• It defines a one-to-many dependency between objects so when one object changes state, all its dependents are notified and updated automatically.
• The usage is to define subjects that must change the state of observers.
  • Subject relies on a list of observers.
  • The subjects and observers are "loosely coupled" because they interact which makes them coupled but also have a little knowledge about each other.
• Events and Delegates, which are first class language features, act as the Subject and Observers respectively as defined in the Observer pattern.
• The Observer pattern facilitates good object-oriented designs as it promotes loose coupling.
• Observers register and unregister themselves with subjects that maintain a list of interested observers.
• The subject does not depend on any particular observer, as long as the delegates are of the correct type for the event.

Participants

• **Subject** (IStock)
  • Knows its observers.
    • Any number of Observer objects may observe a subject.
  • Provides an interface for attaching and detaching Observer objects.
• **ConcreteSubject** (MicrosoftStock)
  • Stores state of interest to **ConcreteObserver**.
  • Sends a notification to its **observers** when its state changes.
• **Observer** (IInvestor)
  • Defines an updating interface for objects that should be notified of changes in a subject.
• **ConcreteObserver** (Investor)
  • Maintains a reference to a ConcreteSubject object.
  • Stores state that should stay consistent with the subject's.
  • Implements the Observer updating interface to keep its state consistent with the subject's.

var microsoftStock = new MicrosoftStock();
microsoftStock.Symbol = "MS";

var investor1 = new Investor();
investor1.Name = "Bill Gates";
var investor2 = new Investor();
investor2.Name = "Berkshire";

microsoftStock.Attach(investor1);
microsoftStock.Attach(investor2);

microsoftStock.Price = 120.10;
microsoftStock.Price = 121.00;
microsoftStock.Price = 120.50;
microsoftStock.Price = 120.75;

public interface IStock
{
	List<IInvestor> Investors { get; }
	string Symbol { get; set; }
	double Price { get; set; }

	void Attach(IInvestor investor);
	void Detach(IInvestor investor);
	void Notify();        
}       

public class MicrosoftStock : IStock
{
	public List<IInvestor> Investors { get; private set; } = new List<IInvestor>();
	public string Symbol { get; set; }

	private double _price;
	public double Price
	{
		get => _price;
		set
		{
			if (_price != value)
			{
				_price = value;
				Notify();
			}
		}
	}

	public void Attach(IInvestor investor) => Investors.Add(investor);
	public void Detach(IInvestor investor) => Investors.Remove(investor);
	public void Notify()
	{
		foreach (IInvestor investor in Investors)
			investor.Update(this);
	}
}

public interface IInvestor
{
	string Name { get; set; }
	void Update(IStock stock);
}

public class Investor : IInvestor
{
	public string Name { get; set; }
	public void Update(IStock stock) => Console.WriteLine("Notified {0} of {1}'s change to {2:C}", Name, stock.Symbol, stock.Price);        
}

Rules of Thumb

• Chain of Responsibility, Command, Mediator, and Observer, address how you can decouple senders and receivers, but with different trade-offs.
  • Chain of Responsibility passes a sender request along a chain of potential receivers.
  • Command normally specifies a sender-receiver connection with a subclass.
  • Mediator has senders and receivers reference each other indirectly.
  • Observer defines a very decoupled interface that allows for multiple receivers to be configured at run-time.
• Mediator and Observer are competing patterns.
  • The difference between them is that Observer distributes communication by introducing "observer" and "subject" objects, whereas a Mediator object encapsulates the communication between other objects.
  • It is easier to make reusable Observers and Subjects than to make reusable Mediators.
  • On the other hand, Mediator can leverage Observer for dynamically registering colleagues and communicating with them.