Iterator模式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。
主要内容
1.Iterator模式解说
2.DotNET中的Iterator模式
……
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:
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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。
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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:
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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。
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/**//// <summary>
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/// 抽象聚集
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/// </summary>
public interface IList
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{
IIterator GetIterator();
}
抽象的迭代器,它是用来访问聚集的类,封装了一些方法,用来把聚集中的数据按顺序读取出来。通常会有MoveNext()、CurrentItem()、Fisrt()、Next()等几个方法让子类去实现。
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/**//// <summary>
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/// 抽象迭代器
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/// </summary>
public interface IIterator
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{
bool MoveNext();
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Object CurrentItem();
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void First();
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void Next();
}
具体的聚集,它实现了抽象聚集中的唯一的方法,同时在里面保存了一组数据,这里我们加上Length属性和GetElement()方法是为了便于访问聚集中的数据。
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/**//// <summary>
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/// 具体聚集
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/// </summary>
public class ConcreteList : IList
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{
int[] list;
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public ConcreteList()
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{
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list = new int[]
{ 1,2,3,4,5};
}
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public IIterator GetIterator()
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{
return new ConcreteIterator(this);
}
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public int Length
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{
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get
{ return list.Length; }
}
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public int GetElement(int index)
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{
return list[index];
}
}
具体迭代器,实现了抽象迭代器中的四个方法,在它的构造函数中需要接受一个具体聚集类型的参数,在这里面我们可以根据实际的情况去编写不同的迭代方式。
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/**//// <summary>
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/// 具体迭代器
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/// </summary>
public class ConcreteIterator : IIterator
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{
private ConcreteList list;
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private int index;
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public ConcreteIterator(ConcreteList list)
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{
this.list = list;
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index = 0;
}
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public bool MoveNext()
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{
if (index < list.Length)
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return true;
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else
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return false;
}
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public Object CurrentItem()
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{
return list.GetElement(index) ;
}
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public void First()
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{
index = 0;
}
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public void Next()
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{
if (index < list.Length)
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{
index++;
}
}
}
简单的客户端程序调用:
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/**//// <summary>
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/// 客户端程序
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/// </summary>
class Program
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{
static void Main(string[] args)
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{
IIterator iterator;
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IList list = new ConcreteList();
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iterator = list.GetIterator();
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while (iterator.MoveNext())
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{
int i = (int)iterator.CurrentItem();
Console.WriteLine(i.ToString());
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iterator.Next();
}
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Console.Read();
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}
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}
一个简单的迭代器示例就结束了,这里我们并没有利用任何的.NET特性,在C#中,实现Iterator模式已经不需要这么麻烦了,已经C#语言本身就有一些特定的实现,下面会说到。
.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:
public interface IEumerator
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{
object Current
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{
get;
}
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bool MoveNext();
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void Reset();
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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。
public interface IEnumerable
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{
IEumerator GetEnumerator();
}
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。
public class Persons : IEnumerable
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{
public string[] m_Names;
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public Persons(params string[] Names)
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{
m_Names = new string[Names.Length];
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Names.CopyTo(m_Names,0);
}
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private string this[int index]
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{
get
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{
return m_Names[index];
}
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set
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{
m_Names[index] = value;
}
}
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public IEnumerator GetEnumerator()
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{
return new PersonsEnumerator(this);
}
}
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public class PersonsEnumerator : IEnumerator
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{
private int index = -1;
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private Persons P;
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public PersonsEnumerator(Persons P)
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{
this.P = P;
}
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public bool MoveNext()
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{
index++;
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return index < P.m_Names.Length;
}
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public void Reset()
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{
index = -1;
}
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public object Current
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{
get
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{
return P.m_Names[index];
}
}
}
来看客户端代码的调用:
class Program
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{
static void Main(string[] args)
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{
Persons arrPersons = new Persons("Michel","Christine","Mathieu","Julien");
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foreach (string s in arrPersons)
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{
Console.WriteLine(s);
}
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Console.ReadLine();
}
}
程序将输出:
Michel
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Christine
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Mathieu
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Julien
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:
class Program
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{
static void Main(string[] args)
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{
Persons arrPersons = new Persons("Michel","Christine","Mathieu","Julien");
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IEnumerator e = arrPersons.GetEnumerator();
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while (e.MoveNext())
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{
Console.WriteLine((string)e.Current);
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}
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Console.ReadLine();
}
}
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:
public class Persons : IEnumerable
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{
string[] m_Names;
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public Persons(params string[] Names)
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{
m_Names = new string[Names.Length];
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Names.CopyTo(m_Names,0);
}
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public IEnumerator GetEnumerator()
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{
foreach (string s in m_Names)
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{
yield return s;
}
}
}
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class Program
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{
static void Main(string[] args)
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{
Persons arrPersons = new Persons("Michel","Christine","Mathieu","Julien");
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foreach (string s in arrPersons)
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{
Console.WriteLine(s);
}
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Console.ReadLine();
}
}
程序将输出:
Michel
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Christine
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Mathieu
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Julien
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。
参考资料
Erich Gamma等,《设计模式:可复用面向对象软件的基础》,机械工业出版社
Robert C.Martin,《敏捷软件开发:原则、模式与实践》,清华大学出版社
阎宏,《Java与模式》,电子工业出版社
Alan Shalloway James R. Trott,《Design Patterns Explained》,中国电力出版社
MSDN WebCast 《C#面向对象设计模式纵横谈(18):Iterator 迭代器模式(行为型模式)》
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