{C#}设计模式辨析.迭代器

背景

• 隐藏遍历方法，确保foreach的实现

示例

using System.Collections;
using System.Collections.Generic;
using static System.Console;

namespace DesignPattern_Enumerator
{
    // 逆向迭代器类，指定遍历方法
    public class ReversedIterator : IEnumerator
    {
        private readonly Aggregate aggregate;
        private int current;

        public ReversedIterator(Aggregate aggregate)
        {
            this.aggregate = aggregate;
            Reset();
        }

        // 调用前，先MoveNext()
        public object Current => aggregate.All[current];

        public bool MoveNext()
        {
            if (current == 0) return false;

            current--;
            return true;
        }

        public void Reset() => current = aggregate.All.Count;
    }

    // 集合类，IEnumerable仅包含IEnumerator
    public class Aggregate : IEnumerable
    {
        public List<string> collection = new List<string>();

        // foreach必须实现IEnumerator
        public IEnumerator GetEnumerator()
            => new ReversedIterator(this);

        public void Add(string item) => collection.Add(item);

        public List<string> All => collection;
    }

    class Program
    {
        static void Main(string[] args)
        {
            var alphabets = new Aggregate();
            alphabets.Add("a");
            alphabets.Add("b");
            alphabets.Add("c");

            foreach (var alphabet in alphabets)
            {
                WriteLine(alphabet);
            }

            ReadKey();
        }
    }
}