面向对象编程思想-状态模式

一、引言

上篇博客中学习了中介者模式，我们留下了一个问题，当出现多个玩家需要输赢状态条件判断时，可不可以不去修改中介者类，因为如果每新增一个条件判断，就要修改中介者类，破坏了封装，违背开闭原则。今天我们学习的内容就是要解决这种业务场景，状态模式

二、状态模式

定义：当一个对象的内在状态改变时允许改变其行为，这个对象看起来像是改变了其类

下面是状态模式的结构图：

下面是代码demo：

     //维护一个ConcreteState子类的实例，这个实例定义当前状态
    class Context
    {
        private State state;
        //定义Context的初始状态
        public Context(State state)
        {
            this.State = state;
        }
        //可读写的State
        public State State
        {
            get { return state; }
            set
            {
                state = value;
                Console.WriteLine($"当前状态为{this.State.GetType().Name}");
            }
        }
        //对请求做处理，并设置下一个状态
        public void Request()
        {
            state.Handle(this);
        }
    }
     //抽象状态类 定义一个接口以封装与Context的一个特定状态相关的行为
    abstract class State
    {
        public abstract void Handle(Context context);
    }
     class ConcreteStateA : State
    {
        //设置ConcreteStateA的下一个状态是ConcreteStateB
        public override void Handle(Context context)
        {
            context.State= new ConcreteStateB();
        }
    }
     class ConcreteStateB : State
    {
        public override void Handle(Context context)
        {
            context.State = new ConcreteStateA();
        }
    }
     class Program
    {
        static void Main(string[] args)
        {
            Context context = new Context(new ConcreteStateA());
            context.Request();
            context.Request();
            context.Request();
            Console.Read();
        }
    }

分析：上面Context类中State不同时对应不同类ConcreteState中的行为，即将与特定状态相关的行为局部化，并且将不同状态的行为分割开来。

1.确保了新增状态及对应的行为时，不是去修改很长一大串if...else..逻辑判断。消除庞大的分支语句

2.把各种状态转移逻辑分布到State类的子类中，更加利于扩展

下面是大话设计模式中的例子：

场景：上午状态好，中午想睡觉，下午渐恢复，晚上苦煎熬

     class Work
    {
        public State current;
        public Work()
        {
            current = new ForenoonState();
        }
        public int hour;
        public int Hour
        {
            get { return hour; }
            set { hour = value; }
        }
        public bool finish;
        public bool TaskIsFinish
        {
            get { return finish; }
            set { finish = value; }
        }
        //设置当前状态
        public void SetState(State state)
        {
            current = state;
        }
        public void WriteProgram()
        {
            current.WriteProgram(this);
        }
    }
     abstract class State
    {
        public abstract void WriteProgram(Work work);
       
    }
    class ForenoonState : State
    {
        public override void WriteProgram(Work work)
        {
            if(work.Hour<12)
            {
                Console.WriteLine("当前时间为{0}点，上午工作，精神充沛",work.Hour);
            }
            else
            {
                work.SetState(new NoonState());
                work.WriteProgram();
            }
        }
    }
    class NoonState : State
    {
        public override void WriteProgram(Work work)
        {
            if (work.Hour < 13)
            {
                Console.WriteLine("当前时间为{0}点，肚子饿了，午休，犯困",work.Hour);
            }
            else
            {
                work.SetState(new AfternoonState());
                work.WriteProgram();
            }
        }
    }
     class AfternoonState: State
    {
        public override void WriteProgram(Work work)
        {
            if (work.Hour < 17)
            {
                Console.WriteLine("当前时间为{0}点，下午状态还不错，继续努力", work.Hour);
            }
            else
            {
                work.SetState(new EveningState());
                work.WriteProgram();
            }
        }
    }
    class EveningState: State
    {
        public override void WriteProgram(Work work)
        {
            if (work.TaskIsFinish)
            {
                work.SetState(new ResetState());
                work.WriteProgram();
            }
            else
            {
                if(work.Hour<21)
                {
                    Console.WriteLine("当前时间为{0}点，要加班哦,疲累至极", work.Hour);
                }
                else
                {
                    work.SetState(new SleepingState());
                    work.WriteProgram();
                }               
            }
        }
    }
     class SleepingState: State
    {
        public override void WriteProgram(Work work)
        {
            Console.WriteLine("当前时间为{0}点，不行了，睡着了", work.Hour);
        }
    }
      class ResetState: State
    {
        public override void WriteProgram(Work work)
        {
            Console.WriteLine("当前时间为{0}点，下班了，回家了", work.Hour);
        }
    }
     class Program
    {
        static void Main(string[] args)
        {
            Work work = new Work();
            work.Hour = 9;
            work.WriteProgram();
            work.Hour = 12;
            work.WriteProgram();
            work.Hour = 13;
            work.WriteProgram();
            work.Hour = 17;
           // work.TaskIsFinish = true;
            work.WriteProgram();
            work.Hour = 19;
            work.WriteProgram();
            work.Hour = 23;
            work.WriteProgram();
            Console.Read();
        }
    }

