迭代器模式分析、结构图及基本代码

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。
适用地方：当需要访问一个聚集对象，而且不管这些对象是什么都需要遍历的时候，就应该考虑用迭代器模式。或者当需要对聚集有多种方式遍历时，可以考虑使用迭代器模式。

尽管我们不需要显式地引用迭代器，但系统本身还是通过迭代器来实现遍历的。总的来说，迭代器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。迭代器模式在访问数组、集合、列表等数据时，尤其是数据库数据操作时，是非常普遍的应用。

结构图：



基本代码：

using System;

using System.Collections.Generic;

using System.Text;

namespace 迭代器模式

{

    class Program

    {

        static void Main(string[] args)

        {

            ConcreteAggregate a = new ConcreteAggregate();

            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);

            //Iterator i = new ConcreteIteratorDesc(a);

            object item = i.First();

            while (!i.IsDone())

            {

                Console.WriteLine("{0} 请买车票!", i.CurrentItem());

                i.Next();

            }

            Console.Read();

        }

    }

    abstract class Aggregate

    {

        public abstract Iterator CreateIterator();

    }

    class ConcreteAggregate : Aggregate

    {

        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

    {

        public abstract object First();

        public abstract object Next();

        public abstract bool IsDone();

        public abstract object CurrentItem();

    }

    class ConcreteIterator : Iterator

    {

        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

    {

        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;

        }

    }

}