设计模式 之 状态模式

前传：

一个类内部存在多个条件分支时，你应该想一下，是不是可以用状态模式给替换。

那什么是状态模式呀？

state 把对象的内部状态独立出来，封装内部状态变化。

实现：

那怎么封装呀？想想应该有两个类：主类，辅助类（即状态类）。

主类开始得有一个状态呀。激发状态变化，但实现状态变化的细节，却通过状态类来实现。

状态类修改主类的状态。根据主类的当前状态指定主类（跳转）到下一状态类。

主类和状态类的双向互动：主类通过函数推动状态类变化；状态类负责修改主类的状态。

用户根本就不知道状态类的存在。只有程序员才知道具体的实现。（这也是传说中的“封装”）

关键词：

封装 内部状态

网上一个兄弟的例子很说明问题：

<p style="margin: 10px auto; padding-top: 0px; padding-bottom: 0px; color: rgb(51, 51, 51); font-family: verdana, Arial, Helvetica, sans-serif; font-size: 14px; line-height: 25px; ">电灯有两个状态，开（亮）与关（不亮），下面就用状态模式来实现对电灯的控制。</p><p style="margin: 10px auto; padding-top: 0px; padding-bottom: 0px; color: rgb(51, 51, 51); font-family: verdana, Arial, Helvetica, sans-serif; font-size: 14px; line-height: 25px; ">　 类图</p><p style="margin: 10px auto; padding-top: 0px; padding-bottom: 0px; color: rgb(51, 51, 51); font-family: verdana, Arial, Helvetica, sans-serif; font-size: 14px; line-height: 25px; ">　　<img src="http://pic002.cnblogs.com/images/2012/155937/2012071614433983.png" alt="" style="margin: 0px; padding: 0px; border: 0px; " /></p><p style="margin: 10px auto; padding-top: 0px; padding-bottom: 0px; color: rgb(51, 51, 51); font-family: verdana, Arial, Helvetica, sans-serif; font-size: 14px; line-height: 25px; ">　　实现代码</p><div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); font-family: 'Courier New' !important; border: 1px solid rgb(204, 204, 204); overflow: auto; "><div class="cnblogs_code_toolbar" style="margin: 5px 0px 0px; padding: 0px; "><span class="cnblogs_code_copy" style="margin: 0px; padding: 0px 5px 0px 0px; line-height: 1.8; "><a target=_blank href="http://www.cnblogs.com/wangjq/archive/2012/07/16/2593485.html" title="复制代码" style="margin: 0px; padding: 0px; text-decoration: none; color: rgb(0, 0, 0); border: none !important; "><img src="http://common.cnblogs.com/images/copycode.gif" alt="复制代码" style="margin: 0px; padding: 0px; border: none !important; " /></a></span></div><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important; ">    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 电灯类，对应模式中的Context类    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">class</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> Light    {        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">private</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> LightState state;        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> Light(LightState state)        {            </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">this</span>.state =<span style="margin: 0px; padding: 0px; line-height: 1.8; "> state;        }        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 按下电灯开关        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">void</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> PressSwich()        {            state.PressSwich(</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">this</span><span style="margin: 0px; padding: 0px; line-height: 1.8; ">);        }        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> LightState State        {            </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">get</span> { <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">return</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> state; }            </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">set</span> { state =<span style="margin: 0px; padding: 0px; line-height: 1.8; "> value; }        }        }    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 抽象的电灯状态类，相当于State类    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">abstract</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">class</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> LightState    {        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">abstract</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">void</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> PressSwich(Light light);    }    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 具体状态类, 开    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">class</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> On : LightState    {        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 在开状态下，按下开关则切换到关的状态。        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><param name="light"></param></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">override</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">void</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> PressSwich(Light light)        {            Console.WriteLine(</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">"</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">Turn off the light.</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">"</span><span style="margin: 0px; padding: 0px; line-height: 1.8; ">);            light.State </span>= <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">new</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> Off();        }    }    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 具体状态类，关    </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>    <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">class</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> Off: LightState    {        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 128, 0); "> 在关状态下，按下开关则打开电灯。        </span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "></summary></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); ">///</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 128, 128); "><param name="light"></param></span>        <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">public</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">override</span> <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">void</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> PressSwich(Light light)        {            Console.WriteLine(</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">"</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">Turn on the light.</span><span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(128, 0, 0); ">"</span><span style="margin: 0px; padding: 0px; line-height: 1.8; ">);            light.State </span>= <span style="margin: 0px; padding: 0px; line-height: 1.8; color: rgb(0, 0, 255); ">new</span><span style="margin: 0px; padding: 0px; line-height: 1.8; "> On();        }    }</span>

　客户端代码

    class Program    {        static void Main(string[] args)        {            // 初始化电灯，原始状态为关            Light light = new Light(new Off());            // 第一次按下开关，打开电灯            light.PressSwich();            // 第二次按下开关，关闭电灯            light.PressSwich();            Console.Read();        }    }