人生如棋、我愿为卒、行动虽缓、从未退过

概述

迭代器（Iterator）模式，又叫做游标（Cursor）模式。GOF给出的定义为：提供一种方法访问一个容器（container）对象中各个元素，而又不需暴露该对象的内部细节。 从定义可见，迭代器模式是为容器而生。很明显，对容器对象的访问必然涉及到遍历算法。你可以一股脑的将遍历方法塞到容器对象中去；或者根本不去提供什么遍历算法，让使用容器的人自己去实现去吧。这两种情况好像都能够解决问题。

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。

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)。 

结构图

实例

// Iterator

/* Notes:
 * Here wish wish to separate node traversal from the nodes themselves. 
 * STL in ISO C++ is a highly successful application of this pattern.
 * Generic programming is a great way to implement iterators. As this is 
 * not yet in C#, we use inheritance. 
 *  
 */
 
namespace Iterator_DesignPattern
{
    using System;
    using System.Collections;

    class Node 
    {
        private string name;
        public string Name 
        {
            get 
            {
                return name;    
            }
        }
        public Node(string s)
        {
            name = s;
        }
    }
    
    class NodeCollection 
    {
        private ArrayList list = new ArrayList();
        private int nodeMax = 0;
        
        // left as a student exercise - implement collection
        // functions to remove and edit entries also
        public void AddNode(Node n)
        {
            list.Add(n); 
            nodeMax++;            
        }        
        public Node GetNode(int i)
        {
            return ((Node) list[i]);
        }

        public int NodeMax 
        {            
            get 
            {
                return nodeMax;
            }
        }
    }

    /*
     * The iterator needs to understand how to traverse the collection 
     * It can do that as way it pleases - forward, reverse, depth-first, 
     */
    abstract class Iterator 
    {
        abstract public Node Next();        
    }

    class ReverseIterator : Iterator
    {
        private NodeCollection nodeCollection;
        private int currentIndex;

        public ReverseIterator (NodeCollection c)
        {
            nodeCollection = c;            
            currentIndex = c.NodeMax -1; // array index starts at 0!
        }

        // note: as the code stands, if the collection changes,
        // the iterator needs to be restarted 
        override public Node Next()
        {
            if (currentIndex == -1)
                return null;
            else 
                return(nodeCollection.GetNode(currentIndex--));
        }
    }
    
    /// <summary>
    ///    Summary description for Client.
    /// </summary>
    public class Client
    {
        public static int Main(string[] args)
        {   
            NodeCollection c = new NodeCollection();
            c.AddNode(new Node("first"));
            c.AddNode(new Node("second"));
            c.AddNode(new Node("third"));

            // now use iterator to traverse this
            ReverseIterator i = new ReverseIterator(c);

            // the code below will work with any iterator type
            Node n;
            do 
            {
                n = i.Next();
                if (n != null) 
                    Console.WriteLine("{0}", n.Name);
            } while (n != null);
                
            return 0;
        }
    }
}