设计模式 迭代器模式

Gof定义

提供一种方法顺序访问一个聚合对象中的各个元素， 而又不暴露该对象的内部表示。

理解

我们实际上经常用到迭代器模式，例如FOREACH语句，就是对迭代器模式的实现，泛型像LIST也是对迭代器模式的实现。关于迭代器模式在实际应用中很广泛，但他已经渐渐的融入到我们的编程语言语法中的其中一部份，所以实际上可能不需要再自己要动手写一个迭代器模式了。

但是大概去了解一下内部实现原理，我觉的还是很值得的。

示例代码 

/// <summary>
/// 集合结构接口
/// </summary>
public interface Aggregate
{
    Iterator CreateIterator();
}
/// <summary>
/// 迭代器接口
/// </summary>
public interface Iterator
{
    object First();
    object Next();
    bool IsDone();
    object CurrentItem();
}
/// <summary>
/// 集合结构的具体类
/// </summary>
class ConcreteAggregate : Aggregate
{
    private List<object> items = new List<object>();
    public Iterator CreateIterator()
    {
        return new ConcreteIterator(this);
    }
    public int Count
    {
        get { return items.Count; }
    }
    public object this[int index]
    {
        get { return items[index]; }
        set { items.Insert(index, value); }
    }
}
/// <summary>
/// 具体的迭代器类
/// </summary>
class ConcreteIterator : Iterator
{
    private ConcreteAggregate _aggregate;
    private int _current = 0;
    public ConcreteIterator(ConcreteAggregate aggregate)
    {
        this._aggregate = aggregate;
    }
    public object First()
    {
        return _aggregate[0];
    }
    public object Next()
    {
        object r = null;
        _current++;
        if (_current < _aggregate.Count)
        {
            r = _aggregate[_current];
        }
        return r;
    }
    public bool IsDone()
    {
        return _current >= _aggregate.Count ? true : false;
    }
    public object CurrentItem()
    {
        return _aggregate[_current];
    }
}
/// <summary>
/// 客户端调用
/// </summary>
class Program
{
    static void Main(string[] args)
    {
        ConcreteAggregate ca = new ConcreteAggregate();
        ca[0] = "AspNet3.5 揭秘";
        ca[0] = "重构：改善既有代码的设计";
        ca[2] = "设计模式";
        ca[3] = "人月神话";
        ca[4] = "代码大全2";
        Iterator i = new ConcreteIterator(ca);
        while (!i.IsDone())
        {
            Console.WriteLine("要读的书:" + i.CurrentItem());
            i.Next();
        }
    }
}

去掉Aggregate类，简单实现

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            ConcreteAggregate ca = new ConcreteAggregate();
            ca[0] = "高等数学";
            ca[1] = "C语言程序设计";
            ca[2] = "操作系统";
            ca[3] = "数据结构";
            ca[4] = "算法原理";
            ca[5] = "编译原理";


            Iterator i = new ConcreteIterator(ca);

            while (i.IsDone())
            {
                Console.WriteLine("要读的书:" + i.CurrentItem());
                i.Next();
            }



            Console.ReadLine();
        }
    }

    public interface Iterator
    {
        object First();
        void Next();
        bool IsDone();
        object CurrentItem();
    }

    class ConcreteAggregate
    {
        private List<object> items = new List<object>();

        public int Count
        {
            get { return items.Count; }
        }

        public object this[int index]
        {
            get { return items[index]; }
            set { items.Insert(index, value); }
        }

    }

    class ConcreteIterator : Iterator
    {

        private ConcreteAggregate _aggregate;
        private int _current = 0;
        public ConcreteIterator(ConcreteAggregate aggregate)
        {
            this._aggregate = aggregate;
        }

        public object First()
        {
            return _aggregate[0];
        }

        public void Next()
        {
            if(_current < _aggregate.Count)
                _current++;
        }

        public bool IsDone()
        {
            if (_current < _aggregate.Count)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        public object CurrentItem()
        {
            return _aggregate[_current];
        }

    }


}