C# 3.0下有限状态机的一种优雅的实现

C# 3.0下有限状态机的一种优雅的实现

实现状态机有多种模式，其中最灵活而强大的方式是通过迁移表来实现，该方式的缺点之一是需要编写大量小块代码去支持迁移表。而在C#3.0中，可以以一种非常优雅的方式实现。

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

namespace StateMachine
{
    class Program
    {
        static void Main(string[] args)
        {
            var door = new Door(State.Open);

            while (true)
            {
                string s = Console.ReadLine();
                Operation op = string.IsNullOrEmpty(s) ? Operation.Push : Operation.Pull;
                door.Process(op);
            }
        }
    }

    enum Operation
    {
        Push, Pull
    }

    enum State
    {
        Open, Closed
    }

    class Door
    {
        public State State { get; set; }

        Dictionary<State, Dictionary<Operation, Action>> rule;
        public Door(State state)
        {
            this.State = state;

            rule = new Dictionary<State, Dictionary<Operation, Action>>();
            foreach (var e in Enum.GetValues(typeof(State)))
            {
                rule[(State)e] = new Dictionary<Operation, Action>();
            }

            InitOperationRule();
        }

        void InitOperationRule()
        {
            //正常操作
            rule[State.Closed][Operation.Push] = () => { Console.WriteLine("门被推开了"); State = State.Open; };
            rule[State.Open][Operation.Pull] = () => { Console.WriteLine("门被拉上了"); State = State.Closed; };

            ////加入几种特殊情况的处理
            //rule[State.Closed][Operation.Pull] = () => Console.WriteLine("门是关上的，拉了也白拉");
            //rule[State.Open][Operation.Push] = () => Console.WriteLine("门是开的，不用推了，直接进去吧");
        }

        public void Process(Operation op)
        {
            try
            {
                rule[State][op]();
            }
            catch (KeyNotFoundException)
            {

                Console.WriteLine(string.Format("门在{0}状态下不允许{1}操作", State, op));
            }
            
        }
    }
}

从代码中可以看到，通过lambda表达式，可以简化迁移表的构造，并且更加直观。

通过迁移表构造状态机的一种不足在于查询速度，在本例中每个操作都要进行两次查询才能进行状态转换操作。如果状态较多则非常费时，这里我把它改进了一下，使得每次操作只需要查询一次即可。

    class DoorPlus
    {
        State state;
        public State State
        {
            get { return state; }
            set
            {
                if (state != value)
                    currentOpRule = rule[value];
                state = value;
            }
        }

        Dictionary<Operation, Action> currentOpRule;
        Dictionary<State, Dictionary<Operation, Action>> rule;
        public DoorPlus(State state)
        {
            this.State = state;

            rule = new Dictionary<State, Dictionary<Operation, Action>>();
            foreach (var e in Enum.GetValues(typeof(State)))
            {
                rule[(State)e] = new Dictionary<Operation, Action>();
            }

            currentOpRule = rule[State];

            InitOperationRule();
        }

        void InitOperationRule()
        {
            //正常操作
            rule[State.Closed][Operation.Push] = () => { Console.WriteLine("门被推开了"); State = State.Open; };
            rule[State.Open][Operation.Pull] = () => { Console.WriteLine("门被拉上了"); State = State.Closed; };

            ////加入几种特殊情况的处理
            //rule[State.Closed][Operation.Pull] = () => Console.WriteLine("门是关上的，拉了也白拉");
            //rule[State.Open][Operation.Push] = () => Console.WriteLine("门是开的，不用推了，直接进去吧");
        }

        public void Process(Operation op)
        {
            try
            {
                currentOpRule[op]();
            }
            catch (KeyNotFoundException)
            {

                Console.WriteLine(string.Format("门在{0}状态下不允许{1}操作", State, op));
            }
        }
    }