在没有接触设计模式时,都是针对具体实现编程,读取数据库新闻时直接写一个基于数据库的方法,读取推荐新闻时再写一个基于XML的方法,两种方法之间没有任何的关系.当时觉的这样实现也没什么不好。可是你总会发现这两种方法在实现上有很多相同的地方,都是读取出一个新闻标题集合来绑定数据源,只是取数据源的方法不同而已。为了方便管理,我们可以定义一个统一的接口来约束这两种方法。这种做法也足够满足读取不同载体的新闻要求。可是如果在读取新闻时要做其它的操作呢?例如:给读取出来的新闻的人气加一。这个时候我们就要修改原程序,这样有背于"对扩展开放,对修改关闭"的编程原则。如何解决呢?这就是装饰者模式出场的时候了。
在不必改变原类文件和使用继承的情况下，动态地扩展一个对象的功能。它是通过创建一个包装对象，也就是装饰来包裹真实的对象。
装饰者模式把每个要装饰的功能放在单独的类中，并让这个类包装它要装饰的对象，因此，当需要执行特殊行为时，客户端代码就可以在运行的时候根据需要有选择地、按顺序地使用装饰功能包装对象了。
图1
使用场景
设想一下，如果我们需要创建一个在不同场合有不同着装的学生，例如：在学校学生需要穿上校服，在舞会学生需要穿上正装，在家学生可以裸装（有点变态），当然，还可以学习超人把底裤穿在外面。这时候问题来了，难道我们要为每种场合编写一个不同穿着的学生类吗？如果我们的童鞋想要一个穿着校服裤子、正装上衣外露的底裤怎么办？StudentWithSchoolUniform、StudentWithFormalWear、StudentWithNaked、StudentWithSchoolUniformAndOutSideUnderWear..................绵绵无尽的类~~~累！是的，如果这样就造成类爆炸了，需求增加，类就不断的增加，整个系统的维护难度可想而知。
所以这时候，装饰者模式就可以发挥它的作用了，底裤、正装、校服、鞋子、眼镜等等都是具体的装饰者，学生是具体的被装饰的对象，被装饰的对象和装饰者的抽象类都继承者同一个父类。为学生穿上不同的服装，其实就是使用装饰者类（服装）包裹被装饰者类（学生），形象的说这是一个穿衣的过程。
类和接口
- Component（被装饰对象基类，对应例子的Person类）
- ConcreteComponent（具体被装饰对象，对应例子的Student类）
- Decorator（装饰者基类，对应例子的Costume）
- ContreteDecorator（具体的装饰者类，对应例子的Pants、Shirt等）
例子
图2
Person.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Person.php </span><span> 5</span> <span>* 被装饰基类 </span><span> 6</span> <span>*</span><span>*/</span> <span> 7</span> <span>abstract</span> <span>class</span><span> Person{ </span><span> 8</span> <span> 9</span> <span>public</span> <span>abstract</span> <span>function</span><span> show(); </span><span>10</span> <span>11</span> }
View Code
Student.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Student.php </span><span> 5</span> <span>* 具体被装饰对象 </span><span> 6</span> <span>*</span><span>*/</span> <span> 7</span> <span>class</span> Student <span>extends</span><span> Person{ </span><span> 8</span> <span> 9</span> <span>private</span> <span>$name</span><span>; </span><span>10</span> <span>11</span> <span>public</span> <span>function</span> __construct(<span>$name</span><span>){ </span><span>12</span> <span>$this</span>->name = <span>$name</span><span>; </span><span>13</span> <span> } </span><span>14</span> <span>15</span> <span>public</span> <span>function</span><span> show(){ </span><span>16</span> <span>echo</span> '我是学生',<span>$this</span>-><span>name; </span><span>17</span> <span> } </span><span>18</span> }
View Code
Costume.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Costume.php </span><span> 5</span> <span>* 装饰者基类 </span><span> 6</span> <span>*</span><span>*/</span> <span> 7</span> <span>abstract</span> <span>class</span> Costume <span>extends</span><span> Person{ </span><span> 8</span> <span> 9</span> <span>10</span> }
View Code
Shirt.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Shirt.php </span><span> 5</span> <span>* 具体的装饰者类 </span><span> 6</span> <span>*</span><span>*/</span> <span> 7</span> <span>class</span> Shirt <span>extends</span><span> Costume{ </span><span> 8</span> <span> 9</span> <span>private</span> <span>$person</span><span>; </span><span>10</span> <span>11</span> <span>public</span> <span>function</span> __construct(Person <span>$person</span><span>){ </span><span>12</span> <span>13</span> <span>$this</span>->person = <span>$person</span><span>; </span><span>14</span> <span>15</span> <span> } </span><span>16</span> <span>17</span> <span>public</span> <span>function</span><span> show(){ </span><span>18</span> <span>19</span> <span>echo</span> <span>$this</span>->person->show(),',穿着衬衫'<span>; </span><span>20</span> <span> } </span><span>21</span> <span>22</span> }
View Code
