.NET Design Patterns

Observer


 Definition
 UML diagram
 Participants
 Structural code in C#
 Real-world code in C#
 .NET Optimized code in C#



Definition

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, object initializers, automatic properties, etc. You can find an example on our Singleton pattern page.

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