设计模式 10 (转)
吕震宇
结构模式(Structural Pattern)描述如何将类或者对象结合在一起形成更大的结构。结构模式描述两种不同的东西:类与类的实例。根据这一点,结构模式可以分为类的结构模式和对象的结构模式。
后续内容将包括以下结构模式:
- 适配器模式(Adapter):Match interfaces of different classes
- 合成模式(Composite):A tree structure of simple and composite objects
- 装饰模式(Decorator):Add responsibilities to objects dynamically
- 代理模式(Proxy):An object representing another object
- 享元模式(Flyweight):A fine-grained instance used for efficient sharing
- 门面模式(Facade):A single class that represents an entire subsystem
- 桥梁模式(Bridge):Separates an object interface from its implementation
一、 适配器(Adapter)模式
适配器模式把一个类的接口变换成客户端所期待的另一种接口,从而使原本接口不匹配而无法在一起工作的两个类能够在一起工作。
名称由来
这很像变压器(Adapter),变压器把一种电压变换成另一种电压。美国的生活用电电压是110V,而中国的电压是220V。如果要在中国使用美国电器,就必须有一个能把220V电压转换成110V电压的变压器。这个变压器就是一个Adapter。
Adapter模式也很像货物的包装过程:被包装的货物的真实样子被包装所掩盖和改变,因此有人把这种模式叫做包装(Wrapper)模式。事实上,大家经常写很多这样的Wrapper类,把已有的一些类包装起来,使之有能满足需要的接口。
适配器模式的两种形式
适配器模式有类的适配器模式和对象的适配器模式两种。我们将分别讨论这两种Adapter模式。
二、 类的Adapter模式的结构:
由图中可以看出,Adaptee类没有Request方法,而客户期待这个方法。为了使客户能够使用Adaptee类,提供一个中间环节,即类Adapter类,Adapter类实现了Target接口,并继承自Adaptee,Adapter类的Request方法重新封装了Adaptee的SpecificRequest方法,实现了适配的目的。
因为Adapter与Adaptee是继承的关系,所以这决定了这个适配器模式是类的。
该适配器模式所涉及的角色包括:
目标(Target)角色:这是客户所期待的接口。因为C#不支持多继承,所以Target必须是接口,不可以是类。
源(Adaptee)角色:需要适配的类。
适配器(Adapter)角色:把源接口转换成目标接口。这一角色必须是类。
三、 类的Adapter模式示意性实现:
下面的程序给出了一个类的Adapter模式的示意性的实现:
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四、 对象的Adapter模式的结构:
从图中可以看出:客户端需要调用Request方法,而Adaptee没有该方法,为了使客户端能够使用Adaptee类,需要提供一个包装(Wrapper)类Adapter。这个包装类包装了一个Adaptee的实例,从而将客户端与Adaptee衔接起来。由于Adapter与Adaptee是委派关系,这决定了这个适配器模式是对象的。
该适配器模式所涉及的角色包括:
目标(Target)角色:这是客户所期待的接口。目标可以是具体的或抽象的类,也可以是接口。
源(Adaptee)角色:需要适配的类。
适配器(Adapter)角色:通过在内部包装(Wrap)一个Adaptee对象,把源接口转换成目标接口。
五、 对象的Adapter模式示意性实现:
下面的程序给出了一个类的Adapter模式的示意性的实现:
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六、 在什么情况下使用适配器模式
在以下各种情况下使用适配器模式:
1、 系统需要使用现有的类,而此类的接口不符合系统的需要。
2、 想要建立一个可以重复使用的类,用于与一些彼此之间没有太大关联的一些类,包括一些可能在将来引进的类一起工作。这些源类不一定有很复杂的接口。
3、 (对对象适配器而言)在设计里,需要改变多个已有子类的接口,如果使用类的适配器模式,就要针对每一个子类做一个适配器,而这不太实际。
七、 一个实际应用Adapter模式的例子
下面的程序演示了Class Adapter与Object Adapter的应用。
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八、 关于Adapter模式的讨论
Adapter模式在实现时有以下这些值得注意的地方:
1、 目标接口可以省略,模式发生退化。但这种做法看似平庸而并不平庸,它可以使Adaptee不必实现不需要的方法(可以参考Default Adapter模式)。其表现形式就是父类实现缺省方法,而子类只需实现自己独特的方法。这有些像模板(Template)模式。
2、 适配器类可以是抽象类。
3、 带参数的适配器模式。使用这种办法,适配器类可以根据参数返还一个合适的实例给客户端。
参考文献:
阎宏,《Java与模式》,电子工业出版社
[美]James W. Cooper,《C#设计模式》,电子工业出版社
[美]Alan Shalloway James R. Trott,《Design Patterns Explained》,中国电力出版社
[美]Robert C. Martin,《敏捷软件开发-原则、模式与实践》,清华大学出版社
[美]Don Box, Chris Sells,《.NET本质论 第1卷:公共语言运行库》,中国电力出版社http://www.dofactory.com/Patterns/Patterns.aspx