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 |