概述
在面向对象的软件设计中,我们经常会遇到一类集合对象,这类集合对象的内部结构可能有着各种各样的实现,但是归结起来,无非有两点是需要我们去关心的:一是集合内部的数据存储结构,二是遍历集合内部的数据。面向对象设计原则中有一条是类的单一职责原则,所以我们要尽可能的去分解这些职责,用不同的类去承担不同的职责。Iterator模式就是分离了集合对象的遍历行为,抽象出一个迭代器类来负责,这样既可以做到不暴露集合的内部结构,又可让外部代码透明的访问集合内部的数据。
意图
提供一种方法顺序访问一个聚合对象中各个元素, 而又不需暴露该对象的内部表示。[GOF 《设计模式》]
结构图
Iterator模式结构图如下:
图1 Iterator模式结构图
生活中的例子
迭代器提供一种方法顺序访问一个集合对象中各个元素,而又不需要暴露该对象的内部表示。在早期的电视机中,一个拨盘用来改变频道。当改变频道时,需要手工转动拨盘移过每一个频道,而不论这个频道是否有信号。现在的电视机,使用[后一个]和[前一个]按钮。当按下[后一个]按钮时,将切换到下一个预置的频道。想象一下在陌生的城市中的旅店中看电视。当改变频道时,重要的不是几频道,而是节目内容。如果对一个频道的节目不感兴趣,那么可以换下一个频道,而不需要知道它是几频道。
图2 使用选频器做例子的Iterator模式对象图
Iterator模式解说
在面向对象的软件设计中,我们经常会遇到一类集合对象,这类集合对象的内部结构可能有着各种各样的实现,但是归结起来,无非有两点是需要我们去关心的:一是集合内部的数据存储结构,二是遍历集合内部的数据。面向对象设计原则中有一条是类的单一职责原则,所以我们要尽可能的去分解这些职责,用不同的类去承担不同的职责。Iterator模式就是分离了集合对象的遍历行为,抽象出一个迭代器类来负责,这样既可以做到不暴露集合的内部结构,又可让外部代码透明的访问集合内部的数据。下面看一个简单的示意性例子,类结构图如下:
图3 示例代码结构图
首先有一个抽象的聚集,所谓的聚集就是就是数据的集合,可以循环去访问它。它只有一个方法GetIterator()让子类去实现,用来获得一个迭代器对象。
.NET中的Iterator模式
在.NET下实现Iterator模式,对于聚集接口和迭代器接口已经存在了,其中IEnumerator扮演的就是迭代器的角色,它的实现如下:
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属性Current返回当前集合中的元素,Reset()方法恢复初始化指向的位置,MoveNext()方法返回值true表示迭代器成功前进到集合中的下一个元素,返回值false表示已经位于集合的末尾。能够提供元素遍历的集合对象,在.Net中都实现了IEnumerator接口。
IEnumerable则扮演的就是抽象聚集的角色,只有一个GetEnumerator()方法,如果集合对象需要具备跌代遍历的功能,就必须实现该接口。
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下面看一个在.NET1.1下的迭代器例子,Person类是一个可枚举的类。PersonsEnumerator类是一个枚举器类。这个例子来自于http://www.theserverside.net/,被我简单的改造了一下。
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来看客户端代码的调用:
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程序将输出:
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现在我们分析编译器在执行foreach语句时到底做了什么,它执行的代码大致如下:
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可以看到这段代码跟我们最前面提到的示例代码非常的相似。同时在这个例子中,我们把大部分的精力都花在了实现迭代器和可迭代的类上面,在.NET2.0下面,由于有了yield return关键字,实现起来将更加的简单优雅。下面我们把刚才的例子在2.0下重新实现一遍:
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程序将输出:
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实现相同的功能,由于有了yield return关键字,变得非常的简单。好了,关于.NET中的Iterator模式就说这么多了,更详细的内容大家可以参考相关的资料。
效果及实现要点
1.迭代抽象:访问一个聚合对象的内容而无需暴露它的内部表示。
2.迭代多态:为遍历不同的集合结构提供一个统一的接口,从而支持同样的算法在不同的集合结构上进行操作。
3.迭代器的健壮性考虑:遍历的同时更改迭代器所在的集合结构,会导致问题。
适用性
1.访问一个聚合对象的内容而无需暴露它的内部表示。
2.支持对聚合对象的多种遍历。
3.为遍历不同的聚合结构提供一个统一的接口(即, 支持多态迭代)。
总结
Iterator模式就是分离了集合对象的遍历行为,抽象出一个迭代器类来负责,这样既可以做到不暴露集合的内部结构,又可让外部代码透明的访问集合内部的数据。