分析：如果老板哪天要求“20点以后强制下班”，只需要新增加一个强制下班状态，稍微修改下傍晚状态类就可以了，而不会影响到其它状态类

下面是上篇博客中留下的问题，结合状态模式和中介者模式处理，下面是代码demo：

    abstract class Mediator
    {
        public IList<Colleague> lstcolleague = new List<Colleague>();
        public State State
        {
            get;
            set;
        }
        public Mediator(State state)
        {
            this.State = state;
        }
        public void Add(Colleague colleague)
        {
            lstcolleague.Add(colleague);
        }
        public void Remove(Colleague colleague)
        {
            lstcolleague.Remove(colleague);
        }
        public void Win(int number)
        {
            State.Win(number);
        }
    }
     class ConcreteMediator : Mediator
    {
        public ConcreteMediator(State state) : base(state)
        {
        }
    }
    abstract class State
    {
        public Mediator mediator;
        public abstract void Win(int number);
    }
     class ConcreteStateAWin : State
    {
        public ConcreteStateAWin(Mediator mediator)
        {
            this.mediator = mediator;
        }
        public override void Win(int number)
        {
            foreach (Colleague colleague in mediator.lstcolleague)
            {
                ConcreteColleagueA concreteColleagueA= colleague as ConcreteColleagueA;
                if(concreteColleagueA!=null)
                {
                    concreteColleagueA.Number += number;
                }
                else
                {
                    colleague.Number -= number;
                }
            }
        }
    }
    class ConcreteStateBWin : State
    {
        public ConcreteStateBWin(Mediator mediator)
        {
            this.mediator = mediator;
        }
        public override void Win(int number)
        {
            foreach (Colleague colleague in mediator.lstcolleague)
            {
                ConcreteColleagueB concreteColleagueB = colleague as ConcreteColleagueB;
                if (concreteColleagueB != null)
                {
                    concreteColleagueB.Number += number;
                }
                else
                {
                    colleague.Number -= number;
                }
            }
        }
    }
    class InitState : State
    {
        public InitState()
        {
            Console.WriteLine("游戏才刚刚开始");
        }
        public override void Win(int number)
        {
           
        }
    }
     abstract class Colleague
    {
        public int Number { get; set; }
        public abstract void Win(int number,Mediator mediator);
    }
     class ConcreteColleagueA : Colleague
    {
        public override void Win(int number, Mediator mediator)
        {
            mediator.Win(number);
        }
    }
     class ConcreteColleagueB : Colleague
    {
        public override void Win(int number, Mediator mediator)
        {
            mediator.Win(number);
        }
    }
     class Program
    {
        static void Main(string[] args)
        {
            ConcreteColleagueA concreteColleagueA = new ConcreteColleagueA();
            ConcreteColleagueB concreteColleagueB = new ConcreteColleagueB();
            //初始化
            concreteColleagueA.Number = 100;
            concreteColleagueB.Number = 100;
            ConcreteMediator concreteMediator = new ConcreteMediator(new InitState());
            //玩家A和B 进入游戏
            concreteMediator.lstcolleague.Add(concreteColleagueA);
            concreteMediator.lstcolleague.Add(concreteColleagueB);
            //玩家A赢了
            concreteMediator.State = new ConcreteStateAWin(concreteMediator);
            concreteMediator.Win(5);

            Console.WriteLine($"A的数量为{concreteColleagueA.Number}");
            Console.WriteLine($"B的数量为{concreteColleagueB.Number}");
            Console.Read();


        }
    }

分析：ok，解决了上面博客中的提出的两个问题，具体问题见上篇博客

优点：

1.将状态判断逻辑分到每个状态里面，减少了相互依赖，简化逻辑

2.当有新的状态出现时，可以通过新增状态来进行扩展，扩展性好

缺点：

1.当状态较多时，ConcreteState类较多，增加系统开销

适用场景：

1.当一个对象的行为取决于它的状态，并且必须在运行时刻根据状态改变行为时，就可以考虑状态模式

2.一个操作中有庞大的分支结构，并且这些分支结构取决于它的状态

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