7 Observer模式
7.1 概述
Observer模式,就是建立一种机制,使多个对象依赖于某个特定对象,当这个特定的对象改变时,能通知到依赖它的所有对象作出反应。
7.2 设计
这里就有2个问题:一个是这个特定对象如何知道它要通知那些对象,二是如何通知?先来看第一问题:如何知道要通知那些对象?那么就需要对象在生成或需要被通知时,主动告诉这个特定的对象:我是你通知的对象。这个特定对象把它们都“记”起来,到时一个个通知。如何通知呢?就需要被通知的对象公布一个接口,这个特定对象调用这个接口,通知发生改变。但是如果有很多对象需要通知,每个对象各有一个接口,就会导致这个特定对象需要记住很多接口,特麻烦!好在它很聪明,让所有的对象统一接口,否则就不通知它。
7.3 实现
UML图:
示例代码为:
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using System;
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using System.Collections ;
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namespace Example
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{
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/// <summary>
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/// 示例
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/// </summary>
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class Example
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{
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/// <summary>
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/// 应用程序的主入口点。
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/// </summary>
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[STAThread]
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static void Main(string[] args)
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{
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Observer o1 = new O1() ;
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Observer o2 = new O2() ;
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Subject s = new Subject() ;
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s.Notify() ;
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}
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/// <summary>
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/// 被观察的对象
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/// </summary>
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public class Subject
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{
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/// <summary>
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/// 静态数组保存观察者
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/// </summary>
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private static ArrayList list = new ArrayList() ;
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/// <summary>
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/// 把观察者加入到数组中
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/// </summary>
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/// <param name="o">被加入的观察者</param>
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public static void Attach( Observer o )
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{
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list.Add( o ) ;
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}
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/// <summary>
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/// 把观察者退出到数组中
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/// </summary>
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/// <param name="o">被退出的观察者</param>
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public static void Detach( Observer o )
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{
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list.Remove( o ) ;
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}
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/// <summary>
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/// 逐个通知发生改变
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/// </summary>
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public void Notify()
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{
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foreach( Observer o in list )
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{
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o.Update() ;
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}
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}
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}
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/// <summary>
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/// 观察者
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/// </summary>
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public class Observer
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{
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public Observer()
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{
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Subject.Attach( this ) ;//在生成时加入观察者列
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}
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~Observer()
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{
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Subject.Detach( this ) ;//在注销时退出观察者列
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}
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public virtual void Update() {}//当观察的对象改变时
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}
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public class O1 : Observer
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{
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public override void Update()
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{
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Console.WriteLine( "O1 Update Now!" ) ;
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}
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}
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public class O2 : Observer
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{
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public override void Update()
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{
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Console.WriteLine( "O2 Update Now!" ) ;
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}
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}
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}
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}
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