Observer Pattern

  • Behavioral pattern
  • The intent of this pattern is to define a one to many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Example Observables in angular, service communications in microservice architectures
  • Observer:- defines an updating interface for objects that should be notified of changes in a subject.
  • Subject :- knows its observers. Provides an interface for attaching and detaching them.
  • ConcreteObserver :- store state that must remain consists with the subject’s state. They implement the Observer updating interface to keep state consistent.
  • ConcreteSubject:- stores state of interest to ConcreteObserver objects, and sends a notification to its Observers when its state changes.
  • Use case :- When a change to one object requires changing others, and you dont know in advance how many objects need to be changed
  • It allows subjects and observers to vary independently : subclasses can be added and change without having to change others: OC principle is not broken
  • Subject and Observers are loosely coupled : Open closed principle is not broken
  • Some of the related patterns:-  Chain of Responsibility, Command, Mediator
				
					namespace observerpattern
{
    public interface ISubject
    {
        void Register(IObserver observer);
        void UnRegister(IObserver observer);
        void Notify();
    }

    public interface IObserver
    {
        void Update(ISubject subject);
    }

    public class ZeeNews : IObserver
    {
        private readonly string _newsChannelName;

        public ZeeNews(string newsChannelName)
        {
            _newsChannelName = newsChannelName;
        }
        public void Update(ISubject subject)
        {
            if (subject is EarthQuakeMeasurement earthQuakeMeasurement)
            {
                Console.WriteLine($"Breakihng news from {_newsChannelName} :: Danger of EarthQuake, seismo records {earthQuakeMeasurement.SeismoMagnitude} , be safe.");
            }
        }
    }

    public class AajTakNews : IObserver
    {
        private readonly string _newsChannelName;

        public AajTakNews(string newsChannelName)
        {
            _newsChannelName = newsChannelName;
        }

        public void Update(ISubject subject)
        {
            if(subject is EarthQuakeMeasurement earthQuakeMeasurement)
            {
                Console.WriteLine($"Big bulletin news from {_newsChannelName} :: Danger of EarthQuake , seismo records {earthQuakeMeasurement.SeismoMagnitude} , be safe.");
            }
        }
    }

    public class EarthQuakeMeasurement : ISubject
    {
        private decimal _seismoMagnitude;
        public decimal SeismoMagnitude {
            get { return _seismoMagnitude; }
            set
            {
                _seismoMagnitude = value;
                Notify();
            }
        }

        private readonly List<IObserver> _observers = new();
        public void Register(IObserver observer)
        {
            _observers.Add(observer);
        }

        public void UnRegister(IObserver observer)
        {
            _observers.Remove(observer);
        }

        public void Notify()
        {
            _observers.ForEach(o => o.Update(this));
        }
    }
}

				
			
				
					using observerpattern;

EarthQuakeMeasurement publisherObj = new EarthQuakeMeasurement();
publisherObj.Register(new AajTakNews("AAJ TAK"));
publisherObj.Register(new ZeeNews("ZEE NEWS"));
publisherObj.SeismoMagnitude = 7.1m;

Big bulletin news from AAJ TAK :: Danger of EarthQuake , seismo records 7.1 , be safe.
Breakihng news from ZEE NEWS :: Danger of EarthQuake, seismo records 7.1 , be safe.

https://github.com/AnuragDaCoder/observer

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