设计模式之观察者Observer

设计模式涵义

　　设计模式描述了软件设计过程中某一类常见问题的一般性的解决方案。面向对象设计模式描述了面向对象设计过程中、特定场景下、类与相互通信的对象之间常见的组织关系。

观察者模式定义

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

　　定义对象间的一种一对多的依赖关系，以便当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并自动更新．

uml类图

using System;

using System.Collections;

namespace DoFactory.GangOfFour.Observer.Structural

　Output
Observer X's new state is ABC

Observer Y's new state is ABC

Observer Z's new state is ABC

实例代码演示了当股票发生变化时．将会通知股东(投资人)．

Code

using System;

using System.Collections;

namespace DoFactory.GangOfFour.Observer.RealWorld

{

// MainApp test application

class MainApp

{

static void Main()

{

// Create investors

Investor s = new Investor("Sorros");

Investor b = new Investor("Berkshire");

// Create IBM stock and attach investors

IBM ibm = new IBM("IBM", 120.00);

ibm.Attach(s);

ibm.Attach(b);

// Change price, which notifies investors

ibm.Price = 120.10;

ibm.Price = 121.00;

ibm.Price = 120.50;

ibm.Price = 120.75;

// Wait for user

Console.Read();

}

}

// "Subject"

abstract class Stock

{

protected string symbol;

protected double price;

private ArrayList investors = new ArrayList();

// Constructor

public Stock(string symbol, double price)

{

this.symbol = symbol;

this.price = price;

}

public void Attach(Investor investor)

{

investors.Add(investor);

}

public void Detach(Investor investor)

{

investors.Remove(investor);

}

public void Notify()

{

foreach (Investor investor in investors)

{

investor.Update(this);

}

Console.WriteLine("");

}

// Properties

public double Price

{

get{ return price; }

set

{

price = value;

Notify();

}

}

public string Symbol

{

get{ return symbol; }

set{ symbol = value; }

}

}

// "ConcreteSubject"

class IBM : Stock

{

// Constructor

public IBM(string symbol, double price)

: base(symbol, price)

{

}

}

// "Observer"

interface IInvestor

{

void Update(Stock stock);

}

// "ConcreteObserver"

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);

}

// Property

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