Click here to Skip to main content
11,484,162 members (70,686 online)
Click here to Skip to main content

The Observer Pattern

, 19 Feb 2013 CPOL 3.6K 183
Rate this:
Please Sign up or sign in to vote.
The "Observer Pattern" defines a One-To-Many dependency between objects.


The "Observer Pattern" defines a One-To-Many dependency between objects.

when one object changes state, all of its dependentsare notified and updated automatically.

ONE Subject to the MANY Observers !!!!
Subject can be anything which interests Observers.
Observers are objects who are interested in some Subject.

EXAMPLE : Newpaper Subscription where "Newspaper is Subject" and "Subscribers are Observers".
          ONE Newspaper to MANY Subscribers.


Design Principal  : Strive for loosely coupled designs between objects that interact.

Loosely coupled designs allow us to build flexible system that can handle changes becoz they minimize interdependency.

Note : Observer Pattern can be implemented using Events & Delegates in .Net, .Net 4.0 has now IObservable & IObserver interfaces to facilitate the Observer Pattern.
This Example has implementations using Custom Interface & .Net in-house IObservable interface.

Using the code

// Custom IObserver Interface

    public interface IObserver
        void Update(float temp, float hum, float press);

 // Observer Class which Observes or Subscribe from Observable Data

    public class CurrentConditionsDisplay : IObserver,IDisplayElement
        private float temperature;
        private float humidity;
        private ISubject weatherData;

        public CurrentConditionsDisplay(ISubject weatherData)
            this.weatherData = weatherData;
            if(weatherData != null)

        public void RemoveObservation()

        public void Update(float temp, float hum, float press)
            this.temperature = temp;
            this.humidity = hum;

        public void Display()
            Console.WriteLine("Current Conditions: " + temperature + "F Degrees and " + humidity + "% Humidity");


// Custom IObservable Interface

    public interface ISubject
        void RegisterObserver(IObserver observer);
        void RemoveObserver(IObserver observer);
        void NotifyObservers();


// Observable class which can be observed by Observer Class

    public class WeatherData : ISubject
        private ArrayList Observers;
        private float _temperature;
        private float _humidity;
        private float _pressure;

        public WeatherData()
            Observers = new ArrayList();

        #region ISubject Members

        public void RegisterObserver(IObserver observer)

        public void RemoveObserver(IObserver observer)
            int i = Observers.IndexOf(observer);
            if (i >= 0)

        public void NotifyObservers()
            foreach (IObserver o in Observers)
                o.Update(_temperature, _humidity, _pressure);


        public void MeasurementChanged()

        public void SetMeasurements(float temperature, float humidity, float pressure)
            this._temperature = temperature;
            this._humidity = humidity;
            this._pressure = pressure;

Points of Interest

This Pattern can be implemented by directly using IObserver<T> & IObservable<T> provided by .Net Library also, attached code includes implementation with .Net in-house interface as well.


Version 1


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Chandra Shekhar Joshi
Software Developer (Senior) ABB
India India
Working in Software Development since 2007.
Work in .Net/VC++/COM.
work in Process Automation domain.
Follow on   Twitter

Comments and Discussions

Question[My vote of 1] My vote of 1. Pin
r v19-Feb-13 5:01
memberr v19-Feb-13 5:01 
AnswerRe: [My vote of 1] My vote of 1. Pin
Chandra Shekhar Joshi19-Feb-13 16:03
memberChandra Shekhar Joshi19-Feb-13 16:03 
QuestionNot an article Pin
OriginalGriff19-Feb-13 0:05
mvpOriginalGriff19-Feb-13 0:05 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Terms of Use | Mobile
Web01 | 2.8.150520.1 | Last Updated 19 Feb 2013
Article Copyright 2013 by Chandra Shekhar Joshi
Everything else Copyright © CodeProject, 1999-2015
Layout: fixed | fluid