观察者（Observer）模式

这个或许叫源-监听器（Source/Listener）模式更加形象。

观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态上发生变化时，会通知所有观察者对象，使它们能够自动更新自己。

 

participants

    The classes and/or 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

 

// Observer pattern -- Real World example

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