【设计模式】迭代器模式

一，概述

定义：提供一种方法顺序访问一个聚合对象中各个元素，而又不暴漏该对象的内部表示。

作用：客户端可以不必知道数据集合的结构，而通过迭代器的接口来操作数据，不仅保护了数据结构，同时可以自定义数据的访问方式。

其实C++中的STL模板类中大量用到了此种设计模式！

二，示例

问题：售票员清点上车人数，然后挨个人遍历，让其买票。

 class Program //客户端
    {
        static void Main(string[] args)
        {
            ConcreteAggregate a = new ConcreteAggregate();//聚集类

            a[0] = "大鸟";
            a[1] = "小菜";
            a[2] = "行李";
            a[3] = "老外";
            a[4] = "公交内部员工";
            a[5] = "小偷";

            Iterator i = new ConcreteIterator(a);//迭代器
            //Iterator i = new ConcreteIteratorDesc(a);
            object item = i.First();
            while (!i.IsDone())//遍历聚集类
            {
                Console.WriteLine("{0} 请买车票!", i.CurrentItem());
                i.Next();
            }

            Console.Read();
        }
    }

    abstract class Aggregate//抽象聚集类
    {
        public abstract Iterator CreateIterator();
    }

    class ConcreteAggregate : Aggregate//具体聚集类
    {
        private IList<object> items = new List<object>();
        public override 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); }
        }
    }

    abstract class Iterator//抽象迭代器类（方便实现各种迭代策略）
    {
        public abstract object First();
        public abstract object Next();
        public abstract bool IsDone();
        public abstract object CurrentItem();
    }

    class ConcreteIterator : Iterator//具体迭代器类(升序访问)
    {
        private ConcreteAggregate aggregate;
        private int current = 0;

        public ConcreteIterator(ConcreteAggregate aggregate)
        {
            this.aggregate = aggregate;
        }

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

        public override object Next()
        {
            object ret = null;
            current++;

            if (current < aggregate.Count)
            {
                ret = aggregate[current];
            }

            return ret;
        }

        public override object CurrentItem()
        {
            return aggregate[current];
        }

        public override bool IsDone()
        {
            return current >= aggregate.Count ? true : false;
        }

    }

    class ConcreteIteratorDesc : Iterator//具体迭代器类（降序访问）
    {
        private ConcreteAggregate aggregate;
        private int current = 0;

        public ConcreteIteratorDesc(ConcreteAggregate aggregate)
        {
            this.aggregate = aggregate;
            current = aggregate.Count - 1;
        }

        public override object First()
        {
            return aggregate[aggregate.Count - 1];
        }

        public override object Next()
        {
            object ret = null;
            current--;
            if (current >= 0)
            {
                ret = aggregate[current];
            }

            return ret;
        }

        public override object CurrentItem()
        {
            return aggregate[current];
        }

        public override bool IsDone()
        {
            return current < 0 ? true : false;
        }

    }