Pants.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Pants.php </span><span> 5</span> <span>*</span><span>*/</span> <span> 6</span> <span>class</span> Pants <span>extends</span><span> Costume{ </span><span> 7</span> <span> 8</span> <span>private</span> <span>$person</span><span>; </span><span> 9</span> <span>10</span> <span>public</span> <span>function</span> __construct(Person <span>$person</span><span>){ </span><span>11</span> <span>12</span> <span>$this</span>->person = <span>$person</span><span>; </span><span>13</span> <span>14</span> <span> } </span><span>15</span> <span>16</span> <span>public</span> <span>function</span><span> show(){ </span><span>17</span> <span>18</span> <span>echo</span> <span>$this</span>->person->show(),',穿着裤子'<span>; </span><span>19</span> <span> } </span><span>20</span> <span>21</span> }
View Code
Glasses.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* Glasses.php </span><span> 5</span> <span>*</span><span>*/</span> <span> 6</span> <span>class</span> Glasses <span>extends</span><span> Costume{ </span><span> 7</span> <span> 8</span> <span>private</span> <span>$person</span><span>; </span><span> 9</span> <span>10</span> <span>public</span> <span>function</span> __construct(Person <span>$person</span><span>){ </span><span>11</span> <span>12</span> <span>$this</span>->person = <span>$person</span><span>; </span><span>13</span> <span>14</span> <span> } </span><span>15</span> <span>16</span> <span>public</span> <span>function</span><span> show(){ </span><span>17</span> <span>18</span> <span>echo</span> <span>$this</span>->person->show(),',带着眼镜'<span>; </span><span>19</span> <span> } </span><span>20</span> <span>21</span> }
View Code
UnderWear.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>/*</span><span>* </span><span> 4</span> <span>* UnderWear.php </span><span> 5</span> <span>*</span><span>*/</span> <span> 6</span> <span>class</span> UnderWear <span>extends</span><span> Costume{ </span><span> 7</span> <span> 8</span> <span>private</span> <span>$person</span><span>; </span><span> 9</span> <span>10</span> <span>public</span> <span>function</span> __construct(Person <span>$person</span><span>){ </span><span>11</span> <span>12</span> <span>$this</span>->person = <span>$person</span><span>; </span><span>13</span> <span>14</span> <span> } </span><span>15</span> <span>16</span> <span>public</span> <span>function</span><span> show(){ </span><span>17</span> <span>18</span> <span>echo</span> <span>$this</span>->person->show(),',穿着DK'<span>; </span><span>19</span> <span> } </span><span>20</span> <span>21</span> }
View Code
Client.php
<span> 1</span> <?<span>php </span><span> 2</span> <span> 3</span> <span>require_once</span> 'Person.php'<span>; </span><span> 4</span> <span>require_once</span> 'Costume.php'<span>; </span><span> 5</span> <span>require_once</span> 'Student.php'<span>; </span><span> 6</span> <span>require_once</span> 'UnderWear.php'<span>; </span><span> 7</span> <span>require_once</span> 'Shirt.php'<span>; </span><span> 8</span> <span>require_once</span> 'Pants.php'<span>; </span><span> 9</span> <span>require_once</span> 'Glasses.php'<span>; </span><span>10</span> <span>11</span> <span>//</span><span> Student继承Person</span> <span>12</span> <span>$jc</span> = <span>new</span> Student('JC'<span>); </span><span>13</span> <span>$jc</span>->show(); <span>//</span><span> 我是学生JC</span> <span>14</span> <span>echo</span> '<br>'<span>; </span><span>15</span> <span>16</span> <span>//</span><span> 用UnderWear类装饰Person</span> <span>17</span> <span>$underwear</span> = <span>new</span> UnderWear(<span>$jc</span><span>); </span><span>18</span> <span>$underwear</span>->show(); <span>//</span><span> 我是学生JC,穿着DK</span> <span>19</span> <span>echo</span> '<br>'<span>; </span><span>20</span> <span>21</span> <span>//</span><span> 再用Pants类装饰Person</span> <span>22</span> <span>$pants</span> = <span>new</span> Pants(<span>$underwear</span><span>); </span><span>23</span> <span>$pants</span>->show(); <span>//</span><span> 