Observer

Summary: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Frequency of use: High

UML class diagram

Participants

The classes and objects participating in this pattern are:

Subject ( Stock ) knows its observers. Any number of Observer objects may observe a subject provides an interface for attaching and detaching Observer objects.

( ) ConcreteSubject ( IBM ) 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

( )

Structural code in C#

This structural code demonstrates the Observer pattern in which registered objects are notified of and updated with a state change.

using System; using System.Collections.Generic; namespace DoFactory.GangOfFour.Observer.Structural { /// <summary> /// MainApp startup class for Structural /// Observer Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { // Configure Observer pattern ConcreteSubject s = new ConcreteSubject(); s.Attach(new ConcreteObserver(s, "X")); s.Attach(new ConcreteObserver(s, "Y")); s.Attach(new ConcreteObserver(s, "Z")); // Change subject and notify observers s.SubjectState = "ABC"; s.Notify(); // Wait for user Console.ReadKey(); } } /// <summary> /// The 'Subject' abstract class /// </summary> abstract class Subject { private List<Observer> _observers = new List<Observer>(); public void Attach(Observer observer) { _observers.Add(observer); } public void Detach(Observer observer) { _observers.Remove(observer); } public void Notify() { foreach (Observer o in _observers) { o.Update(); } } } /// <summary> /// The 'ConcreteSubject' class /// </summary> class ConcreteSubject : Subject { private string _subjectState; // Gets or sets subject state public string SubjectState { get { return _subjectState; } set { _subjectState = value; } } } /// <summary> /// The 'Observer' abstract class /// </summary> abstract class Observer { public abstract void Update(); } /// <summary> /// The 'ConcreteObserver' class /// </summary> class ConcreteObserver : Observer { private string _name; private string _observerState; private ConcreteSubject _subject; // Constructor public ConcreteObserver( ConcreteSubject subject, string name) { this._subject = subject; this._name = name; } public override void Update() { _observerState = _subject.SubjectState; Console.WriteLine("Observer {0}'s new state is {1}", _name, _observerState); } // Gets or sets subject public ConcreteSubject Subject { get { return _subject; } set { _subject = value; } } } }

Output

Observer X's new state is ABC

Observer Y's new state is ABC

Observer Z's new state is ABC



Real-world code in C#

This real-world code demonstrates the Observer pattern in which registered investors are notified every time a stock changes value.

using System; using System.Collections.Generic; namespace DoFactory.GangOfFour.Observer.RealWorld { /// <summary> /// MainApp startup class for Real-World /// Observer Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { // Create IBM stock and attach investors IBM ibm = new IBM("IBM", 120.00); ibm.Attach(new Investor("Sorros")); ibm.Attach(new Investor("Berkshire")); // Fluctuating prices will notify investors ibm.Price = 120.10; ibm.Price = 121.00; ibm.Price = 120.50; ibm.Price = 120.75; // Wait for user Console.ReadKey(); } } /// <summary> /// The 'Subject' abstract class /// </summary> abstract class Stock { private string _symbol; private double _price; private List<IInvestor> _investors = new List<IInvestor>(); // Constructor public Stock(string symbol, double price) { this._symbol = symbol; this._price = price; } 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); } Console.WriteLine(""); } // Gets or sets the price public double Price { get { return _price; } set { if (_price != value) { _price = value; Notify(); } } } // Gets the symbol public string Symbol { get { return _symbol; } } } /// <summary> /// The 'ConcreteSubject' class /// </summary> class IBM : Stock { // Constructor public IBM(string symbol, double price) : base(symbol, price) { } } /// <summary> /// The 'Observer' interface /// </summary> interface IInvestor { void Update(Stock stock); } /// <summary> /// The 'ConcreteObserver' class /// </summary> class Investor : IInvestor { private string _name; private Stock _stock; // Constructor public Investor(string name) { this._name = name; } public void Update(Stock stock) { Console.WriteLine("Notified {0} of {1}'s " + "change to {2:C}", _name, stock.Symbol, stock.Price); } // Gets or sets the stock public Stock Stock { get { return _stock; } set { _stock = value; } } } }

Output

Notified Sorros of IBM's change to $120.10

Notified Berkshire of IBM's change to $120.10



Notified Sorros of IBM's change to $121.00

Notified Berkshire of IBM's change to $121.00



Notified Sorros of IBM's change to $120.50

Notified Berkshire of IBM's change to $120.50



Notified Sorros of IBM's change to $120.75

Notified Berkshire of IBM's change to $120.75



.NET Optimized code in C#

The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, LINQ, lambda functions, and more. You can find an example on our Singleton pattern page.



All other patterns, and so much more, are available in our Dofactory .NET product.