Pattern.

The definition of the Observer Pattern is as follows:

The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically (Head First Design Patterns-the observer pattern).

Below is the corresponding code in C# for the Observer pattern found in the Head First Design Patterns book:

 

void Main()

{

    WeatherData weatherData = new WeatherData();

            CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);

            weatherData.setMeasurements(80F, 65F, 30.4F);

            weatherData.setMeasurements(82F, 70F, 29.2F);

            weatherData.setMeasurements(78F, 90F, 29.2F);

}

 public class CurrentConditionsDisplay: Observer, DisplayElement

    {

        private float temperature;

        private float humidity;

        private Subject weatherData;

        public CurrentConditionsDisplay(Subject weatherData)

        {

            this.weatherData = weatherData;

            weatherData.registerObserver(this);

        }

        public void update(float temperature, float humidity, float pressure)

        {

            this.temperature = temperature;

            this.humidity = humidity;

            display();

        }

        public void display()

        {

            Console.WriteLine("Current conditions: " + temperature + "F degrees and " + humidity + "% humidity");

        }

    }

public interface Observer

    {

        void update(float temp, float humidity, float pressure);

    }

    public interface DisplayElement

    {

        void display();

    }

public interface Subject

    {

        void registerObserver(Observer o);

        void removeObserver(Observer o);

        void notifyObservers();

    }

public class WeatherData: Subject

    {

        private ArrayList observers;

        private float temperature;

        private float humidity;

        private float pressure;

        public WeatherData()

        {

            observers = new ArrayList();

        }

        public void registerObserver(Observer o)

        {

            observers.Add(o);

        }

        public void removeObserver(Observer o)

        {

            int i = observers.IndexOf(o);

            if (i >= 0)

            {

                observers.RemoveAt(i);

            }//if

        }

        public void notifyObservers()

        {

            for (int i = 0; i < observers.Count; i++)

            {

                Observer observer = (Observer)observers[i];

                observer.update(temperature, humidity, pressure);

            }

        }

        public void setMeasurements(float temperature, float humidity, float pressure)

        {

            this.temperature = temperature;

            this.humidity = humidity;

            this.pressure = pressure;

            measurementsChanged();

        }

        private void measurementsChanged()

        {

            notifyObservers();

        }

    }

If you are looking to use .NET's built-in IObservable interface, you can read more about that here: http://msdn.microsoft.com/en-us/library/dd990377%28v=vs.110%29.aspx

For an example of the Observer Design Pattern based on implementing the IObservable interface, you can find that here: http://msdn.microsoft.com/en-us/library/ee850490%28v=vs.110%29.aspx