IBufferTest

using System;
using System.Collections.Generic;
using System.Threading;

/// <summary>
/// http://zrx.zhaojie.me/20140721/
/// 问题在于firstBuffer.Flush的时候会有新的item加到secondBuffer里面，导致一次Flush的时候firstBuffer的元素有可能比secondBuffer的元素少。
/// 解决方法是先把buffer给缓存出去（先缓存secondBuffer再缓存firstBuffer），然后再Flush。
/// </summary>
namespace IBufferTest
{
    public interface IBuffer
    {
        void Cache();
        void Flush();
    }

    public class Item
    {
        private bool _doFirst;
        private readonly int _id;

        public Item(int id)
        {
            _id = id;
        }

        public void First()
        {
            Console.WriteLine(_id + " First");
            _doFirst = true;
        }

        public void Second()
        {
            Console.WriteLine(_id + " Second");
            if (_doFirst)
            {
                return;
            }
            throw new Exception(_id + " Should invoke First() first");
        }
    }

    public class AddBuffer<T> : IBuffer
    {
        private readonly object _gate = new object();
        private List<T> _items;
        private List<T> _toProcess;

        public int CachedItemCount()
        {
            return _items == null ? 0 : _items.Count;
        }

        public void Add(T item)
        {
            lock (_gate)
            {
                if (_items == null)
                {
                    _items = new List<T>();
                }

                _items.Add(item);
            }
        }
 
        public void Cache()
        {
            lock (_gate)
            {
                _toProcess = _items;
                _items = null;
            }
        }

        public void Flush()
        {
            if (_toProcess == null)
            {
                return;
            }
            Process(_toProcess);
        }

        protected virtual void Process(List<T> items) { }
    }

    public class FirstBuffer : AddBuffer<Item>
    {
        protected override void Process(List<Item> items)
        {
            foreach (var item in items)
            {
                item.First();
            }
        }
    }

    public class SecondBuffer : AddBuffer<Item>
    {
        protected override void Process(List<Item> items)
        {
            foreach (var item in items)
            {
                item.Second();
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            var firstBuffer = new FirstBuffer();
            var secondBuffer = new SecondBuffer();

            // Thread A
            new Thread(() =>
            {
                var i = 0;
                while (true)
                {
                    var item = new Item(++i);

                    firstBuffer.Add(item);
                    secondBuffer.Add(item);

                    Thread.Sleep(1);
                }
            }).Start();

            // Thread B
            new Thread(() =>
            {
                while (true)
                {
                    secondBuffer.Cache();
                    firstBuffer.Cache();
                    firstBuffer.Flush();
                    secondBuffer.Flush();

                    Thread.Sleep(100);
                }
            }).Start();
        }
    }
}