我是学生JC,穿着DK,穿着裤子</span> <span>24</span> <span>echo</span> '<br>'<span>; </span><span>25</span> <span>26</span> <span>//</span><span> 再用Shirt类装饰Person</span> <span>27</span> <span>$shirt</span> = <span>new</span> Shirt(<span>$pants</span><span>); </span><span>28</span> <span>$shirt</span>->show(); <span>//</span><span> 我是学生JC,穿着DK,穿着裤子,穿着衬衫</span> <span>29</span> <span>echo</span> '<br>'<span>; </span><span>30</span> <span>31</span> <span>//</span><span> 再用Glasses类装饰Person</span> <span>32</span> <span>$glasses</span> = <span>new</span> Glasses(<span>$shirt</span><span>); </span><span>33</span> <span>$glasses</span>->show(); <span>//</span><span> 我是学生JC,穿着DK,穿着裤子,穿着衬衫,带着眼镜</span> <span>34</span> <span>echo</span> '<br>';
图3 输出结果截图
Symfony2 EventDispatch 组件对装饰者模式的应用
图4 Symfony2 EventDispatch组件使用装饰模式
图5 Framework配置EventDispatcher
- Symfony\Component\EventDispatcher\EventDispatcherInterface 是被装饰的接口
- Symfony\Component\EventDispatcher\EventDispatcher 和 Symfony\Component\EventDispatcher\ContainerAwareEventDispatcher 是被装饰的具体对象
- Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcherInterface 装饰者接口
- Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcher 装饰者基类
- Symfony\Component\HttpKernel\Debug\TraceableEventDispatcher 具体的装饰者对象
具体装饰者对象Symfony\Component\HttpKernel\Debug\TraceableEventDispatcher::dispatch()方法，核心依旧是调用被装饰的具体对象Symfony\Component\EventDispatcher\EventDispatcher::dispatch()方法进行工作，但是装饰者对象Symfony\Component\HttpKernel\Debug\TraceableEventDispatcher::dispatch()方法添加了相应的功能，例如在调用Symfony\Component\EventDispatcher\EventDispatcher::dispatch()方法前后分别调用了preProcess()、preDispatch()和postDispatch()、postProcess():
<span> 1</span> <span>/*</span><span>* </span><span> 2</span> <span> * {@inheritdoc} </span><span> 3</span> <span>*/</span> <span> 4</span> <span>public</span> <span>function</span> dispatch(<span>$eventName</span>, Event <span>$event</span> = <span>null</span><span>) </span><span> 5</span> <span> { </span><span> 6</span> <span>if</span> (<span>null</span> === <span>$event</span><span>) { </span><span> 7</span> <span>$event</span> = <span>new</span><span> Event(); </span><span> 8</span> <span> } </span><span> 9</span> <span>10</span> <span>//</span><span> 装饰者对象增加的功能</span> <span>11</span> <span>$this</span>->preProcess(<span>$eventName</span><span>); </span><span>12</span> <span>$this</span>->preDispatch(<span>$eventName</span>, <span>$event</span><span>); </span><span>13</span> <span>14</span> <span>$e</span> = <span>$this</span>->stopwatch->start(<span>$eventName</span>, 'section'<span>); </span><span>15</span> <span>16</span> <span>//</span><span> 核心依旧是调用被装饰的具体对象Symfony\Component\EventDispatcher\EventDispatcher::dispatch()方法</span> <span>17</span> <span>$this</span>->dispatcher->dispatch(<span>$eventName</span>, <span>$event</span><span>); </span><span>18</span> <span>19</span> <span>if</span> (<span>$e</span>-><span>isStarted()) { </span><span>20</span> <span>$e</span>-><span>stop(); </span><span>21</span> <span> } </span><span>22</span> <span>23</span> <span>//</span><span> 装饰者对象增加的功能</span> <span>24</span> <span>$this</span>->postDispatch(<span>$eventName</span>, <span>$event</span><span>); </span><span>25</span> <span>$this</span>->postProcess(<span>$eventName</span><span>); </span><span>26</span> <span>27</span> <span>return</span> <span>$event</span><span>; </span><span>28</span> }
优点
- 通过组合而非继承的方式，实现了动态扩展对象的功能的能力。
- 有效避免了使用继承的方式扩展对象功能而带来的灵活性差，子类无限制扩张的问题。
- 充分利用了继承和组合的长处和短处，在灵活性和扩展性之间找到完美的平衡点。
- 装饰者和被装饰者之间虽然都是同一类型，但是它们彼此是完全独立并可以各自独立任意改变的。
- 遵守大部分GRASP原则和常用设计原则，高内聚、低偶合。
缺点
- 装饰链不能过长，否则会影响效率。
- 只在必要的时候使用装饰者模式，否则会提高程序的复杂性，增加系统维护难度。
- 装饰者对象和被装饰者对象都继承Component，如果Component内部发生变化，所有的子类都要改变。
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